Welded structural joint between a high-performance thermoplastic matrix composite material and an elastomer by powder functionalization

Abstract

A method for producing, by welding, a joint between a thermoplastic matrix composite material and an elastomeric material. The elastomeric material is functionalized by forming a thin layer of elastomeric material incorporating particles of thermoplastic material and melting the thin layer onto the surface of the elastomeric material during the pressure-vulcanization of the elastomer. The functionalized surface of the elastomeric material is welded to the thermoplastic material of the composite. The welding operation includes interposing a metal fabric coated with thermoplastic material between the surfaces of the elastomer and the composite that are welded to each other, and passing an electric current through same, resulting in the surface melting of the two materials.

Claims

1. A method for producing a welded structural joint between a thermoplastic matrix composite and an elastomer, comprising the steps of: providing a functionalized elastomeric material layer by incorporating a predetermined proportion of a powder of a thermoplastic material in a thin layer of raw elastomeric material; applying the functionalized elastomeric material layer onto a main layer of the elastomeric material; fusing the functionalized elastomeric material layer with the main layer of the elastomeric material during a step of vulcanization of both the functionalized elastomeric material layer and the main layer of the elastomeric material under pressure; and welding a thermoplastic composite material of the thermoplastic matrix composite to the functionalized elastomeric material layer.

2. The method as claimed in claim 1, wherein the step of functionalizing comprises the steps of: cleaning various parts of a vulcanizing mold; preparing a pre-functionalized thin layer of raw elastomer by incorporating homogeneously the predetermined proportion of the powder of the thermoplastic material in the pre-functionalized thin layer of elastomer; applying the pre-functionalized thin layer of raw elastomer on a surface of the elastomeric material; vulcanizing the elastomeric material under a pressure so that the pre-functionalized thin layer of raw elastomer becomes incorporated in the main layer of the elastomeric material, at least at the surface, during the vulcanizing process.

3. The method as claimed in claim 1, further comprising the step of selecting the thermoplastic material incorporated in the thin layer of the raw elastomeric that is both weldable to a thermoplastic material forming the thermoplastic matrix composite and compatible with the elastomeric material.

4. The method as claimed in claim 3, wherein the thermoplastic material incorporated in the thin layer of raw elastomeric material is identical to the thermoplastic material forming the thermoplastic matrix composite.

5. The method as claimed in claim 1, wherein the thermoplastic composite material is a carbon/polyetheretherketone composite; wherein the elastomeric material is of a hydrogenated nitrile butadiene rubber type; and wherein the thermoplastic material incorporated in the thin layer of raw elastomeric material is a polyetherimide.

6. The method as claimed in claim 5, further comprising the step of incorporating the thermoplastic material in the raw elastomeric material in a form of particles with a diameter between 20 m and 60 m, in a proportion between 10% and 25% of the mixture by weight.

7. The method as claimed in claim 1, wherein the step of welding comprises the steps of heating faying surfaces of two materials to be welded by interposing a woven metal wire cloth between the faying surfaces of the two materials, the woven metal wire cloth functions as a heating resistance and the woven metal wire cloth is impregnated with a thermoplastic material.

8. The method as claimed in claim 7, further comprising the step of preparing the faying surfaces of the two materials to be assembled prior to the step of welding.

9. The method of claim 8, wherein the step of preparing comprises the steps of degreasing using a solvent, and cleaning the faying surfaces of the two materials with the solvent.

10. The method of claim 8, wherein the step of preparing comprises the steps of grinding a functionalized surface of the elastomeric material forming the elastomer, and cleaning the surface of the functionalized layer with a solvent.

11. The method as claimed in claim 1, wherein the thermoplastic composite material is a carbon/polyetheretherketone composite; wherein the elastomeric material is of a hydrogenated nitrile butadiene rubber type; and wherein the step of functionalizing the elastomeric material comprises the steps of: setting up the following elements on a bottom plate of a vulcanizing mold and covering the elements with a top plate of the vulcanizing mold to provide a stack: a first set of two superposed layers of Teflon-coated glass cloth; a sheet of raw elastomeric material in which a polyetherimide powder has been incorporated therein; sheets of unvulcanized raw elastomeric material; a set of edging blocks; and a second set of two superposed layers of Teflon-coated glass cloth; and placing the stack on plate of a heating press preheated to a temperature .sub.2 of 140 C.; hot pressing the stack by applying a nominal pressing cycle of the elastomer in question is applied; removing the vulcanizing mold from the heating press which maintains the temperature .sub.2 of 140 C.; and removing the functionalized layer of elastomeric material from the vulcanizing mold to cool to a room temperature.

12. The method as claimed in claim 11, wherein the nominal pressing cycle comprises: a phase of gradual increase in temperature to a high temperature .sub.1 of 230 C. at a gradient of 2.5 C./min; a phase of maintaining a temperature .sub.1 for 10 minutes; and a phase of temperature decrease to the temperature .sub.2 of 140 C. at a gradient of 2.5 C./min.

13. The method as claimed in claim 1, wherein the thermoplastic composite material is a carbon/polyetheretherketone composite; wherein the elastomeric material is of a hydrogenated nitrile butadiene rubber type, and wherein the step of welding comprises the steps of: setting up and arranging the following elements in order on a plate of heat-insulating material to provide a stack: a first film of heat-resistant polyimide; the functionalized elastomeric material; a first film of polyetherimide; a woven metal wire cloth pre-impregnated with polyetherimide; a second film of polyetherimide; the thermoplastic composite material; a second film of heat-resistant polyimide; a layer of heat-insulating material; and a glass cloth; producing a bladder under a vacuum with a third film of heat-resistant polyimide arranged to allow the woven metal wire cloth to stand out and ensuring a hermeticity of the bladder by a high-temperature mastic; placing the bladder under the vacuum, connecting an electric power supply unit to the woven metal wire cloth, and applying different values of current according to a predetermined cycle; removing the third film of heat-resistant polyimide serving as a vacuum enclosure and dismounting the stack.

14. The method as claimed in claim 13, further comprising the steps of applying a current of 15.6 amps to the woven metal wire cloth for 200 seconds, and then applying a current of 10 amps to the woven metal wire cloth for 100 seconds.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The features and advantages of the invention will be better understood from the description that follows, which is based on the appended figures, in which:

(2) FIG. 1 is a schematic illustration of an example of a structure for which assembly of the different elements by welding seems particularly suitable, in view of the mechanical stresses imposed on said assembly;

(3) FIG. 2 is an illustration of an embodiment example of the elastomer functionalization step of the method according to the invention; and

(4) FIG. 3 is an illustration of an embodiment example of the welding operation according to the invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

(5) The principle of the method according to the invention consists mainly of carrying out an operation of preparation of the elastomeric material, before welding proper. This operation consists of assembling, at the raw elastomer stage, i.e. unvulcanized, two separate layers of elastomeric material: a first layer, or main layer, of homogeneous raw elastomeric material, and a so-called prefunctionalized second layer. The functionalization operation consists here of modifying the surface, or the actual core, of the elastomer in order to endow it with the necessary properties so that it can be welded to the thermoplastic composite material. Functionalize the elastomeric material means modify the composition of the surface layers of this material by incorporating a thermoplastic material therein.

(6) For this purpose, to produce the so-called functionalized first layer, a given proportion of powder of a thermoplastic material having both compatibility with the elastomeric material and being weldable to the matrix of the thermoplastic composite material to which we wish to assemble the elastomeric material in question, is introduced into this layer of raw elastomeric material.

(7) Compatibility between the elastomeric material and the thermoplastic material forming the powder incorporated in this material mainly means, in this case, physicochemical compatibility between the two materials. This compatibility is reflected in possible interaction between the thermoplastic material of the powder incorporated and the elastomer in which this powder is incorporated, an interaction that a person skilled in the chemistry of plastics is able to characterize by specific criteria, which are rather of a chemical nature: wettability, mutual diffusion between the materials, chemical bonds.

(8) However, we are also interested in compatibility of application between the different materials (physical compatibility), notably regarding the need to be able to hot-vulcanize the elastomer thus functionalized but without causing degradation of the thermoplastic material incorporated in the form of powder.

(9) According to the invention, the process for producing the functionalized elastomer comprises several steps: A) a preliminary step of cleaning the various parts of the vulcanizing mold, said cleaning being carried out for example with methyl ethyl ketone (or MEK). B) a first step during which a thin layer of raw elastomeric material is prepared, prefunctionalized by incorporating a given proportion of powder of thermoplastic material, homogeneously in this layer of elastomer.

(10) According to the invention, the proportion of powder is determined by carrying out mechanical testing on peeling test specimens, adjusting said proportion so as to obtain the desired value of peel strength, with a value equal to at least 20 N/mm.

(11) Moreover, the granulometry of the powder is in its turn selected so as to facilitate mixing with the elastomer.

(12) Also according to the invention, the various constituents of the raw elastomer, constituted for example of process oil-based rubber and silica fillers, and the proportion of powder of thermoplastic material are mixed in a closed mixer of the Werner type. The peroxide vulcanizing agent is then added to the open mixer. The last step consists of calendering the functionalized raw elastomer thus obtained, so as to produce the thin layer with the desired thickness. C) a second step during which the prefunctionalized layer of raw elastomer is applied on the surface of the raw elastomeric material to be assembled with the thermoplastic composite material and, D) a third step during which vulcanization of the elastomeric material under pressure is carried out. According to the invention, the pressure applied is determined such that the two layers of elastomeric material fuse at the surface during the vulcanizing process.

(13) A vulcanized functionalized elastomer is obtained comprising a conventional elastomer base covered with a layer of functionalized elastomer of small thickness, a layer with a thickness of 1 mm for example.

(14) Thus, for example, in a particular embodiment of the invention, more particularly suitable for welding a composite material with a matrix of polyetheretherketone (PEEK) and with carbon fiber reinforcement (carbon/PEEK thermoplastic composite) to an elastomer of the type of hydrogenated nitrile rubbers (hydrogenated butadiene-acrylonitrile), or Buna or else HNBR (hydrogenated nitrile butadiene rubber), step C) of production proper of the functionalized elastomer may itself comprise the following operations: a) A first operation of setting up, on the bottom plate 21 of the vulcanizing mold, the elements necessary for carrying out functionalization of the elastomeric material, namely in the following order, and as illustrated in FIG. 2: two superposed layers 22 and 23 of Teflon-coated glass cloth, whose role consists of allowing mold release of the elastomer after polymerization; the layer of raw elastomer 24 in which polyetherimide powder (or PEI) has been incorporated. The powder selected is a PEI powder, Ultem 1000 for example, which has a median diameter between 20 m and 60 m, a diameter of 45 m for example. As for the proportion of powder incorporated, it has a value between 10% and 25% of the total mixture by weight, a value roughly equal to 20% for example. the sheets of raw elastomeric material (HNBR) 25 (unvulcanized), to be functionalized, a set of edging blocks 26, 27, blocks of type 304L stainless steel for example, two superposed layers 28 and 29 of Teflon-coated glass cloth, performing a role similar to that provided by layers 22 and 23.

(15) After setting up, the whole is covered with the top plate 211 of the vulcanizing mold so that all the elements listed above are positioned between the bottom plate 21 and the top plate 211 of the mold; b) a second operation of placing the stack prepared on the plate of a heating press preheated to a temperature .sub.2 of 140 C.; c) a third operation of hot pressing of the stack, during which the nominal pressing cycle appropriate to the elastomer in question is applied, said cycle comprising: a phase of gradual increase in temperature up to a high temperature .sub.1 (230 C. at 2.5 C./min), a phase of holding at the temperature .sub.1 (230 C. plateau for 10 minutes), a phase of temperature decrease to the temperature .sub.2 (140 C. at 2.5 C./min); d) a fourth operation during which the mold is removed from the press while the latter maintains the temperature .sub.2 (140 C.), the functionalized elastomer is removed from the mold and is left to cool to room temperature.

(16) As already stated above, a surface-functionalized vulcanized elastomer is thus obtained, comprising a conventional elastomer base having a surface layer of functionalized elastomer 24 formed from a mixture of elastomer and thermoplastic material.

(17) In a particular embodiment, the thermoplastic material used for carrying out the operation of functionalization of the elastomer is selected to be the same material as that which constitutes the matrix of the composite material to which the elastomeric material is to be welded. However, it should be noted that the thermoplastic material used may be different, notably to better satisfy the dual requirement of good adhesion of the two thermoplastic materials (i.e. that of the composite material and that used for functionalization of the elastomer) and of good compatibility of the material used for functionalization of the elastomer and of the elastomeric material itself.

(18) Once the polymer material is functionalized, the method according to the invention continues with the welding step proper, an operation which, owing to the functionalization of the polymer material, takes place advantageously similarly to an operation of welding of two parts made of thermoplastic matrix composite. This operation may notably be carried out by induction or by resistance.

(19) It should be noted that, advantageously, the welding operation is not subject to any constraint of delay in being carried out, in contrast to what may happen in the case of assembly by gluing. Since the functionalized elastomeric material is vulcanized before assembly, welding of the two materials may thus be carried out at the time that is considered to be the most suitable, which may be a varying length of time after completion of functionalization.

(20) The resistance method consists of local heating of the interface between the composite material and the functionalized elastomer, to obtain a bond between the thermoplastic matrix of composite material and the thermoplastic part of the functionalized elastomer.

(21) According to the invention, the welding step begins with a preliminary operation I) of preparation of the surfaces to be assembled. Depending on the state of these surfaces, preparation may consist of simple degreasing using a suitable solvent, ethanol for example, or fine grinding of the functionalized surface of the element made of elastomer, with SiC paper abrasive of 400 grain for example, followed by cleaning the surfaces and in particular the ground surface, using the same solvent. Here, suitable solvent means a solvent of fats, which by its nature or its composition does not present any risk of causing dissolution of the material itself.

(22) There then follows the operation II) of welding proper, consisting of heating the faying surfaces of the two materials to be welded by interposing, between these two surfaces, a woven metal wire cloth, which performs the function of heating resistance; said operation being carried out under vacuum.

(23) According to the invention, the woven metal wire cloth is itself impregnated with thermoplastic material, PEI for example.

(24) Thus, for example, in a particular embodiment of the invention, more particularly suitable for welding a composite material with a PEEK matrix and with carbon fiber reinforcement (carbon/PEEK thermoplastic composite) to an elastomer of the HNBR type (Buna), operation II) of welding proper may itself comprise the following steps: a) A first step of setting up, during which, as illustrated in FIG. 3, mainly the following elements are arranged, in the order shown, on a plate 31 of heat-insulating material, NL FIH for example: a first film 32 of heat-resistant polyimide, preferably a film of Upilex, intended to promote disassembly of the assembly after welding, the functionalized elastomeric material 33, a first film 34 of polyetherimide (PEI), of the type Ultem 1000 for example, a woven metal wire cloth 35 preimpregnated with PEI, for example a cloth of reference 102083 manufactured by the company Gantois, a second film 36 of polyetherimide (PEI), the thermoplastic composite material 36 (C/PEEK) coated with PEI, a second film 37 of polyimide Upilex also intended to promote disassembly of the assembly after welding, a layer of heat-insulating material 38 (NL FIH), glass cloth 311, of the type E5555 for example, intended to promote drainage of the air present. b) A second step during which a bladder is produced under vacuum with a film 312 of polyimide (Thermalimide) arranged so as to allow the woven metal wire cloth impregnated with PEI to stand out; hermeticity being obtained with a high-temperature mastic A800 3G. c) A third step during which the bladder 312 thus produced is placed under vacuum, an electric power supply unit is connected to the woven metal wire cloth 35 and different values of current are applied according to an appropriate cycle: for example 15.6 A for 200 seconds and then 10 A for 100 seconds. d) A fourth step during which the polyimide film 312 serving as vacuum enclosure is removed and the stack is dismounted, so as to recover the elastomer-thermoplastic composite assembly thus produced.