METHOD FOR MANUFACTURING SILICON DIOXIDE PREFORMS EMPLOYED IN THE PRODUCTION OF OPTICAL FIBERS

Abstract

The present invention relates to a method for manufacturing a preform of silica for optical fiber production, as well as to a method for the production of optical fibers comprising a step of drawing the optical fiber from such a preform of silica, the method comprising a step of vaporization of a siloxane feedstock added with a compound having the following formula (I): wherein R, R and R, equal or different each other, are an alkyl group having from 1 to 5 carbon atoms, and A is a saturated or unsaturated chain of atoms selected from the group consisting of carbon atom, nitrogen atom, and oxygen atom, said chain A forming with the nitrogen atom linked thereto a saturated, unsaturated or aromatic heterocyclic moiety.

##STR00001##

Claims

1. A method for manufacturing a preform of silica for optical fiber production, the method comprising a step of vaporization of a siloxane feedstock added with a compound having the following formula (I): ##STR00007## wherein R, R and R, equal or different each other, are an alkyl group having from 1 to 5 carbon atoms, and A is a saturated or unsaturated chain of atoms selected from the group consisting of carbon atom, nitrogen atom, and oxygen atom, said chain A forming with the nitrogen atom linked thereto a saturated, unsaturated or aromatic heterocyclic moiety.

2. The method according to claim 1, wherein A is a saturated or unsaturated chain of carbon atoms and one or more nitrogen atoms.

3. The method according to claim 2, wherein said one or more nitrogen atoms is substituted with a Si(R)(R)(R) group.

4. The method according to claim 1, wherein A is a saturated or unsaturated chain of carbon atoms and one or more oxygen atoms.

5. The method according to claim 1, wherein chain A forms with the nitrogen atom linked thereto an aromatic heterocyclic moiety.

6. The method according to claim 1, wherein R, R and R, equal or different each other, are a linear or branched alkyl chain comprising from 1 to 3 carbon atoms.

7. The method according to claim 1, wherein the compound of formula (I) is selected from the group consisting of 1-(trimethylsilyl)-imidazole (TMSI), 1-(triethylsilyl)-imidazole, 1-(ethyldimethylsilyl)-imidazole, 1-(diethylmethylsilyl)-imidazole, 1-(t-butyldimethylsilyl)-imidazole (TBDMSIM), 1-(trimethylsilyl)-pyrazole, 1-(trimethylsilyl)-piperidine, 1-(trimethylsilyl)-piperazine, N-(tri-methylsilyl)-oxazolidine, 3-(trimethylsilyl)-2-oxazolidinone (TMSO), and N-(trimethylsilyl)-morpholine.

8. The method according to claim 1, wherein the siloxane feedstock is selected from hexamethylcyclotrisiloxane (HMCTS), octamethylcyclotetrasiloxane (OMCTS) and decamethylcyclopenta-siloxane (DMCPS).

9. The method according to claim 1, wherein the compound of formula (I) is present in an amount of at least 0.001 vol % with respect to the total volume of siloxane feedstock.

10. The method according to claim 1, comprising the step of adding the compound of formula (I) to the siloxane feedstock at least one hour before the step of vaporization of the siloxane feedstock.

11. The method according to claim 1, wherein the siloxane feedstock comprises a further silylating agent.

12. The method according to claim 11, wherein the further silylating agent is selected from the group consisting of N,O-bis(trimethylsilyl)-acetamide (BSA), N,O-bis(trimethylsilyl)trifluoroacetamide (BSTFA), N-methyl-N-(trimethylsilyl)trifluoroacetamide (MSTFA).

13. The method according to claim 11, wherein the further silylating agent is present in a vol % equal to or lower than that of the compound of formula (I).

14. A method for the production of optical fibers comprising a step of drawing the optical fiber from a preform of silica, wherein said preform of silica is obtained from a siloxane feedstock comprising a compound having the following formula (I): ##STR00008## wherein R, R and R, equal or different each other, are a linear or branched alkyl group having from 1 to 5 carbon atoms, and A is a saturated or unsaturated chain of atoms selected from the group consisting of carbon atom, nitrogen atom, and oxygen atom, said chain A forming with the nitrogen atom linked thereto a saturated, unsaturated or aromatic heterocyclic moiety.

15. A compound for use in manufacturing a preform of silica, said compound having the following formula (I): ##STR00009## wherein R, R and R, equal or different each other, are an alkyl group having from 1 to 5 carbon atoms, and A is a saturated or unsaturated chain of atoms selected from the group consisting of carbon atom, nitrogen atom, and oxygen atom, said chain A forming with the nitrogen atom linked thereto a saturated, unsaturated or aromatic heterocyclic moiety.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0049] The present invention will be better understood by reading the following detailed description, given by way of example and not of limitation, to be read with the accompanying drawings, wherein:

[0050] FIG. 1 shows a schematic view of the apparatus containing the feedstock of the invention for processing

[0051] FIG. 2 shows a schematic representation of an apparatus and process for depositing silica soot on a rotating mandrel to form a porous blank or preform.

DETAILED DESCRIPTION OF THE INVENTION

[0052] The present invention relates to a method for manufacturing a preform of silica, comprising a step of vaporization of a siloxane feedstock, as well as to a method for the production of optical fibers comprising a step of drawing the optical fiber from such a preform of silica, wherein said siloxane feedstock comprises a compound having the above described formula (I).

[0053] Most of the processes being developed by industry today for the manufacture of optical waveguides employ the chemical vapor deposition (CVD) concept or a modified version thereof. The precursor vapors are entrained by a carrier gas stream and then passed through a burner flame, usually a natural gas/oxygen mixture and frequently containing excess oxygen. The vapors in the mixture are converted to their respective oxides upon exiting the burner orifice to form a stream of volatile gases and finely-divided, amorphous, spherical aggregates, called soot. The soot is collected on a mandrel or in a pipe, where it is deposited in thin layers in radial or axial direction. The final product of soot collection, the porous preform, is then subjected to a temperature at which the preform consolidates to a non-porous monolithic glassy body.

[0054] In usual practice, the process for manufacturing optical fibers is a three-stage process.

[0055] The first stage involves oxidizing reactant feedstock(s) to form finely-divided, amorphous spherical particles of soot on a substrate. In the second stage of the process, the blank or preform is subsequently heat treated in inert atmosphere to full consolidation. In the third and final stage, conventional fiber-draw technology is utilized in extracting optical fiber from the preform.

[0056] The first stage of this process can be carried out in a number of different ways, by depositing layers of specially formulated silicon dioxide on the surface of a substrate. The layers are deposited by applying a gaseous stream of pure oxygen added with siloxane feedstocks to the substrate. As the oxygen contacts the hot surface silicon dioxide of high purity is formed.

[0057] In a first embodiment of the first stage, reactant is supplied in liquid form to a flow distributor that delivers the liquid to one end of a vaporization device. The liquid flows down a heated, inclined surface as a thin film toward a second end of the device. When the second end is reached, liquid has been converted to a vapor and is delivered to a burner for oxidation to soot particles.

[0058] A second embodiment of the first stage also involves use of a vaporizer. Here, the vaporizer is a heated, vertically-extending expansion chamber which achieves vaporization when reactant is sprayed onto heated interior walls of the vaporizer.

[0059] In a third embodiment of the first stage, liquid reactant is delivered to a flash vaporization chamber. In that chamber, the liquid assumes the form of a thin film, vaporizes, and mixes with a gas selected from the group consisting of an inert gas, a combustible gas, an oxidizing gas, and mixtures thereof for delivery to an oxidation burner.

[0060] As depicted in FIG. 2, a siloxane feedstock 20 comprising the compound having the above described formula (I) according to the present invention is provided in a container 10. The feedstock 20 is pressurized in the container 10 by an inert gas 30. The pressurized feedstock 20 is vaporized at a temperature of 200250 C. The resulting vapors of the siloxane feedstock 20 are entrained in the carrier gas 30 and transported to the burner 50 fed with a burner flame fuel 40 (for example a methane/oxygen mixture), whereby combustion and oxidation are induced at the burner 50. The resulting soot is deposited on a rotating mandrel 60 thus forming a preform or blank 70 of silica soot.

[0061] The following examples are intended to further illustrate the present invention, without however restricting it in any way.

Example 1

[0062] Fluid D4 (octamethylcyclotetrasiloxane) has been used for the production of silica soot. The feedstock was maintained in a tank as shown in FIG. 1.

[0063] The fluid D4 in the tank 1 was constantly mixed through a recirculating pump 2.

[0064] Through spilling valve 4, a small amount of D4 was sampled for chemical analysis (the hydroxy end-terminated siloxanes were analyzed by using a gas chromatography after pre-concentration by solid phase extraction; the extracted was reacted with bis(trimethyl-silyl)-trifluoroacetamide BSTFA). After standing for 20 minutes at room temperature, the sample was injected into the gas chromatograph. The gas chromatograph used was a GC-MSD Agilent 7890A system.

[0065] Through feeding valve 5, compound addition was carried out.

[0066] During the preform manufacturing process, the feedstock was fed to the burner (50 in FIG. 2) opening the line above the container line (10).

[0067] Different batches of D4 were used containing hydroxy end-terminated linear siloxane impurities L.sub.n as shown in Table 1 (the amounts being expressed as ppm).

[0068] As reported, the linear siloxane impurities L.sub.n content of the batches A and B was mainly due to L2 up to L6 linear molecules. This silandiol concentration is known to be too high, resulting in troublesome gel formation.

[0069] Different batches were added with a passivating agent according to the invention, i.e. a compound of formula (I), in particular trimethylsilyl-imidazole (TMSI), optionally in the presence of another passivating agent (N,O-bis(trimethylsilyl)-acetamide, BSA) and a silylation catalyst (trymethylchlorosilane, TMCS), or according to the prior art, in particular a silazane derivative, hexamethyldisilazane (HMDS). The passivating agents were added to D4 batches and the resulting mixtures were stirred for different reaction times.

[0070] The results are summarized in the following Table 1.

TABLE-US-00001 TABLE 1 Reaction Sample time L2 L3 L4 L5 L6 L7 L8 to L13 Total 1. Batch A 28.2 15.3 6.8 0.3 0.2 <0.1 n.d. 50.8 1a. Batch A + 48 hours 8.8 6.4 5.5 0.1 <0.1 n.d. n.d. 20.8 HMDS (0.005 vol %) 1b. Batch A + 72 hours 1.2 0.6 0.1 <0.1 <0.1 n.d. n.d. 1.9 HMDS (0.01 vol %) 1c. Batch A + 2 hours 0.1 0.1 <0.1 <0.1 n.d. n.d. n.d. 0.2 TMSI + BSA + TMCS 3:2:3 (0.005 vol % of TMSI) Batch B 3.3 11.1 1.9 0.1 <0.1 <0.1 n.d. 16.4 Batch B + 3 hours 0.3 0.8 0.1 <0.1 n.d. n.d. n.d. 1.2 TMSI (0.1 vol %) Batch B + 60 hours 0.1 0.1 <0.1 n.d. n.d. n.d. n.d. 0.2 TMSI (0.1 vol %) n.d.: not detectable

[0071] TMSI (a compound of formula (I) according to the invention), optionally in the presence of another passivating agent (N,O-bis(trimethylsilyl)-acetamide, BSA) and a silylation catalyst, showed to effectively reduce the amount of linear siloxane impurities at concentration and time suitable for the industrial application.

[0072] HMDS (a silazane derivative) had an appreciable effect in reducing the amount of linear siloxane impurities, but it takes a substantially long time because it first reacts with the water present in D4. It should be taken in account that HMDS has two silylating groups and, accordingly, a double passivation capacity with the respect to a compound of formula (I).