FR2680011A1 - Optical fibres which include a glassy carbon layer and method of applying this coating - Google Patents

Abstract

Protective coating consisting of a vitreous carbon film obtained by applying a solution of a solubilizable vitreous carbon precursor in a suitable organic solvent to the surface of a solid substrate sensitive to high temperatures, heat treating at temperatures of between approximately 200 DEG and 600 DEG C. The solubilizable vitreous carbon precursor consits of a polyethynyl-aromatic compound and, in particular, poly-(1,3 diethynyl-5-(trimethylsilyl)-benzene). Application especially for protecting optic fibres from chemical agents in the environment and especially from humidity.

Description

 The present invention relates to optical fibers having a vitreous carbon coating and to a method of applying the coating.

 It is known that the quartz of the optical fibers undergoes the aggressions of the chemical agents of the external environment, in particular of the humidity and more precisely of the OH- ions, these aggressions being responsible for the increased fragility of the optical fibers as well as of the deterioration of their optical qualities and therefore of light transmission.

Among the solutions of the prior art used to protect an optical fiber from moisture and, in general, from the aggression of the chemical agents present in the environment (normally or accidentally), it has been proposed in particular that adoption of
1) a polymer coating, such as an epoxy acrylate resin or a polyimide, as described in particular in patent FR-2,628,847; however, this polymer coating is not perfectly sealed;
2) a silicone coating, which in the long term is unsatisfactory because of the high affinity between the silicones and the water, which makes the initial protection ineffective due to the accumulation of water in this type of coating.

 In addition, a disadvantage common to all solutions of the prior art is the incompatibility between the quartz of the optical fiber and the protective coating adopted for it, resulting in the absence of chemical interaction between the quartz and the coating, the latter therefore being applied loosely on the optical fiber.

The object of the present invention is therefore to provide optical fibers which respond better to the requirements of the practice than optical fibers of the same type and aimed at the same object previously known, in particular in that they comprise a coating which is
- totally waterproof
- mechanically resistant
- chemically compatible with quartz, while interacting with it in such a way as to give an effective interaction with quartz.

 The present invention relates to an optical fiber equipped with a coating, characterized in that this coating comprises a vitreous carbon film.

 The vitreous carbon adopted in the context of the present invention corresponds to the optimal solution of a long-felt need in the technique contemplated by the present invention, because the vitreous carbon is, among the known materials, the one which is the most impermeable to liquid and gas and which is chemically inert, while being very thermally stable up to 3000'C (its use is well known for making crucibles as well as biocompatible material), but which is inapplicable directly on an optical fiber, whose quartz composition has a melting temperature of about 1700 ° C, which is significantly lower than the temperature at which vitreous carbon is formed and which is between 2000 and 3000 C.

 It is true that one of the inventors has already demonstrated (Cf MR CALLSTROM et al., MACROMOLECULES, 1988, vol.21, page 3525) that a polymer of the family of polydiethynylaromatic compounds heat-treated, preferably at 600 C, -fnrme vitreous carbon having the same properties as that formed 2000 3000 C mentioned above (the latter is obtained by step heating of any carbon polymer, the step heating to eliminate one after the other all chemical elements other than carbon); however, also, the vitreous carbon obtained from a polymer of the family of polydiethynylaromatic compounds already known is not directly applicable on an optical fiber, because it is not known to produce it either in a form that is practically usable for coating an optical fiber.

 However, the originality of the present invention lies precisely in the fact that the Applicant has found, unexpectedly, that an appropriate modification of the chemical structure of a polymer of the above-mentioned family of polydiethynylaromatic compounds makes it possible to obtain a carbon vitreous which, while being compatible with the quartz of an optical fiber, is practically applicable thereon.

 The subject of the present invention is therefore also a process for the synthesis of a solubilizable precursor of vitreous carbon from a polymer of the family of polydiethynylaromatic compounds, characterized in that it is obtained by nucleophilic or electrophilic substitution of at least a hydrogen atom of at least one aromatic ring of said polymer.

 In accordance with the invention, the nucleophilic or electrophilic substituent is selected from alkyl groups, aryl groups, halogens, and silanes.

 Also according to the invention, from the silanes, the alkylsilanes and preferably trimethylsilane are chosen.

 The present invention also relates to a saturated solution of the solubilizable precursor of vitreous carbon according to the invention in an aromatic organic solvent, preferably consisting of toluene.

 The solution of this precursor is suitable for direct application to an optical fiber according to a process which is also the subject of the present invention, which consists in applying to the optical fiber a solution film of the vitreous carbon precursor mentioned above. high and submit the assembly to a heat treatment defined by a temperature between 200 and 1700oc in particular between 300 and 1200qu and in particular between 300 and 600 C, and which is preferably chosen to be equal to about 600 C.

 Other characteristics and advantages of the invention will become apparent from the description which follows, given by way of example and with reference to the appended drawing, in which the single figure schematically illustrates the industrial process for applying a vitreous carbon film on an optical fiber.

 The originality of the present invention consists in the fact that it proposes for the first time, to the knowledge of the Applicant, the synthesis of a vitreous carbon precursor polymer which is soluble, so as to easily apply the latter. on an optical fiber.

 The starting point of the synthesis is a polymer -normally non-soluble- of the family of polydiethynylaromatic compounds already known, which is modified by nucleophilic or electrophilic substitution of at least one hydrogen atom of at least one aromatic ring of the polymer supra.

où R et R' sont des groupements aromatiques.This polymer has as general formula

where R and R 'are aromatic groups.

 The nucleophilic or electrophilic substituent may be chosen in particular from alkyl groups or aryl groups or else from halogens or silanes.

In the context of the present invention, the alkylsilanes, and preferably trimethylsilane, are preferably chosen from silanes: in this case, poly (1,3-diethynyl-5- (trimethylsilyl) benzene is obtained as solubilizable precursor of glassy carbon. ), hereinafter

 This means that the modified polymer thus obtained, still belonging to the family of polydiethynylaromatic compounds, comprises a group R consisting of a benzene ring substituted with a silane and a group R 'consisting of a benzene ring.

 The preferential choice mentioned above is dictated by the fact that in this way, in addition to the desired solubility, a good compatibility of the vitreous carbon, obtained according to the process indicated below, with an optical fiber is also ensured.

A synthesis route of poly- (1,3-diethynyl-5 (trimethylsilyl) benzene), a solubilizable precursor polymer of vitreous carbon, preferred in the context of the present invention, is obtained by following the steps indicated below.
1) adding an equivalent of n-butyl lithium to 1,3,5-tribromobenzene followed by the addition of trimethylchlorosilane to form 3,5-dibromo-1-trimethylsilylbenzene;
2) addition of two equivalents of trimethylsilylacetylene (TMSA) to the compound thus formed, and in the presence of copper iodide and a complex of palladium (O), in particular the PdCl 2 (PPh 3) 2 complex, to form 1 trimethylsilyl-3,5-trimethylsilylacetylenebenzene, with a yield of 85%;
3) removal of the trimethylsilyl (TMS) protecting group from acetylene with potassium hydroxide in methanol, with a yield of 95%;
4) polymerization or coupling of acetylene with 1 to 10 equivalents of ethynylbenzene (C8H6), under oxygen and in the presence of copper chloride, pyridine and tetramethylethylenediamine (TMEDA), in quantitative yield;
5) precipitation of the product thus obtained in methanol, for isolating the solubilizable precursor of vitreous carbon according to the invention, which is in the form of a fine powder, aune.

Steps 1) to 4) are illustrated below

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<tb> zb <SEP> P Fw, TMEDA
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A saturated solution of the solubilizable precursor of vitreous carbon thus obtained is prepared by dissolving the aforementioned precursor in an aromatic organic solvent, preferably consisting of toluene.

 The polymer in solution is then deposited on a fiber, whose heat treatment at temperatures of preferably between 300 and 00C cross-links this polymer, thus allowing the formation of a film (or a film) of vitreous carbon on the optical fiber, having a thickness of the order of a few micrometers.

 The single figure appended to this description schematically illustrates the process that goes from the formation of an optical fiber to its coating by a film of vitreous carbon according to the invention.

 A preform 1 of an optical fiber 10 to be coated with glassy carbon is heated in an oven 11.

The fiber 10 comes out of this furnace by an incorporated die having a calibrated outlet orifice, the diameter of the fiber being controlled when it passes inside a device 12 for measuring its diameter, known to technicians in the field. .

 At the exit of the device 12, the fiber 10 passes through another die 13 containing a saturated solution of the vitreous carbon precursor, which makes it possible to apply to the fiber a coating constituted by a film (or a film) of solution 15 .

 The solution-coated fiber 15 is heat-treated in the furnace 14 in an inert atmosphere, especially nitrogen, which is introduced into a quartz tube 18 through an inlet tube 19 and exits through an outlet tube 20, following a closed circuit, the tubes 19 and 20 being heated so as to preheat the nitrogen to a temperature which does not disturb the heat treatment of the fiber.

 The passage of the fiber 10 thus coated in another furnace 14 allows the crosslinking of the polymer and the formation of a vitreous carbon film on the fiber 10, the total diameter of which is controlled in a second measuring device 16 at the exit of the oven 14, before being wound on a drum 17.

 As is apparent from the above, the invention is not limited to those of its modes of implementation, implementation and application which have just been described more explicitly; it embraces, on the contrary, all the variants that may come to the mind of the technician in the field, without departing from the scope or scope of the present invention.

Claims (14)

 An optical fiber having a coating, characterized in that said coating comprises a vitreous carbon film formed from a solubilizable precursor thereof, obtained by nucleophilic or electrophilic substitution of at least one hydrogen atom. at least one aromatic ring of a polymer of the family of polydiethynylaromatic compounds.  2. Process for the synthesis of a solubilizable precursor of vitreous carbon, from a polymer of the family of polydiethynylaromatic compounds, characterized in that it is obtained by nucleophilic or electrophilic substitution of at least one hydrogen atom. at least one aromatic ring of said polymer.  3. Method according to claim 2, characterized in that the nucleophilic or electrophilic substituent is selected from alkyl groups, aryl groups, halogens and silanes.  4. Process according to claim 3, characterized in that among the silanes, the alkylsilanes are chosen.  5. Method according to claim 4, characterized in that among the alkylsilanes is chosen trimethylsilane, to form a solubilizable precursor of vitreous carbon consisting of poly (1,3-diethynyl-5- (trimethylsilyl) -benzene) which meets the formula II below

 1) addition of n-butyl lithiSlm to 1,3,5-tribromobenzene followed by; addition of trimethylchlorosilane to form 3,5-dibromo-1-trimethylsilylbenzene  6.Procédé according to claim 5, characterized in that the synthesis of poly (1,3-diethynyl5- (trimethylsilyl) -benzene) comprises the following steps

 2) addition of trimethylsilylacetylene (TMSA) to the compound thus formed, in the presence of copper iodide and a palladium (0) complex, to form 1-trimethylsilyl-3,5-trimethylsilylacetylenebenzene ::

 3) removal of the trimethylsilyl protecting group from acetylene with potassium hydroxide in methanol

 4) polymerization or coupling of acetylene with 1 to 10 equivalents of ethynylbenzene, under oxygen and in the presence of copper chloride, pyridine and tetramethylethylenediamine (TMEDA) which act as catalysts  5) isolation of poly- (1,3-diethynyl-5 (trimethylsilyl) -benzene), which is the solubilizable precursor of vitreous carbon, by precipitation in methanol of the product obtained in step 4.  7. Polymer of the family of polydiethynylaromatic compounds, characterized in that it corresponds to formula I below

 wherein R and R 'are aromatic groups and in that at least one hydrogen atom of at least one aromatic ring of said polymer is modified by nucleophilic or electrophilic substitution.  8. Solubilizable precursor of vitreous carbon, characterized in that it is instituted by a polydiethynylaromatic polymer according to claim 7.  Solubilizable precursor according to Claim 8, characterized in that it consists of poly (1,3-diethynyl-5- (trimethylsilyl) benzene) of formula  10. Solution of a solubilizable precursor of vitreous carbon obtained by the process according to any one of claims 2 to 6, characterized in that it consists of a saturated solution of said precursor in an aromatic organic solvent, preferably consisting of toluene.  11. A method for applying a vitreous carbon coating to an optical fiber, characterized in that it comprises the following steps  a) synthesis of a solubilizable precursor of vitreous carbon from a polymer of the family of polydiethynylaromatic compounds by nucleophilic or electrophilic substitution of at least one hydrogen atom of at least one aromatic ring of said polymer according to one of any of Claims 2 to 6  b) preparing a solution of said vitreous carbon precursor in an aromatic organic solvent, preferably consisting of toluene  c) application of a film of this solution on the optical fiber  d) heat treating the optical fiber thus coated at a temperature of between 200 and 1780 ° C, to transform said film of vitreous carbon-carbon precursor solution into glassy carbon.  12. The method of claim 11, characterized in that the nucleophilic or electrophilic substituent is selected from alkyl groups, aryl groups and halogens and preferably from alkylsilanes.  13. The method of claim 12, characterized in that the alkylsilanes is preferably chosen trimethylsilane, the solubilizable precursor of vitreous carbon is then poly- (1,3-diethynyl-5- (trimethylsilyl) benzene).  14. The method of claim 11, characterized in that the heat treatment temperature of the optical fiber coated with said glassy carbon precursor solution is chosen in particular between 300 and 1200-C, and in particular between 300 and 600 C, and is preferably about 600 C.