Coating of fibers, especially optical fibers, with boron nitride-based coatings

Abstract

The present invention relates to a fiber (1) comprising a core (11) made of a fiberizable material and having an outer surface (111). The fiber further comprises an outer coating (2) comprising a mixture of hexagonal boron nitride and bentonite, the proportion of bentonite being at least 10% by weight relative to the total weight of the outer coating (2). Furthermore, the present invention relates to an optical component comprising one or more optical fibers (1) according to the present invention. Finally, the present invention also relates to a method for producing a pasty composition for fiber coating.

Description

[Technical Field]

[0001] The present invention relates to the coating of fibers, in particular optical fibers, with boron nitride (BN)-based coatings and to the manufacture of such fibers. Furthermore, the invention relates to optical components comprising such optical fibers. [Background technology]

[0002] Use of boron nitride in protective coatings for optical fibers.

[0003] Therefore, international application WO2020 / 222152 [1] describe a method for coating optical waveguides with boron nitride in the form of nanotubes (BNNTs). However, this method has the drawback that it can only be implemented for short lengths, since it requires the prior application of a primer layer and / or texturing of the waveguide surface (especially by hydrofluoric acid (HF) etching). These deposition methods involving primer layers are not suitable for deposition on long lengths of optical fiber. Furthermore, such methods are very costly, since they require the synthesis of BN nanotubes.

[0004] In addition, silica fiber and sapphire [2、3、4] BN / SiBCN coatings on fibers are known to those skilled in the art. The deposition process carried out in this case is a chemical vapor deposition process that allows the deposition of a thickness of 2.5 μm for a deposition time of 24 hours. This cannot be applied to long fibers.

[0005] Finally, Chinese Patent Application CN106066508 describes a fiber sheathing material comprising a mixture of polyetheretherketone and an inorganic filler, including talc, limestone, calcium carbonate, barium sulfate, boron nitride, silicon dioxide, or bentonite (although bentonite or boron nitride is not described as being used in combination). The purpose of such a coating is to improve the elongation resistance of the fiber. Furthermore, this inorganic sheathing material is not applied directly onto the fiber. The fire resistance of this inorganic sheathing material is considered to be an undesirable property. This material is used as a filler material, not as an actual coating. Finally, there is no mention of the length of the fiber or the thickness of the coating.

[0006] Nevertheless, none of the above documents teaches how to apply an adhesive coating to long fibers and furthermore that is highly resistant to low and high temperatures and to mechanical stress. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] WO2020 / 222152 [Patent Document 2] CN106066508 Summary of the Invention [Means for solving the problem]

[0008] To solve the above-mentioned problems, the applicant has developed a fiber comprising a core made of a fiberizable material and having an outer surface, said fiber further comprising an outer coating comprising a mixture of hexagonal boron nitride and bentonite, the proportion of bentonite being at least 10% by weight relative to the total weight of said outer coating.

[0009] If the bentonite is less than 10% by weight relative to the total weight of the outer coating, the coating will not adhere to the fiber, and if the bentonite is more than 35% by weight relative to the total weight of the outer coating, the fiber so coated will not have sufficient flexibility.

[0010] A fiberizable material is to be understood as a material that allows fiberization, i.e., a material that can undergo deformation of the bulk fiberizable material. The fiberizable material may consist of a glassy material that has a glass transition and is therefore stretchable. Preferably, the core may be made of a material selected from glass transition materials and sapphire glass.

[0011] Advantageously, the core may be made from a material chosen from glass transition materials and sapphire glass.

[0012] According to a first advantageous embodiment of the invention, the outer coating, the fiber according to the invention may further comprise a protective sheath made of a polymer material surrounding the core over at least a portion of the length of the fiber, this protective sheath having an inner surface in contact with the core and an outer surface in contact with the outer coating.

[0013] According to a second advantageous embodiment of the invention, the outer coating can be in direct contact with the core.

[0014] Advantageously, regardless of the embodiment under consideration, the core of the fiber may have a diameter between 100 μm and 10 mm, preferably between 100 μm and 140 μm, and even better still 125 μm.

[0015] Advantageously, regardless of the embodiment under consideration, the outer coating may have a thickness between 5 and 240 μm, and if the core is cylindrical, the thickness of the outer coating will be a radial thickness between 5 and 240 μm.

[0016] In the context of the present invention, the fiber according to the invention may preferably be an optical fiber.

[0017] Another object of the invention is an optical component comprising one or more optical fibers according to the invention.

[0018] Optical components according to the invention may in particular be multicore fibers, microstructured fibers, tapered fibers (in English "tapers"), optical couplers with one or more fibers at the inlet and one or more fibers at the outlet, laser fibers and fiber-based Bragg gratings, but this list is not limiting.

[0019] Applicant has developed a method for producing such fibers.

[0020] For this purpose, the applicant has developed a method for producing a paste-like composition for fiber coating, which comprises the following steps: A) dispersing a dry mixture of hexagonal boron nitride BN and bentonite in water to ensure a proper mixture of bentonite and boron nitride, the dry mixture comprising at least 10 wt. % bentonite relative to the total weight of the dry mixture, to form an aqueous suspension. B) Evaporating the water contained in said aqueous suspension until a powdery dry extract is obtained. C) Dispersing the powdered dry extract in water to form a pasty composition, wherein the proportion of dry extract in water is at least 40% by weight.

[0021] Advantageously, step B) of the method for producing a fiber coating pasty composition according to the invention can be carried out under primary vacuum or under atmospheric pressure and at a temperature between 50°C and 90°C, preferably between 60°C and 80°C, and even more preferably at 60°C.

[0022] Another object of the present invention is a pasty composition for coating fibers obtainable by the above-mentioned manufacturing process.

[0023] Advantageously, the pasty composition according to the invention can further comprise a dopant, which can advantageously be based on carbon, zirconium oxide, titanium oxide, and metal or semiconductor nanoparticles, organic fillers (organic and organometallic molecular compounds), inorganic fillers, and mixtures thereof.

[0024] Another object of the invention is therefore a method for manufacturing a fiber according to the invention, implementing such a pasty composition to achieve the deposition of an external coating on the outer surface of the fiber, said method comprising the following steps: A) Providing or preparing a fiber core made from a fiberizable material, said core being covered or not covered with a protective sheath. B) Providing a paste composition for fiber coating according to the invention. C) coating at least a portion of the fibers with the pasty composition to form a wet layer on the fibers. D) Heat treating the optical fiber coated with the wet layer at a temperature between 100°C and 250°C for a period of time sufficient to form an outer coating layer that can be handled (in this case, wrapped and handled).

[0025] Advantageously, steps C and D may be repeated one or more times until a desired thickness of the outer coating is obtained.

[0026] According to a first advantageous embodiment of the method for producing a fiber according to the invention, step A) can be a step of preparing a fiber comprising a core made of a fiberizable material and surrounded by a protective sheath, whereby steps B to D are carried out after the production of the fiber, and the heat treatment step D) is drying carried out in an oven at 100°C.

[0027] According to this first embodiment, the method for producing a fiber according to the invention may further comprise a step A' of stripping the fiber according to the invention, so as to strip off the protective sheath over at least a portion of the fiber's length. Preferably, this step A' may be carried out by contacting the protective sheath with a dichloromethane solution, if the protective sheath is made of polyacrylate. Other methods of stripping the fiber are possible, such as mechanical stripping using clamps or a razor blade. However, for optical fibers intended to be handled at least once, chemical stripping is preferably considered.

[0028] According to a second advantageous embodiment of the method for producing fibers according to the invention, step A) can consist of producing the fibers on a fiberization tower.

[0029] According to a second embodiment, the step B of coating the pasty composition can be carried out in a die holder arranged below a first vertical furnace in a static state, and the heat treatment step D can be carried out in a second vertical furnace arranged below the first vertical furnace and the die holder.

[0030] Other advantages and features of the invention will become apparent from the following description, given by way of non-limiting example, with reference to the accompanying drawings and examples.

[0031] The following examples, in conjunction with the figures discussed below, illustrate the invention but do not limit the scope of the invention. [Brief explanation of the drawings]

[0032] [Figure 1] 1A and 1B show a cross-sectional view and a perspective view of a first example of a fiber according to the invention (fiber without a protective sheath). [Figure 2] 2A and 2B are cross-sectional and perspective views, respectively, of a second example of a fiber according to the invention (fiber with a protective sheath). [Figure 3] 1 is a schematic diagram showing a device for carrying out a method for producing fibers according to a second embodiment, namely a method in which a coating paste composition is applied to a fiberizing tower. [Figure 4] 4A shows two optical microscope photographs at different focal lengths (4A is the edge of the fiber, and 4B is the surface of the fiber) of the fiber 1 obtained in Example 2 (post-treatment) covered with an outer coating 2 based on hexagonal boron nitride and bentonite after heat treatment at 1,000° C. [Figure 5] FIG. 1 shows the relative change in response of the Bragg grating (Dl / l) over time for a bare fiber (solid line) and a fiber according to the invention coated with three layers of BN (dashed line) over 800 hours at 800° C. [Figure 6A] FIG. 1 shows an optical microscope photograph of fiber 1 obtained in Example 4 (fiberization tower), covered with an outer coating 2 based on hexagonal boron nitride and bentonite and heat-treated at 800°C, corresponding to the fiber obtained immediately after completion of the heat treatment at 800°C. [Figure 6B] 1 shows an optical microscope photograph of fiber 1 obtained in Example 4 (fiberization tower), covered with an outer coating 2 based on hexagonal boron nitride and bentonite and heat-treated at 800°C, corresponding to the fiber obtained at the completion of a 2-hour liquid nitrogen quench at -195.72°C after heat treatment. DETAILED DESCRIPTION OF THE INVENTION

[0033] 1 and 2 will be described in more detail below, and FIGS. 4 to 6 will be described in more detail in the following Examples. These Examples are intended to illustrate the present invention and are not intended to limit the scope of the present invention.

[0034] 1A and 1B show a first example of a fiber 1 according to the invention (fiber without a protective sheath), which comprises a core 11 made of a fiberizable material and having an outer surface 111, which is covered by an outer coating 2 based on hexagonal boron nitride and bentonite.

[0035] 2A and 2B show a second example of a fiber according to the invention (a fiber with a protective sheath), which, unlike the example shown in Figures 1A and 1B, further comprises a protective sheath 12 made of a polymer material surrounding the core 11 over at least a certain portion of the fiber (over the entire length in the example shown in Figure 2), the protective sheath 12 having an inner surface 120 in contact with the core 11 and an outer surface 121 in contact with the outer coating 2.

[0036] (Example) The nature of the products used to produce the fibers according to the invention and the methods carried out and characterization methods are now detailed.

[0037] Products, raw materials - Chemical stripping solvent: dichloromethane, isopropanol, - Hexagonal BN powder, - General formula Al2H2O 12 Si4 bentonite, - an optical fiber sample (in particular made from silica, sapphire or chalcogenides) provided with a protective sheath made from an organic polymer (for example made from polyacrylate); - Glass preforms.

[0038] Structural and Microstructural Characterization Devices and Testing To characterize the applied coating layers, a complete physicochemical characterization was carried out using complementary techniques at different scales. The following methods were used: - optical microscopy, - X-ray diffraction (XRD) analysis, - high temperature resistance test, which comprises heating a fiber sample according to the invention to 1,000°C at 10°C / min using a heat lamp, followed by inertial or flash cooling; - a low temperature resistance test, which comprises heating a fiber sample according to the invention to 800°C using a heat lamp at 10°C / min, followed by cooling to room temperature by gravity, and then quenching in liquid nitrogen to -195.72°C for 2 hours; - Determining the Bragg response behavior of a fiber sample according to the invention by analyzing the reflectance at the Bragg wavelength using a broadband laser source and an optical spectrum analyzer.

[0039] (Example 1: Production of an example of a paste-like composition C for fiber coating) Boron nitride and bentonite (at least 10% by weight of bentonite) are ground using a planetary mill by reversing the direction of rotation every 5 minutes (for a sufficient particle size distribution).

[0040] The pulverized material thus obtained is dispersed in a large amount of water (about 250 mL) to form a suspension.

[0041] The resulting suspension is evaporated to dryness in a 500 mL Schlenk tube using a vacuum / argon manifold under primary vacuum (10 -3 The evaporation is carried out under 50°C (Pa). Throughout the procedure, the Schlenk tube is maintained at 60°C in an oil bath and then in a water bath. After 4-6 hours of evaporation, the resulting dry extract is manually ground (using a mortar and pestle). The powder thus obtained can be stored in an oven at 50°C or in a desiccator for several months.

[0042] For deposition onto the fiber, the resulting powder is dispersed in at least 20 mL of distilled water.

[0043] A pasty composition C according to the present invention is obtained.

[0044] Example 2: Production of coated fibers of the present invention according to the first embodiment with post-treatment Step A A commercially available optical fiber sample (particularly made from silica, sapphire, or chalcogenides) is used, which is provided with a protective sheath made from polyacrylate, which is stripped off in step A'.

[0045] Step A' It should be noted that traditionally, during the manufacture of optical fibers, these optical fibers are protected by organic polymers. Without this protective coating, the optical fiber would be very vulnerable to mechanical contact and difficult to handle. However, this organic coating is inherently unsuitable for deploying optical fibers in harsh environments.

[0046] It is therefore preferable to at least partially strip this coating. Preferably, this stripping operation A' is carried out by chemical attack. The purpose of this step A' is to strip a specific portion of the optical fiber, either at one end or in a predefined area. Typically, at each end of the fiber, the initial coating is retained for a length at least sufficient to hold the fiber in place during the step of depositing the coating without weakening it. These lengths are adjusted depending on the type of application in question.

[0047] In the case of an original protective sheath of polyacrylate type (standard case), the solvent used is dichloromethane.

[0048] If the commercially available optical fiber sample is made of a polymer other than polyacrylate and has a protective sheath that is less susceptible to dichloromethane, another solvent capable of dissolving this polymer is used, for example, if the protective sheath is made of polyimide, hydrochloric acid or hot sulfuric acid is used to dissolve the protective sheath.

[0049] Step A', which involves chemical exfoliation, makes it possible to avoid weakening the fibers, unlike mechanical exfoliation (using clamps or razor blades).

[0050] In some applications, it may be preferable to retain the original protective sheath of the optical fiber (polyacrylate or polyimide deposited during or at a later stage in the manufacture of the optical fiber). The BN and bentonite-based coating 2 according to the invention can then be applied directly to the unstripped fiber. In this case, the stripping step A') is therefore not performed.

[0051] Step B Pasty composition C of Example 1 is used.

[0052] Step C At least a portion of the stripped fiber sample is then coated with the pasty composition C to form a wet layer on the fiber, for example by dipping.

[0053] Step D The sample is then placed in an oven at 100° C. The coating is dry to the touch after 15 seconds. After this treatment, the fiber can be wound onto a standard coil (typically 158 mm diameter).

[0054] Example 3: Production of coated fibers of the invention according to the second embodiment in a fiberization tower (see Figure 3) In some applications, optical fibers are of interest in themselves. The particularity required for this application is the uniform production of the fiber (e.g., a preform with a specific composition that causes Rayleigh scattering). The lengths implemented for these applications are from several tens of meters up to several kilometers. In this case, it is preferable to deposit the BN and bentonite-based coating directly during fiberization of the preform, i.e., during fiberization on a fiberization tower.

[0055] For this purpose, a fiberization tower such as that shown in FIG. 3 is used.

[0056] Step A A glass preform 10 is inserted into an oven F1 heated to a temperature of approximately 2,000°C. Under the influence of heat and gravity, the glass softens, leading to the formation of a "droplet." Once refined, the preform forms a glass fiber 11, which, with its similarity, forms the core of the fiber 1 according to the invention. This fiber is conventionally coated with a polyacrylate injected under pressure and crosslinked by UV, then driven by a capstan at a controlled speed.

[0057] Step B A pasty composition C is used.

[0058] Step C The pasty composition C is applied to the fiber under atmospheric or superatmospheric pressure. A standard die holder PF equipped with a diffuser is used to contain the pasty composition. This diffuser has no other purpose than to reduce the exit diameter of the die holder. The volume required to cover a 100 m length of 125 μm diameter fiber is estimated to be 10 mL.

[0059] Step D The tubular oven F2 is positioned vertically at a position 220 mm below the die holder PF. The hot zone is approximately 250 mm. The oven temperature is 250°C. A diaphragm D is placed over the top outlet of the oven to avoid heating the die holder.

[0060] Fiberization parameters The fiberization parameters to be controlled to ensure proper deposition of the coating are the speed and temperature of the drying oven (here F2). These two parameters are determined by the hardware used. The fiberization speed should be between 4 and 8 m / min. Below 4 m / min, the coating will not adhere to the fiber. Regardless of the speed selected, the temperature should not be below 250°C. If an oven with a heating area greater than 20 cm is used, a faster deposition speed may be considered.

[0061] Example 4: Characterization of coatings according to the present invention Various tests were then carried out to characterize the BN and bentonite coatings according to the present invention.

[0062] These samples are observed under an optical microscope, characterized by XRD and under various temperature conditions in order to reveal any physicochemical changes in the coatings that may be prohibitive for the intended application. Furthermore, the optical-mechanical behavior is investigated.

[0063] The first heat resistance test of the coating formed in Example 3 was carried out at 1,000°C for 500 hours, ramping to 1,000°C at a rate of 10°C / min, followed by inertial cooling. Figure 4 shows the sample after this heat treatment, observed under an optical microscope. These observations show no change in the integrity of the coating (cracks or breaks).

[0064] The behavior of this coating at low temperatures was also investigated. To this end, the coating was heat-treated at 800 °C for stabilization, and then quenched by immersion in liquid nitrogen at -195.72 °C for 2 hours. No chemical or physical degradation was observed, as shown in the photograph in Figure 6.

[0065] Furthermore, other fiber samples with coated Bragg gratings BN will be investigated under various isotherms (high and low temperatures) to verify the criterion of no change in the optical-mechanical properties of the fiber. Indeed, it is important that the sensitivity of the sensor protected by the coating is not altered by the coating. Furthermore, successive heating and cooling cycles will be repeated for samples with and without coating to verify the appropriate dynamic behavior (thermal expansion of various materials).

[0066] Similarly, the behavior of the Bragg response during cycling at 800° C. for over 800 hours is compared between the coated and uncoated cases, as shown in FIG.

[0067] (References) 1. WO2020 / 222152 (2019-08-27) - "Boron nitride nanotube coated optical waveguide and uses thereof" - NRC - NATIONAL RESEARCH COUNCIL CANADA 2. Xin'gang Luan, Xinming Xu, Min Li, Rong Yu, Qiqi Zhang, Sam Zhang and Laifei Cheng. "Design, preparation, and properties of a boron nitride coating of silica optical fiber for high temperature sensing applications". Journal of Alloys and Compounds, Volume 850, 5 January 2021, 156782. 3. Xin'gang Luan, Rong Yu, Qiqi Zhang, Sam Zhang and Laifei Cheng. "Boron nitride coating of sapphire optical fiber for high temperature sensing applications". Surface and Coatings Technology, Volume 363, 15 April 2019, pages 203-209. 4. Xin'gang Luan, Xinming Xu, Rong Yu, Qiqi Zhang, Sam Zhang and Laifei Cheng. "BN / SiBCN light-leakage-proof coatings of silica optical fiber for long term sensors at high temperatures". Chinese Journal of Aeronautics, Volume 34, Issue 5, May 2021, pages 93-102. [Explanation of symbols]

[0068] 1. Fiber 2. External Coating 11 cores 10 Glass preform 11 Glass fiber 12 Protective sheath 111 Outer surface 120 Inner surface 121 Outer surface F1 Oven F2 Tubular Oven PF Standard Die Holder D diaphragm

Claims

1. A fiber (1) comprising a core (11) made of a fiberizable material and having an outer surface (111), said fiber further comprising an outer coating (2) comprising a mixture of hexagonal boron nitride and bentonite, wherein the proportion of bentonite is at least 10% by weight relative to the total weight of said outer coating (2).

2. 2. The fiber (1) according to claim 1, wherein the core (11) is made of a material selected from glass transition materials and sapphire glass.

3. 3. The fiber (1) according to claim 1 or 2, further comprising a protective sheath (12) made of a polymer material surrounding the core (11) over at least a portion of the length of the fiber (1), the protective sheath (12) having an inner surface (120) in contact with the core (11) and an outer surface (121) in contact with the outer coating (2).

4. 3. The fiber (1) according to claim 1 or 2, wherein the outer coating (2) is in direct contact with the core (11).

5. The fiber (1) according to any one of claims 1 to 4, wherein the core (11) of the fiber has a diameter between 100 μm and 10 mm, preferably between 100 μm and 140 μm, preferably a diameter of 125 μm.

6. The fiber (1) according to any one of claims 1 to 5, wherein the outer coating (2) has a thickness between 5 and 240 μm.

7. The fiber (1) according to any one of claims 1 to 6, wherein the fiber (1) is an optical fiber.

8. An optical component comprising one or more optical fibers (1) as defined according to any one of claims 1 to 7.

9. A method for manufacturing a fiber as defined in any one of claims 1 to 7 by depositing an external coating (2) on the outer surface (111, 121) of a fiber (1), comprising: A) providing or making a fiber core (11) made from a fiberizable material, said core (11) being covered or not covered with a protective sheath (12); B) preparing a fiber coating paste composition obtained according to the method for producing a fiber coating paste composition; C) coating at least a portion of the fibers (1) with the paste-like composition to form a wet layer (21) on the fibers (1); D) heat treating the optical fiber (1) coated with the wet layer (21) at a temperature between 100°C and 250°C for a time sufficient to form a handleable outer coating layer (2); A method comprising:

10. 10. The method of claim 9, wherein steps C and D are repeated one or more times until a desired thickness of the outer coating (2) is obtained.

11. Step A) is the step of providing a fiber (1) made of a fiberizable material and comprising a core (11) surrounded by a protective sheath (12), 11. The method according to claim 9 or 10, wherein steps B to D are carried out after the production of the fiber (1), and the heat treatment step D) consists of drying carried out in an oven at 100°C.

12. 12. The method according to claim 11, wherein the protective sheath (12) is at least partially removed over a certain length of the fiber (1) during step A'), the fiber according to the invention before step B, removing the protective sheath over at least a portion of the length of the fiber.

13. 11. The method according to claim 9 or 10, wherein step A) is a step of producing a fiber (1) on a fiberization tower.

14. The step (B) of preparing a paste composition for fiber coating comprises: i. dispersing a dry mixture of hexagonal boron nitride BN and bentonite in water, with a proportion of bentonite being at least 10% by weight relative to the total weight of the dry mixture, to form an aqueous suspension; ii. evaporating the water contained in said aqueous suspension until a powdery dry extract is obtained; iii. dispersing the powdered dry extract in water to form a pasty composition, wherein the proportion of dry extract in water is at least 40% by weight; The method of any one of claims 9 to 13, comprising:

15. i. dispersing a dry mixture of hexagonal boron nitride BN and bentonite in water, with a proportion of bentonite being at least 10% by weight relative to the total weight of the dry mixture, to form an aqueous suspension; ii. evaporating the water contained in said aqueous suspension until a powdery dry extract is obtained; iii. dispersing the powdered dry extract in water to form a pasty composition, wherein the proportion of dry extract in water is at least 40% by weight; A pasty composition (20) for fiber coating obtained by a method comprising:

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