electrically insulating composition based on polybenzimidazole and liquid crystal polymer, process for its manufacture and material comprising

Abstract

The disclosure relates to a composition comprising at least one polybenzimidazole and at least one liquid crystal polymer in a mass ratio (RM, mass polybenzimidazole to mass liquid crystal polymer) between 0.01 and 99, and an electrically insulating material comprising said composition. Abstract: Figure 2

Description

Title of the invention: Electrically insulating composition based on polybenzimidazole and liquid crystal polymer, method for manufacturing it and material comprising it. Technical field

[0001] This disclosure falls within the field of electrically insulating polymer compositions. In particular, this disclosure relates to electrically insulating materials, especially for their use in electrical cables. Prior art

[0002] Polyimide, better known by its trade name Kapton®, and developed by DuPont, was widely used as an insulating material for electrical cables. Invented in 1955, it was quickly accepted as an excellent insulating material with an exceptional combination of thermal stability, mechanical toughness, and chemical resistance. Consequently, manufacturers used a polyimide tape wrapped around an electrical conductor to create an insulated cable. Kapton® made rapid inroads into commercial and military aircraft, and even space shuttles, in the 1970s. However, the popularity of Kapton® wiring ended in the early 1980s when the U.S. military observed that some aircraft fires and accidents were attributed to electrical short circuits. Polyimide was implicated in several aviation incidents and became a threat to the aerospace industry.Indeed, polyimides degrade rapidly when exposed to a combination of heat, humidity, and mechanical stress. The US Navy banned polyimide-insulated cables for aeronautical applications in 1992. Therefore, to maintain the exceptional mechanical and thermal performance of Kapton®, the polyimides were bonded to a PTFE top layer, better known by its trade name Teflon®, developed by DuPont. In the late 1990s, Airbus began using Teflon®-coated Kapton®, known generically as "KT" cable, for Kapton®-Teflon®. Subsequently, a cable called "TKT" was installed in Boeing 757 and 737 aircraft built after 1992. Since mid-2006, Airbus has been using its own version of the TKT shown in [Fig. 1], comprising at its core a copper conductor, covered with a layer of PTFE, covered with a layer of polyimide, covered with a layer of PTFE, covered with a PTFE sheath.

[0003] While these cables offer a good technical solution, evolving European regulations necessitate the development of alternatives. Indeed, on February 7, 2023, the European Chemicals Agency (ECHA) published a regulatory proposal. aiming to restrict the manufacture, placing on the market, and use of per- and polyfluoroalkyl substances (PFAS) in the European Union. This includes a total ban on more than 10,000 PFAS, including PTFE. Therefore, the use of PTFE could be regulated in the coming years.

[0004] At first glance, liquid crystal polymers (LCPs) could represent an interesting alternative to the aforementioned fluorinated insulators. Indeed, one of the characteristics of LCPs is their molten state. When melted or solvated, LCP molecules tend to align themselves lengthwise like a crystal. This liquid crystal state results from the rigid nature of LCP molecules, which tend to stack like parallel logs. There is an intermediate temperature at which the LCP becomes fluid without breaking the crystalline structure. This crystalline structure prevents the massive absorption of moisture or other gases.

[0005] Due to their thermoplastic nature, LCPs can be shaped by processes such as molding or extrusion. For example, Vectra® A950 has a melting point of approximately 280°C, compatible with the extrusion process. LCPs also possess very good thermal resistance. Thermogravimetric analysis of Vectra® A950 shows a mass loss of 5% at 482°C in air with a temperature ramp of 10°C / minute. The physicochemical, mechanical, and dielectric properties of Vectra® A950 (European Polymer Journal, 2023, Volume 196, page 112302) noted that the dielectric constant of Vectra® A950 is of the same order of magnitude as polyimide-type insulators (i.e., > 3).2)

[0006] Furthermore, Vectra® A950 exhibits good resistance to chemicals such as acetone, chromic acid, dimethylformamide, motor oil, ethanol, methanol, ethyl acetate, 37% aqueous hydrochloric acid, silicone oil, R22 refrigerant, 10% aqueous sodium hydroxide, 50% aqueous sulfuric acid, toluene, and water (10 days at 121°C).

[0007] However, extruded LCPs generally exhibit poor abrasion resistance, as the LCP layer tends to fray into fibers, and they generally lack flexibility. While these properties may not be problematic in some applications, they are problematic for the manufacture of electrical cables, which are typically produced by extrusion. Solutions to this problem have been proposed, notably in US patent 2010 / 0326696, which discloses a material comprising LCP and other polymers, with the addition of an extruded layer over the LCP layer. However, the other polymers proposed in this patent are of the PFAS type.

[0008] There is therefore a need to propose a new material that can be used, in particular as insulation in electrical cables.

[0009] Thus, the present invention aims to achieve at least one of the following objectives: -01- Propose an electrically insulating material that does not contain PFAS, -02- Propose an electrically insulating material that can be extruded without loss of mechanical or chemical performance. -03- Propose an electrical cable free of PFAS, -04- Propose an electrical cable that can be used in an aeronautical environment. Summary

[0010] Surprisingly, at least one of the aforementioned objectives is achieved through a composition comprising at least one polybenzimidazole and at least one liquid crystal polymer in a mass ratio RM, mass polybenzimidazole to mass liquid crystal polymer of between 0.01 and 99.

[0011] This disclosure also relates to a process for preparing the composition mentioned above, comprising the following successive steps: E1: prepare an aqueous solution of polybenzimidazole having a pH between 1 and 3, E2: heat the aqueous solution obtained in step E1 to a temperature between 60°C and 100°C, preferably between 70°C and 90°C, for example 80°C, in order to obtain a solution, E3: lower the temperature of the solution obtained at the end of step E2 to room temperature, for example between 15°C and 30°C, preferably 25°C, E4: add to the solution obtained at the end of step E3 an amount of liquid crystal polymers to obtain a mass ratio RM, mass polybenzimidazole to mass liquid crystal polymer of between 0.01 and 99, and stir at room temperature, for example between 15°C and 30°C, preferably 30°CE5: Pour the solution obtained at the end of step E4 into a sufficient volume of water to precipitate the polybenzimidazole and the liquid crystal polymer to form a solid polymer mixture. E6: Filter, rinse, and dry the polymer mixture obtained at the end of step E5.

[0012] Furthermore, the present disclosure relates to an electrically insulating material characterized in that it comprises, preferably is made of, the composition mentioned above.

[0013] In addition, the present disclosure is directed to an electrical cable comprising an electrically conductive material 1, said wire being covered with at least one layer 3 of the electrically insulating material mentioned above and optionally at least one layer 2 of an electrically insulating material different from the insulating material of layer 3.

[0014] The features described in the following paragraphs may optionally be implemented independently of each other or in combination with each other. Brief description of the drawings

[0015] Other features, details and advantages will become apparent from reading the detailed description below and from analyzing the accompanying drawings, in which: Fig. 1

[0016] [Fig.1] Fig.1 is a cross-sectional representation of a TKT type electrical cable, in accordance with the prior art. Fig. 2

[0017] [Fig.2] Fig.2 shows an electrical cable according to one embodiment. Fig. 3

[0018] [Fig.3] Fig.3 shows an electrical cable according to another embodiment. Detailed description

[0019] The composition

[0020] As mentioned above, the present disclosure relates to a composition comprising at least one polybenzimidazole and at least one liquid crystal polymer in a mass ratio RM, mass polybenzimidazole to mass liquid crystal polymer of between 0.01 and 99.

[0021] Polybenzimidazole

[0022] Polybenzimidazoles (PBIs) are amorphous thermoplastics initially marketed as fibers due to their high strength, high modulus, and excellent flame-retardant properties. They are generally prepared by high-temperature polycondensation from an aromatic tetraamine and an aromatic dicarboxylic acid or a derivative thereof. They are obtained, for example, as disclosed in US patent 2014 / 357831, by a condensation reaction of diphenyl isophthalate with 3,3-diaminoenzidine, which undergo spontaneous cyclization at temperatures of approximately 350°C to 400°C under an inert atmosphere.

[0023] [Chem.l]

[0024] Polybenzimidazoles (PBI), in particular Celazole®, are engineering plastics that offer high thermal resistance combined with a high service temperature. Due to their amorphous nature, they exhibit a temperature of glass transition, often very high, generally in the order of more than 400°C, and do not have a melting point.

[0025] They are, generally, also endowed with exceptional chemical resistance, in particular with respect to alcohols, hydrocarbons, chlorinated solvents, hydrogen sulfide, weak acids and bases and many other chemicals.

[0026] Their decomposition temperature can exceed 500°C. PBIs are also excellent electrical and thermal insulators and have exceptional wear resistance. However, PBIs cannot be injection-molded or extruded, and their shaping can generally only be achieved by compression molding.

[0027] Although most PBIs can be used in the composition, in a preferred embodiment, said polybenzimidazole is chosen from the group comprising, preferably consisting of, low molar mass polybenzimidazole, it may be for example Celazole®.

[0028] Preferably, said polybenzimidazole has a weight average molar mass between 10 KDa and 100 KDa, preferably between 25 KDa and 35 KDa.

[0029] The liquid crystal polymer

[0030] One of the objectives being the possibility of using the composition in an electrically insulating material, particularly for aeronautics, it is important that certain characteristics are respected by the composition.

[0031] For example, liquid crystal polymers (LCPs) typically exhibit moisture absorption that is generally 100 times lower than that of polyimides (approximately 0.03 versus 3%).

[0032] Furthermore, LCPs generally exhibit a limiting oxygen index (LOI) between approximately 40% and 50%, like polyimides. The LOI is a measure of the lowest percentage of oxygen in the oxygen / nitrogen mixture required to promote the combustion of plastics. The LOI is a widely used quality control indicator for detecting the relative flammability of polymeric materials. The higher the LOI value, the greater the non-flammability.

[0033] It can also be noted that UL 94 is a flammability standard for plastics published by Underwriters Laboratories (USA). This standard classifies plastics according to their combustion in different orientations and part thicknesses, from least to most flame-resistant, in six categories. Like PTFE, Vectra® LCPs comply with UL 94 V-0 standards for thicknesses as low as 0.2 mm in many grades. The combustion products of Vectra® A950 grade are primarily carbon dioxide, carbon monoxide, and water. These characteristics comply with US federal regulation FAR25.853 (A1), Part IV, Annex F, governing materials used in aircraft.

[0034] Thus, although many LCPs can be implemented within the scope of this disclosure, such as those described in the article "Advanced Industrial and Engineering Polymer Research 3 (2020) 160-174", in one embodiment, said liquid crystal polymer is preferably a p-hydroxybenzoic acid (HBA) 6-hydroxy-2-naphthalic acid (HNA) copolyester.

[0035] The p-hydroxybenzoic acid (HBA) 6-hydroxy-2-naphthalic acid (HNA) copolyester, Vectra® A950, has the following formula:

[0036] [Chem.2]

[0037] Preferably, said liquid crystal polymer has an average molar mass by weight of between 10 KDa and 100 KDa, preferably between 25 KDa and 35 KDa.

[0038] The mass ratio RM

[0039] According to this disclosure, the composition has a mass ratio RM, mass polybenzimidazole to mass liquid crystal polymer of between 0.01 and 99.

[0040] The mass ratio RM can be adjusted according to the desired characteristics. The more desirable the properties of the PBI, the higher the mass ratio RM will be. Conversely, the more desirable the properties of the LCP, the lower the mass ratio RM will be.

[0041] Thus, in one embodiment, the mass ratio RM is between 0.10 and 9, preferably between 0.25 and 4, preferably between 0.40 and 2.5, even more preferably between 0.6 and 1.7, and particularly preferably about 1.

[0042] Optional Additives

[0043] In a particular embodiment, the composition according to this disclosure further comprises at least one additive selected from the group comprising, preferably consisting of, graphite, carbon fiber, molybdenum disulfide, mica, alumina, silicon dioxide, talc, zinc oxide, tungsten carbide, carbon black, particulate polyimide, boron nitride, aramid, quartz, potassium titanate, barium titanate.

[0044] These additives may be advantageous in cases where it would be beneficial to improve mechanical, anti-abrasive and / or thermal conductivity properties.

[0045] The preparation process

[0046] As previously stated, this disclosure also relates to a process for preparing the composition mentioned above, comprising the following successive steps: El: prepare an aqueous solution of polybenzimidazole with a pH between 1 and 3. E2: Heat the aqueous solution obtained in step E1 to a temperature between 60°C and 100°C. E3: Lower the temperature of the solution obtained at the end of step E2 to room temperature, for example between 15°C and 30°C, preferably 25°C. E4: Add to the solution obtained at the end of step E3 a quantity of liquid crystal polymers to obtain a mass ratio RM, mass polybenzimidazole to mass liquid crystal polymer of between 0.01 and 99, and stir at room temperature, for example between 15°C and 30°C, preferably 30°C. E5: Pour the solution obtained at the end of step E4 into a sufficient volume of water to precipitate the polybenzimidazole and the liquid crystal polymer to form a solid polymer mixture, E6: Filter, rinse and dry the polymer mixture obtained at the end of step E5.

[0047] The El stage

[0048] Advantageously, polybenzimidazole is introduced in powder form.

[0049] The aqueous solution having a pH between 1 and 3 may contain, for example, sulfuric acid.

[0050] In one embodiment, the pH is between 1 and 3, preferably 1.

[0051] Step E2

[0052] In one embodiment, the suspension is heated to a temperature between 70°C and 90°C, for example 80°C.

[0053] Step E3

[0054] As previously stated, step E3 consists of lowering the temperature. This lowering can be achieved by allowing the solution to cool or by accelerating the cooling process, for example, using an ice bath or dry ice.

[0055] Step E4

[0056] The addition of liquid crystal polymer can be done in powder form or in liquid form.

[0057] The quantity introduced will depend on the desired mass ratio RM.

[0058] Heating is not required during step E4, but it is possible to accelerate homogenization by heating slightly, for example to around 30°C.

[0059] Step E5

[0060] Step E5 consists of precipitating the resulting polymer mixture. Precipitation can be carried out by any means known to those skilled in the art. A particularly suitable method is the addition of a sufficient quantity of water.

[0061] Precipitation can be accelerated by lowering the temperature. This can be achieved by allowing the solution to cool or by accelerating the cooling process, for example, using an ice bath or dry ice.

[0062] Step E6

[0063] Step E6 consists of recovering the polymer mixture obtained. This can be done by any means known to those skilled in the art. In particular, filtration will be carried out with a suitable filter, rinsing with an aqueous solvent in which the polymer mixture is not soluble, and drying can be done in an oven.

[0064] Optional step E4bis

[0065] In embodiments where the composition further includes at least one additive, the additive(s) are added between step E4 and step E5, in the desired quantity.

[0066] The electrically insulating material

[0067] As previously disclosed, the present disclosure also relates to an electrically insulating material characterized in that it comprises, preferably is made of, the composition detailed above.

[0068] The material is advantageously in the form of a film. Advantageously, the film has a thickness of between 100 µm and 400 µm, measured with a mechanical probe.

[0069] Alternatively, the material can be formed directly on the object to be electrically insulated, for example by hot extrusion from granules of the polymer mixture.

[0070] The electrical cable

[0071] As previously mentioned, the present disclosure is, moreover, directed to an electrical cable comprising an electrically conductive material 1, said wire being covered with at least one layer 3 of the electrically insulating material mentioned above and optionally at least one layer 2 of an electrically insulating material different from the electrically insulating material of layer 3.

[0072] In other words, the electrical cable according to this disclosure may comprise a conductive material 1 covered with one or more layers of insulating material. The number of layers of insulating material is not particularly limited, but it is preferable that the layers of insulating material be alternated, i.e., that there are no two adjacent layers composed of the same insulating material.

[0073] In a first embodiment, the electrical cable consists of an electrically conductive material 1 simply covered with a layer 3 of the electrically insulating material, in accordance with this disclosure.

[0074] In a second embodiment, illustrated in [Fig.2], the electrical cable may consist of an electrically conductive material 1, covered with a layer 2 of an electrically insulating material, itself covered with a layer 3 of the electrically insulating material, according to the present disclosure, different from the electrically insulating material of layer 2.

[0075] In a third embodiment, illustrated in [Fig.3], the electrical cable may consist of an electrically conductive material 1, covered with a layer 3 of the electrically insulating material, according to this disclosure, which is covered with a layer 2 of an electrically insulating material, different from the electrically insulating material of layer 3, which is covered with a layer 3 of the electrically insulating material, according to this disclosure.

[0076] As an alternative to the second embodiment, the electrical cable may consist of an electrically conductive material 1, covered with a layer 3 of the electrically insulating material, according to this disclosure, itself covered with a layer 2 of an electrically insulating material different from the electrically insulating material of layer 3.

[0077] The conductive material 1 can be of any nature, such as aluminium, aluminium alloy, copper, copper alloy, or any combination thereof, and of various shapes; in particular, it can be a single-strand material such as a metal wire or a multi-strand conductor such as a plurality of metal wires.

[0078] In some embodiments, the insulating material of layer 2 may be polyimide (PI, Kapton®), polyamideimide (PAI), polyetherimide (PEI), polyesterimide (PEIs), or an insulator from the polyaryletherketone (PAEK) family such as polyetheretherketone (PEEK) or polyetherketoneketone (PEKK). Examples

[0079] Example 1: preparation of a polymer blend conforming to this disclosure with an RM ratio of 1

[0080] PBI (5 g, Celazole®, PBI Performance products) is mixed with sulfuric acid (100 mL, H2SO4 95-98%, Merck) at a temperature of 80°C for 2 hours. The solution is cooled to 25°C. Then, LCP (5 g, Vectra A950®, Resinex) is added to this solution, which is stirred for 2 hours at 20°C. The solution is poured into excess water (500 mL) to precipitate the polymer mixture. The polymer mixture is recovered by filtration and then washed with water (3 x 100 mL), before being dried at 150°C under vacuum (10 mbar) for 4 hours.

[0081] Example 2: Moisture absorption test of the polymer mixture according to Example 1

[0082] The moisture absorption of the polymer mixture obtained in Example 1 was measured according to the international standard ISO 62 "Plastics. Determination of water absorption". Before starting the test, three identical test specimens (10 mm x 10 mm x 1 mm) were dried for 24 hours at a temperature of 23 °C and a relative humidity of 50% in a climate chamber until a constant mass mb was obtained. The specimens were then immersed in distilled water (400 mL per specimen) at 23 °C. The water was stirred at least once a day, for example, using a magnetic stirrer and a magnetic stir bar. After 24 hours of immersion, the specimens were removed from the water. The surface water was removed using filter paper. Each test tube is weighed again to the nearest 0.1 mg, within one minute of being removed from the water in order to obtain a mass m2.The amount of water absorbed by the test tube is determined by measuring its mass change, that is, the difference between its mass after exposure to water and its initial mass, and it is expressed as a percentage of the initial mass.

[0083] While commercial PBI / PEEK blends (Celazole® T, PBI Performance Products) are known to exhibit very high moisture absorption (> 6.5%) incompatible with electrical insulation applications, PBI / LCP blends exhibit moisture absorption of less than 1% due to the presence of LCP (a highly crystalline structure that prevents massive moisture absorption).

Claims

Demands

1. Composition comprising at least one polybenzimidazole and at least one liquid crystal polymer in a mass ratio RM, mass polybenzimidazole to mass liquid crystal polymer of between 0.01 and 99.

2. Composition according to claim 1, characterized in that said polybenzimidazole is selected from the group comprising, preferably consisting of, low molecular weight polybenzimidazole having a weight average molar mass between 10 KDa and 100 KDa, preferably between 25 KDa and 35 KDa.

3. Composition according to any one of claims 1 or 2, characterized in that said liquid crystal polymer is a p-hydroxybenzoic acid (HBA) 6-hydroxy-2-naphthalic acid (HNA) copolyester, preferably said liquid crystal polymer has a weight average molar mass between 100 KDa and 100 KDa, preferably between 25 KDa and 35 KDa.

4. Composition according to any one of claims 1 to 3, characterized in that it further comprises at least one additive selected from the group comprising, preferably consisting of, graphite, carbon fiber, molybdenum disulfide, mica, alumina, silicon dioxide, talc, zinc oxide, tungsten carbide, carbon black, particulate polyimide, boron nitride, aramid, quartz, potassium titanate, barium titanate.

5. A process for preparing the composition according to any one of claims 1 to 4 comprising the following successive steps: E1: preparing an aqueous solution of polybenzimidazole in an aqueous solution having a pH between 1 and 3, E2: heating the aqueous solution obtained in step E1 to a temperature between 60°C and 100°C, preferably between 70°C and 90°C, for example 80°C, to obtain a solution, E3: lowering the temperature of the solution obtained at the end of step E2 to room temperature, for example between 15°C and 30°C, preferably 25°C, E4: adding to the solution obtained at the end of step E3 an amount of liquid crystal polymers to obtain a mass ratio RM, mass of polybenzimidazole to mass of liquid crystal polymer between 0.01 and 99, and stir at room temperature, for example between 15°C and 30°C, preferably 20°C, E5: pour the solution obtained at the end of step E4 into a volume of water sufficient to precipitate the polybenzimidazole and the liquid crystal polymer to form a solid polymer mixture, E6: filter, rinse and dry the polymer mixture obtained at the end of step E5.

6. A preparation process according to claim 5, characterized in that step E4 further comprises an addition, subsequent to the addition of said quantity of liquid crystal polymers, of at least one additive selected from the group comprising, preferably consisting of, graphite, carbon fiber, molybdenum disulfide, mica, alumina, silicon dioxide, talc, zinc oxide, tungsten carbide, carbon black, particulate polyimide, boron nitride, aramid, quartz, potassium titanate, barium titanate.

7. Electrically insulating material characterized in that it comprises, preferably is made of, the composition according to any one of claims 1 to 4.

8. Electric cable comprising an electrically conductive material 1, said electrically conductive material 1 being covered with at least one layer 3 of the electrically insulating material according to claim 7 and optionally at least one layer 2 of an electrically insulating material different from the insulating material of layer 3.

9. Electric cable according to claim 8, characterized in that the electrically insulating material of layer 2 is selected from the group comprising, preferably consisting of, polyimide, polyamideimide (PAI), polyetherimide (PEI), polyesterimide (PEIs), or an insulator from the family of polyaryletherketones (PAEK) such as polyetheretherketone (PEEK) or polyetherketoneketone (PEKK).

10. Electric cable according to any one of claims 8 or 9, characterized in that the electrically conductive material 1 is a single-strand conductor such as a metallic wire or a multi-strand conductor such as a plurality of metallic wires.

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