MXPA96003820A - Method for purifying polyaquilsiloxanes and the proputs result - Google Patents

Abstract

The present invention relates to a method for purifying polyalkylsiloxanes, characterized in that it comprises the steps of: providing a polyalkylsiloxane starting material containing impurities having boiling points, under atmospheric conditions, of more than 250 ° C, in a total concentration of at least 14 ppm, and distilling the polyalkylsiloxane starting material under effective conditions to produce a purified polyalkylsiloxane composition, having a boiling point, under atmospheric conditions, of less than 250 ° C, and a total concentration of less than of 14 ppm of said impurities having boiling points, under atmospheric conditions, of more than 250

Description

METHOD FOR PURIFYING POLIALOUILSTI OXANOS AND THE RESULTING PRODUCTS FIELD OF THE INVENTION The present invention relates to a method for purifying polyalkyl-siloxanes, to a method for producing silica glass from purified polyalkylsiloxanes and to the resulting purified polyalkylsiloxanes and to the fused silica glass products made therefrom. In the production of metal oxides from reagents in the vapor state, SiCl. *, GeCla, POCl *, ZrCl *., TiCl-t and Bel are mainly used as steam sources, even though these materials have certain chemical properties. undesirable. Sid * is generally used for the production of high purity silica glass. As described in the patent of E.U.H. No. 3,698,936 one of several reactions can be carried out to produce high purity fused silica by the oxidation of SiCl_v7 namely: (1) Sid + 0a - SiO2 + 2C1 (2) SiCU + 2/3 0a - SiO + 2C1, or (3) SiCU + 2Ha0 - SiOat + 4HC1 characterized in that burners or nozzles are used to feed the reactive gases and vapors to a reaction space. Reaction (2) rarely occurs or is used. There are economic disadvantages inherent in each of these reactions. Moreover, these reactions, in which SiCU is oxidized through pyrolysis and hydrolysis, have the disadvantage of producing harmful and corrosive hydrochloric acid or chlorine byproducts. While the first two reactions occur theoretically, an auxiliary fuel is usually needed to achieve the pyrolysis temperature. With respect to the third reaction, hydrolysis of SiCU results in the formation of hydrochloric acid (HCl), a by-product that detrimentally affects not only many deposition substrates and reaction equipment, but is also harmful to the environment. It has been proven that emission abatement systems are very expensive due to the short duration, loss and maintenance of the equipment caused by the corrosive capacity of HCl. Despite problems in the handling and disposal of hydrochloric acid byproduct, hydrolysis of SiCU tends to be the preferred commercial method for producing fused silica for economic reasons. Although hydrolysis of SiCU has been the industry's preferred method of producing high purity fused silica over the years, the emphasized global sensitivity to environmental protection has led to stricter government regulation of peak source emissions. to a search for environmentally less pernicious supply materials. Regulations for peak source emissions require that HCl, the byproduct of SiCU hydrolysis, be cleaned from exhaust gases prior to release to the atmosphere. The economic consequences of complying with these regulations have made the commercial production of fused silica from halide-based supply materials less attractive to the industry. As an alternative, high purity fused quartz or silica has also been produced by the thermal decomposition and oxidation of silanes. Silanes are silicone-containing compounds analogous to hydrocarbons where silicon is replaced by carbon. (For clarity, it is noted that siloxanes are compounds that have an oxygen atom attached to two silicon atoms, for example Si - 0 - Si). However, thermal decomposition and oxidation require taking safety measures in handling because of the violent reaction that results from the introduction of air into a closed container of silanes. Silanes react with carbon dioxide, nitrous oxide, oxygen or water to produce high purity materials that are potentially useful for producing, among other things, semiconductor devices. However, it has been proven that silanes are too expensive and reactive to be considered for commercial use except possibly for applications of extremely high purity, on a small scale. A number of patents disclose the production of high purity metal oxides, particularly fused silica, from chlorides-based supply material, see U.S. Pat. 4,491,604, 3,666,414, 3,486,913, 2,269,059, 3,416,890, 2,239,551, 2,326,059, 2,272,342 and 4,501,602. Several patents where silanes have been used to produce high purity fused silica are also cited in the art, see Japanese Patent Application No. 90838-1985, the patents of E.U.R. 3,117,838, 4,810,673 and 4,242,487. It would be highly desirable for economic and environmental reasons to find halide-free silicone compounds that would replace the silicon halide supply materials typically used to produce high purity silica glass. Such halide-free starting materials would produce carbon dioxide and water, rather than harmful and corrosive Cl and HCl as by-products of the glassmaking process. Reference is made to the patents of E.U.fl. 5,043,002, 5,043,002, 5,152,819, 5,043,002 and 5,152,819. Although this may prevent the formation of HCl, some problems remain, particularly when the glass is intended to be used for the formation of waveguide optical fibers. Applicants have found that the presence of high boiling impurities in, for example, a polyalkylsiloxane supply material, can result in the formation of gel deposits in the line that carries the reagents in the vapor state to the burner or within the same burner. This leads to a reduction in the deposition rate of the soot preform that is consolidated subsequently to a template from which an optical fiber is extracted. Moreover, high molecular weight particulates, high boiling impurities, can be deposited in the fiber optic template, resulting in "defect" or "cluster defect" imperfections that adversely affect the quality of the fiber subsequently extracted and may require discarding a complete template. The defects have small bubbles (i.e. 0.1 to 4.0 m in diameter) in a glass body. They can be formed in the fused silica by an impurity, such as a polyalkylsiloxane in the form of a gel not consumed by combustion. A very small particle of siloxane gel may be the starting site of a defect. The siloxane decomposes at high temperature after it is deposited in the body of the glass, releasing gases that cause the formation of the defect. Cluster defects are the largest glass defects that are found in the preforms of optical fibers. These are composed of a series of defects in the form of a line or a cluster in the form of funnel or flower. A large gel particle may be the starting point of a cluster defect. After the gel particle collides with the porous preform, it causes a raised area to protrude from the surface of the preform. Because the cluster defect is a high site, there is more heat transfer to this site, because of this increased heat transfer, more ter oresores occur at this site, causing the imperfection to grow and stop behind. yes a series of defects. As a result of the cluster defect, the affected portion of the fiber optic preform can not be consolidated normally and the consequent irregularity in the template produces a defective fiber optic. Thus, there is a need for a purified polyalkylsiloxane composition containing a very low concentration of high boiling impurities to serve as a supply material for the production of high purity fused silica glass and articles made from of this, including waveguide optical fibers. The present invention is directed to satisfy this need.

SUMMARY PE Lfl INVENTION The present invention is directed to a purified polyalkylsiloxane composition having a boiling point, under atmospheric conditions, of less than 25 ° C, at a total concentration of less than 14 parts per million. ("ppm"). The present invention is further directed to a method for producing a purified polyalkylsiloxane composition, having a boiling point, under atmospheric conditions, of less than 250 ° C, by distilling a polyalkyl-siloxane starting material containing high impurities. boiling that have boiling points ba or atmospheric conditions, of more than 250 ° C at a total concentration of at least 14 ppm, under conditions effective to produce a purified polyalkylsiloxane composition with a boiling point under atmospheric conditions, of less than 250 ° C and with high boiling impurities of more than 250 ° C in a total concentration of 14 ppm. In accordance with the preferred embodiments, relatively volatile, low-boiling chemical species that dye a molecular weight less than the hexamethyl cyclotrisiloxane (D3), including low molecular weight silanols and preferably D3 itself, are also removed. . The present invention is further directed to a method for producing fused silica glass by glass conversion of the purified polyalkylsiloxane composition containing high boiling impurities, boiling under atmospheric conditions, greater than 250 ° C in a total concentration of less than 14 ppm.

BRIEF DESCRIPTION OE LQS DRAWINGS Figure 1 is a gel penetration chromatographic trace showing the bimodal distribution of high molecular weight, high boiling impurities in an ilcycotetrasiloxane octame sample. Figure 2 is a schematic representation of an apparatus and method for forming large masses of fused silica. Figure 3 is a perspective view of a first embodiment of vaporizer useful in the production of optical waveguides. Figure 4 is a schematic view of a second vaporizer mode useful in the production of optical waveguides. Figure 5 is a partial cross-sectional view of a third embodiment of vaporizer useful in the production of optical waveguides. Figures 6fl and 6B are schematic representations of an apparatus and method for depositing silica soot on a rotating lathe shaft to form a porous template or preform. Figure 7 is a schematic representation of a heating chamber characterized in that the porous template burns in an atmosphere of helium and chlorine until complete consolidation of a non-porous body. Figure 8 is a schematic cross-sectional view of a vertical evaporator for silicone-containing compounds.

DETAILED DESCRIPTION OF THE INVENTION Although the polysiloxanes as a supply material for the production of silica glass provide, as discussed above, important advantages over the commonly used SiCU, substantial practical difficulties have been encountered in its use. Commercially available siloxanes such as, for example, octamethylcyclotetrasiloxane contain high boiling impurities whose concentrations can reach up to 200 ppm or more. The presence of these impurities in the siloxane supply material results in the formation of deposits in the lines of the steam supply system, causing a substantial reduction in the rate of deposition of soot from the burner. This problem is aggravated when an oxidizing vehicle gas such as oxygen is included in the reactive vapor stream, since the oxidants can catalyze the polymerization of the siloxane supply material. In addition, carrying the impurities to the soot can result in the formation of defects in the templates of the glass. In the case of a typical lOOk consolidated template, which has a diameter of 70 millimeters (mm) and a length of 0.8 meters (), the presence of a cluster defect on the surface of the template will typically result in the loss of 5 km of fiber in extraction. In the case of a longer consolidated workforce, the negative impact of a single cluster defect is proportionately higher. In a consolidated 250-km template, which has a diameter of 90 mm and a length of 1.8 m, a cluster-shaped defect on the surface of the template will typically result in the loss of 8 km of fiber in the extraction. The presence of high boiling impurities in a polyalkylsiloxane supply material for the production of silica glass will be problematic when the partial pressures of the impurities are greater than the vapor pressure of the vaporizing polyalkylsiloxane. Under these conditions, all impurities do not vaporize and tend to form gels on the surfaces of the vaporizer. On the other hand, when the partial pressures of the impurities are less than the vapor pressure of the polyalkylsiloxane that is vaporized, contamination is not a problem, since the vaporization conditions are sufficient to completely vaporize the supply material. In conventional polyalkylsiloxane synthesis processes, the hydrolysis of an alkylhalosilane compound produces a mixture of polyalkylsiloxanes and silanols. The polyalkylsiloxane-silanol mixture is distilled to remove most silanols from the mixture. This can be achieved in a single distillation, or can be achieved with two or more sequential distillations. The resulting distillate is passed through a carbon filter and after a molecular sieve bed, thereby producing the polyalkylsiloxane starting material which will subsequently be purified from the removal of the high boiling impurities. Contacting silanes and eloxanes with charcoal and molecular sieves to remove impurities of platinum contaminants from the catalyst is described in the U.S. Patent. No. 4,156,689 issued to Flshby, et al., The description of which is incorporated herein for reference. While not wishing to be bound by theory, it is believed that the carbon filtration steps and the molecular sieve treatment according to the present invention result, as a side effect, in the formation of at least a portion of the high boiling impurities in the starting material of polyalkyl-siloxane. It is believed that these increased impurities result from condensation reactions involving low boiling compounds, possibly including silanols, to produce linear or high boiling cyclic compounds. Such condensation reactions occur as follows: HO - CSÍ (CHa) 3a "O] - H + HO - CSÍ (CHa) a - 0] - HO -CSKCHa)]» 0U ..- H + Hβ0 It is the presence of these impurities in concentrations of 14 ppm or more that causes the above observed contamination and product quality problems. It is an object of the present invention to reduce the concentration of such impurities to less than 14 ppm (and reduce them as much as 0.25 ppm, or less) by distillation.

In the preferred embodiments, the polyalkysiloxane is first passed through a bed of carbon and then through a bed of molecular sieves. As is known in the art, the carbon bed can catalyze the polymerization of the silanols to produce high molecular weight compounds. The molecular sieve then traps these compounds because of their greatly increased molecular size. In accordance with the present invention, starting material of polyalkyl-siloxane containing high boiling impurities having boiling points, under atmospheric conditions, of more than 250 ° C is distilled to produce a purified polyalkyl-siloxane composition which It has a boiling point, under atmospheric conditions, of less than 250 ° C. The total concentration of the high boiling impurities in the starting material is at least 14 ppm and can be up to 200 ppm or even higher. The distillation, which is carried out at a temperature where the vapor pressure of the polyalkyl-siloxane product exceeds the total pressure for the distillation, can be carried out either at substantially atmospheric pressure or under reduced pressure under vacuum. This produces a purified polyalkylsiloxane composition having a total concentration of high boiling impurities of less than 14 ppm, preferably less than 6 pprn, more preferably less than 2 ppm and most preferably less than 0.25 ppm. . The purified polyalkylsiloxane composition may have a linear (eg, hexamethyldisiloxane) or cyclic molecular structure but is preferably a polyethylocyclosiloxane selected from the group consisting of hexamethylcyclotrisiloxane (D3), octarnetylcyclotetrasiloxane (D4), deca ethylcyclopentasiloxane (D5), dodecamethylcyclohexasiloxane ( D6), and mixtures of them. (The terms D3, D4, D5, D6, etc. are examples of the siloxane notation of the General Electric Co., where D represents the group (CCH) Sil-O-). Octamethylcyclotetrasiloxane (D4) is particularly preferred for use in accordance with the present invention. The dodeca ethylcyclohexasiloxane (D6) is useful, but is the least preferred, among the named cyclic species. The preparation of polyrnethylcyclosiloxanes is described in the patent of E.U.fl. No. 4,689,420 granted to Baile etal., Description of which is incorporated herein for reference. The polyalkylsiloxanes of the present invention also encompass polyrnenylhydrocyclosiloxanes having hydrogen groups as well as methyl group attached to the silicone atoms. For example, half of the methyl groups in octamethylcyclotetrasiloxane can be replaced with hydrogen groups to form tetra-ethylcyclotetrasiloxane. Additionally, the ethyl groups may be substituted for all of the methyl or hydrogen groups. When the purified polyalkyl-siloxane composition is predominantly octamethylcyclotetrasiloxane, the distillation is preferably carried out under conditions wherein the vapor pressure of octamethylcyclotetrasiloxane, in Torr, exceeds exp (15.56055 - 3128.52 / (T-98.093)), and , in Pascal, exceeds exp (20.4534 - 3128.52 / (T-98.093)), where T is the distillation temperature in degrees Kelvin. The separated high boiling impurities typically include a bimodal distribution of components whose median weight is greater than 900 grams / mole. This distribution in a typical octamethylcyclotetrasiloxane sample is shown in FIG. 1 where the relative peak height of the concentration is plotted of the species having a given molecular weight as the polyalkylsiloxane sample passes through the chromatography column against time in minutes. The relatively high molecular weight species pass rapidly through the gel. Relatively low molecular weight species pass more slowly through the gel. The concentration of high boiling impurities in a polyalkylsiloxane material can be determined by two alternative methods. In the first procedure, a heavy amount of the polyalkylsiloxane material is placed in a round bottom flask attached to a rotary evaporator. The bottle is placed in a bath of water or hot oil and the material is concentrated under vacuum to a small amount of oily residue. This residue is transferred to a smaller bottle using a small volume of toluene and the material dissolved in toluene is again concentrated to oil. After a second transfer with the aid of toluene to a vessel without tare followed by a concentration, the oily residue is weighed, thus allowing the concentration of high boiling impurities in the polyalkylsiloxane material to be determined. An aliquot of this residue dissolved in toluene is subjected to gel permeation chromatography to determine the molecular weight distribution of the high boiling impurities. The second alternative method for determining the concentration of high boiling impurities in a polyalkylsiloxane material uses an evaporative light scattering detector (ELSD). Commercial equipment available for this method includes a Varex MKIII fllltech detector, a fllltech 426 high pressure liquid chromatography pump and a fllltech 570 automatic sampler. A sample of the polyalkylsiloxane material in a pressurized gas such as nitrogen is nebulized. and the nebulized material is passed through the detector, which is connected to the pump. The tow tube of the light scattering detector, which is provided with a laser source, is maintained at a temperature range of 65-85 ° C, preferably at 5 ° C. The laser light detected by the particles in the nebulized sample is detected and an analog signal proportional to the light intensity is generated and detected. The detector response is a peak that can be displayed with either a chart recorder or a chromatographic integrator. A height or peak area can be used for the quantification of the response; the integration area is preferred. The method by ELSD is fast and convenient but does not provide information such as the molecular weight distribution of the impurities. As noted above, the high boiling impurities may be in the form of siloxanes or siloxanes terminated in silanol. These siloxane impurities can include cyclic polyalkylsiloxanes which have boiling points, under atmospheric conditions, greater than 250 ° C. The dodeca ethylcyclohexasiloxane (D6) has a boiling point lower than 250 ° C and, therefore, does not constitute such an impurity, while the tetradecametilcicloheptasiloxane (D7), whose boiling point is higher than 250 ° C, constitutes such impurity . In the presence of water, a cyclic polyalkylsiloxane may undergo a ring opening reaction to form a linear silanol-terminated siloxane, as shown in the following general equation: C 'R "YES0J) < + Ha0 - H0- [R'R" YES0] > < -H where R 'and R "are alkyl groups The disilanol-siloxane compounds produced by the ring hydrolytic opening of the preferred polyethycylcylosiloxanes such as D4 or D5 (the X equals 4 or 5 respectively in the above equation) they would, of course, have substantially reduced volatility compared to the latter compounds and could therefore be expected to result in harmful gel deposits in the reactive steam lines or in the burner of the silica glass production equipment. Moreover, the diethylamine compounds produced in the hydrolysis reaction described above are highly reactive intermediates that readily undergo condensation reactions with their precursor cyclosiloxanes, resulting in the formation of high boiling concentration products, as shown in the following general equation : HO-CR'R "SÍOJM: -H + CR'R" YES0] HO-CR 'R' ^ iOH ^ -H It is extremely likely that such high-boiling, low volatility materials, which may have a cyclic or non-cyclic structure, form gel deposits in the production equipment. Therefore, in accordance with the invention, it is necessary to provide polyalkylsiloxane supply materials which contain very low concentrations of these high boiling impurities to ensure the efficient production of soot. The low boiling compounds which remain in the polyalkyl-siloxane composition may include hexamethylcyclotrisiloxane (D3), and low molecular weight eilanols, all having a molecular weight of 250 grams / mole or less. The silanol materials form SiOa deposits and polyalkylsiloxane gel very quickly in the exhaust pipes and burners. D3 has a greater tendency than D4, D5 or D6 to decompose, forming, among other things, SiOs, which can then be deposited in the exhaust pipes and in the burners. In cases where the burners that are being used are located and do not traverse the full length of the target for soot deposition, the soot deposits are non-uniform, which leads to non-uniform products. In the preferred embodiments, these low molecular weight silanols are removed as an integral part of the distillation process according to the present invention. Preferably, D3 is also removed, although it is believed that silanols present a comparatively greater problem. These relatively volatile materials will be vaporized at the beginning of the distillation process and then discarded. Preferably, the concentration of D3 and low molecular weight silanols is reduced to less than 7000 ppm. More preferably, the concentration is reduced to less than 100 ppm. Current technology does not facilitate the separate measurement of concentrations of D3 and low molecular weight silanols. In addition, in accordance with the present invention, a method for producing fused silica glass is provided. A purified polyalkylsiloxane composition having a boiling point, under atmospheric conditions, greater than 250 ° C is provided in a total concentration of less than 14 ppm, preferably less than 6 ppm, more preferably less than 2 ppm and most preferably lower 0.25 ppm and converted to fused silica. The purified polyalkylsiloxane composition that is required to produce fused silica is obtained as described above. The purified polyalkylsiloxane composition can be contaminated by mixing it with a compound capable of being converted by oxidation or heat hydrolysis to Pa-Oa and / or metal oxide with a metal component selected from the group consisting of the Ifl, IB Group, Ilfl, IIB, Illfl, IIIB, IVA, IVB, VA, the series of rare earths and mixtures of them. Preferred dopant oxides include AlaOa, Ba02, GeOs, Pa0a and TiOA. In the preferred embodiments, the purified polyalkylsiloxane is further stabilized against degradation prior to its final use, by rolling it with an inert gas, e.g. nitrogen, in order to remove dissolved oxygen and water. Oxygen and water can, of course, react with and degrade to the polyalkylsiloxane, which not only leads to the presence of undesirable low molecular weight species in the composition, but can give rise to reactive species that could polymerize and form a gel. The purified polyalkylene oxane composition is generally converted to fused silica glass by providing a gas stream containing a purified polyalkylsiloxane entrained composition. It is noted, incidentally, that the polyalkyl-siloxane composition can alternatively be supplied in liquid form to the flame of a burner, as described in the US patent application registered on the same date together with this in the name of Daniel U. Ha tof, Greg E. Smith and Eric H. Urruti entitled "Method and flpparat? S for Forming F? Sed Silica by Combustion of Liquid Reactants", the content of which is incorporated herein for reference. Returning to the discussion, the gas stream is then oxidized to convert the polyalkylsiloxane composition into a finely divided amorphous soot, which is deposited to produce a porous mass. The gas stream is provided by nebulizing or vaporizing the purified polyalkylsiloxane composition in a carrier gas such as an oxidizing gas, a combustible gas, an inert gas or mixtures thereof. Particularly suitable gases are hydrogen, nitrogen, oxygen or mixtures thereof. Where the selected vehicle is not inert and especially if it is an oxidant such as oxygen, the carrier gas must be combined with the polyalkylsiloxane compound immediately before combustion.

The gas stream is oxidized by passing the gas stream through the combustion flame of a burner in the presence of oxygen. The soot can be deposited on or within a fixed bait or any other known conventional target. Preferably, the soot is deposited on a mandrel. The resulting porous mass can then be converted into a consolidated mass of fused silica glass by heating it in a furnace preferably containing helium and chlorine. Also in accordance with the present invention, the fused silica glass product can be further treated, for example, by extracting an optical fiber. In the conventional process of forming a "pear" shaped mass, a vehicle gas is bubbled through a supply material that is maintained at a specific temperature. The reagent in the vapor state is entrained in the vehicle gas and then transported to the reaction site. The reaction site comprises a number of burners that combust with and oxidize the reagent in the vapor state, typically at a temperature greater than 1700 ° C. The aforementioned system is illustrated in FIG. 2 wherein the supply material 201 comprises the purified polyalkylsiloxane composition of the invention which is to be combusted in a commercial furnace to produce high purity fused silica "pears". An inert gas, an oxidizing gas or mixtures thereof are used as the vehicle gas 200, and the bypass stream 202 of a gas chosen from the same group is introduced to prevent saturation of the stream in the vapor state. The reagent is vaporized by the action of the gas vehicle 200 bubbled through the supply material 201, using apparatuses such as that described in the references mentioned above. The reagent is then passed through a dispensing mechanism 203 to a reaction site having a number of burners 204 present in extreme proximity to an oven crown 205. The reagent is combined with a fuel / oxygen mixture. in these burners and burns and oxidizes at a temperature higher than 1700 ° C. The high purity metal oxide soot is directed downwardly through the refractory furnace crown 205, where it is immediately deposited and consolidated to a non-porous mass in a bait 206. It is well known in the art that the Processing of the supply material requires an apparatus and transfer system capable of vaporizing the supply material and supplying it to the burner in the vapor state. Most of the procedures currently being developed in the industry for the manufacture of optical waveguides use the concept of chemical vapor deposition (CVD) or a modified version thereof. The precursor vapors are entrained in a vehicle gas stream and then passed through the flame of a burner, usually a mixture of natural gas / oxygen and often containing excess oxygen. The vapors present in the mixture are converted to their respective oxides after leaving the burner orifice to form a stream of volatile gases and finely divided, amorphous, spherical aggregates called soot. The soot is collected in a baboon or in a bait where it is deposited in thin layers. The final product of the soot collection, the porous preform, is then subjected to a high temperature at which it consolidates in a monolithic non-porous glassy body. The steps of oxidation, soot deposition, and consolidation can be carried out either sequentially or simultaneously as taught in U.S. Pat. No. 5,043,002 issued to Dobbins et al., And the patent of E.U.fl. No. 5,152,819 issued to Blackwell et al., Descriptions of which are included herein for reference. In usual practice, the method for producing the optical waveguide is a three-step procedure. The first step includes supply material (s) to form finely divided, amorphous spherical particles of soot on a substrate. In the second step of the process, the template or preform is subsequently treated with heat in a helium / chlorine atmosphere until complete consolidation. In the third and last step, conventional fiber extraction technology is used to extract the waveguide fiber from the preform. The first step of this procedure can be carried out in different ways.

In a first embodiment of the first step, reagent is supplied in liquid form to a flow distributor that delivers the liquid to the end of a vaporization device. The liquid flows through a sloped, heated surface, like a thin film towards the second end of the device. When the second end is reached, the liquid has been converted into steam and is supplied to a burner for oxidation to soot particles. This modality is described in the patent of E.U.fl. No. 5,356,451 issued to Cain et al. which is incorporated herein for reference and is shown in FIG. 3. With respect to Figure 3, the vaporizer 315 comprises a vaporization chamber 321 enclosed by an upper wall 322, a lower wall 323, side walls 324 and 325 or end walls 326 and 327. A flow distributor 328 which it has plurality of U-shaped channels 329 located near the wall 326, the space between the flow distributor 328 and the wall 326 constituting a reservoir of liquids 330. The liquid inlet orifice 322 is located at the bottom of the reservoir 330, and the steam outlet hole 335 is located in the upper wall 322 near the wall 327. The reactive liquid is fed into the orifice 322. When the liquid level rises over the dotted line 333, it begins to flow through the channels 329 and evenly distributed through the chamber 321 between the walls 324 and 325. The support means 336 raises one end of the vaporizer so that it is oriented at an angle the alpha with respect to the horizontal and the liquid flows towards the wall 327. A film 338 is formed, whose maximum thickness £ depends on parameters such as surface tension, density, viscosity and the alga angle. In contrast to those vaporizers that confine the film within a space between two parallel surfaces, the film 338 is such that the vapor and liquid phases of the reagent are separated by a surface that is parallel to or closely parallel to the bottom surface. The thickness i. The film should be thin enough so that bubbles can not form inside the liquid film 338. A second embodiment of the first step also includes the use of a vaporizer. Here, the vaporizer is a heated vertical expansion chamber which achieves vaporization when the reagent is sprayed onto the heated interior walls of the vaporizer. This procedure is described in the American patent application Serial No. 08 / 368,318 issued to Williams, registered on December 30, 1994, entitled "VERTICAL VflPORIZER FOR HALIDE-FREE SILICON CONTAINING COMPOUNDS" which is incorporated herein for reference and it is shown in FIG. 4. As shown in Figure 4, pre-heated liquid reagent is supplied to the expansion chamber 420 via the conduit 452 and the vertical hollow bar 442. The reagent is sprayed from the bar 442 on the heated walls of the chamber 420 at locations 401 through holes 445. A portion of the liquid reagent vaporizes upon entering the interior of chamber 420 due to the pressure drop between the interior of the bar and the interior of the chamber. The remnant of the liquid reaction forms the dew 454 which passes through the interior 424 of the chamber and comes in contact with the wall 422. This vaporizes the remaining liquid, and the vapors are discharged through the exhaust 450. The chamber 420 includes the gel collection zone 426, the average vaporization zone 428 and the superheating zone 430. The zone 426 has the 480 port for the removal of high molecular weight materials which may be capable of forming gels. In the third modality of the first step, liquid reagent is added to an instantaneous vaporization chamber. In that chamber, the liquid assumes the form of a thin film, vaporizes, and is mixed with a gas selected from the group consisting of an inert gas., a combustible gas, an oxidizing gas and mixtures of them to supply towards an oxidation burner. This is described in the patent of E.U.fl. No. 5,078,092 issued to Antos, etal., Which is incorporated herein by reference and is described in Figure 5. As shown in Figure 5, the liquid is supplied to an instantaneous vaporization chamber 501 by a vertical tube. 516 inside the outer tube 512 directly on the inner surfaces 517 of the heating element 506, which is formed inside the flash chamber 501. The liquid is fed at the upper end of the tube 516 from the feed line 507 at a controlled flow rate and is supplied to form a thin film directly on the surface 517 resulting in a soft vaporization, free of oscillations. The tube 516 may comprise a vertical tube 5.12 of 0.156 cm internal diameter connected to a T of 0.625 cm internal diameter and a pipe 514 to supply the liquid directly on the surface 517 as a thin film. The flash chamber 501 is heated by the heating element 506 and the cylinder of the chamber 519. The cylinder of the chamber 519 may comprise several different configurations, for example, a rod in a cylinder or paired parallel flat plates. The temperature of the heating element 506 is maintained below the temperature at which the liquid boiling or nucleation occurs. Additional embodiments of vaporizers are described in the co-pending patent application of E.U.A. No. 08 / 368,319 granted to Cain, registered on December 30, 1994 entitled "VERTICAL, PACKED-BED FILM EVAPORFlTOR FOR HflLIDE-FREE, SILICON-CONTAINING COMPOUNDS", which is incorporated herein for reference. That application describes, as shown in FIG. 8, a vaporizer (film evaporator) 813 for liquid reagents containing halide-free silicone used in the production of preforms. The vaporizer includes a plurality of packed bed columns 822 surrounding the central tube 824. A mixture of liquid reagent, e.g., is sprayed. octamethylcyclotetrasiloxane, and gas, e.g. oxygen, on top surfaces 824 of columns 822 by a set of nozzles 832. The liquid reagent evaporates in the gas until the dew point temperature is reached, at which point all liquid reagent will have become in steam. The vapor / gas mixture leaves the lower surfaces 856 of the columns 822, where its flow direction changes from bottom to top. This change in the direction of flow separates any molecular species of high molecular weight 846 present in the octamethylcyclotetrasiloxane from the vapor / gas mixture. The vapor / gas mixture leaves the vaporizer 813 through the central tube 824 and is supplied to the soot-producing burners. Alternatively, in the first step of fiber optic manufacture, a carrier gas is bubbled through a liquid supply material that is maintained at a constant temperature. Optionally, each of the liquid components is heated separately until having a constant temperature at which sufficient vapor pressure is generated to produce a reasonable rate of deposition, and then the individual vapors are combined. The reagent in the resulting vapor state is transported by the carrier gas, which can be an inert gas, a fuel gas, an oxidizing gas or mixtures thereof, to a reaction site such as a burner, wherein the gas stream It burns in the flame of a burner. In the presence of oxygen, the reactants in the vapor state become their respective oxides, leaving the burner orifice to form a gas stream containing amorphous, eeféricae, finely divided particles of soot that are deposited on a substrate., forming a porous template or preform of opaque white silica soot. Co or is drawn in FIG. 6fl, the supply material 607 comprises the purified polyalkylsiloxane composition of the present invention in a standard process used in the manufacture of optical waveguides. The gae vehicle 611 can be, for example,? Fuel gas, an oxidizing gas,? N inert gas or mixtures thereof. The nitrogen is preferably used as the carrier gas 611 and a methane / oxygen mixture 612 is used as the fuel for the burner flame, whereby combustion and oxidation are induced in the burner 608. The resulting soot is deposited in the burner. a rotating mandrel 609, making so preform or 610 silica soot template shown in FI6. 6B. The preform is then heat treated, as shown in FIG. 7, in a consolidation furnace 713 preferably with a He / Cl 714 atmosphere to effect complete consolidation. Conventional fiber extraction techniques can then be used to make waveguide optical fiber from the consolidated preform. Alternatively, a waveguide optical component according to the invention can be formed. In an exemplary process, the soot is applied to a flat substrate to form a base covering layer, and then the additional soot is deposited in two successive steps using different reactive compositions to produce the center and coating layers on the layer. of covering that is in the stratum. The resulting mass is then consolidated. The design for the trajectories of the optical waveguides of the component is then applied lithographically followed by etching to leave the desired waveguides superimposed on the substrate. Then, the component is coated and the coating is consolidated. In a second and similar procedure, the final coating is suitably impurified so that its refractive index is the same as that of the previously deposited revetment, but its melting point is lower so that it flows easily in the consolidation at a temperature that will not risk damaging previously deposited waveguides. Still in a third procedure, a preform is forced into a rod where the waveguides are then applied as described above (see EU Patent No. 5,125,946 to Bhagavatula and US Patent No. 5,253,319 to Bhagavatula both incorporated here for your reference). In addition, the fused silica glass product of the present invention can be used to produce stepped lenses or other optical elements, e.g. conventional lenses.

The examples and examples further illustrate the present invention.

Example 1 Pressure distillation of octamethylcyclotetrasiloxane to the mosphere A five-liter flask equipped with a heating mantle, a distillation head and two thermometers for measuring the flask and head temperatures were charged with 4000 ml of octamethylcyclotetrasiloxane (D-244), commercially available from Dow Corning, and containing 110 ppm of impurities of molecular weights greater than 250 grams per mole. The concentration of such high boiling impurities is determined by the rotary evaporation process described above.

The distillation is carried out at atmospheric pressure (at 350 meters above sea level) using a nitrogen purge to exclude oxygen and the following fractions were collected: Temperature Temoeratur »« Je head bottle Volume Fraction (.) (Mi) before up to 173 to 166 500 1 173 166 - 167 850 2 173 - 174 166 - 167.5 850 3 174 167.5 850 4 174 - 175 166.5 - 171 750 afterwards until 189 to 174 60 The analysis of the distillation fractions 1 to 4 shows concentrations of high boiling impurities in the following concentrations: ppm of impurities of high fraction or molecular weight 1 0.2 2 0.2 3 0.1 4 2.1 In this way, distillation at atmospheric pressure produced a purified polyalkylsiloxane composition (i.e. fractions 1-4) with a greatly reduced level of high boiling impurities compared to the parent material.

Eie PO 2 :: yes ació Atmospheric pressure of octa and Iciclotetrasiloxane A distillation process is carried out at atmospheric pressure (at 350 meters above sea level) of approximately 2.5 liters of octamethylcyclotetrasiloxane D-244 from Dow Corning using substantially the same distillation apparatus as described in Example 1. The following distillation fractions were obtained: Temperature Bottle temperature 4e Head Volume Fraction (° C) (° C) (mi) 1 174 144 - 162 30 2 174 162 - 167 270 3 175 - 176 168 - 170 990 4 176 170 15 5 177 - 179 170.7 - 172.7 780 6 180 - 187 170.5 - 166 160 7 190 162 60 8 192 158.5 15 9 195 152 8 10 199 146 6 11 204 - 210 140 - 134 10 Fraction 3, collected at a rate of 15.5 ml / min, is divided into two portions, which are concentrated under reduced pressure using a one liter bottle connected to a rotary evaporator up to a volume of 5 ml. This residue is transferred to a 50 ml flask using two toluene rinses. The toluene solution is reconcentrated in the rotary evaporator until an oil is obtained. The gel penetration chromatographic analysis of this oil by the previously described procedure showed no detectable amount of high boiling material. Thus, atmospheric pressure distillation of octamethylcyclotetrasiloxane is effective in removing the high boiling impurities present.

Eiemolo 3 - Pr? < IMPROVED JUSTICE < j Silica glass stencil to part r s octamethylcyclotetrasloxane mirifi a or having low concentrations of high boiling impurities Silica glass stencils are produced from octamethylcyclotetrasiloxane supply materials containing varying concentrations of high boiling impurities. For each of the supply materials, changes in speed and pressure per template are measured.

In addition, the number of templates that could have been produced before brushing the burners is required. The appearance of the steam tubes of the burners is also observed after the combustion of the supply materials. Later the results of these tests are shown: Material Impurities Speed Change of No. of High of change / pressure / spaces Supply of boiling spaces between spaces LPJ = m (Krtt / hr) < "" "Ho) brushed 199 2 161 -4.2 0. 82 4 3 109 -2.2 0. 59 8 4 17 -1.8 0. 37 10 5 7 -1.9 0. 31 15 6 4 0 0. 08 - - 7 0.6 - 0.9 0. 11 22 The data listed above show the substantial improvements in velocity and pressure changes per template as the concentration of high boiling impurities in the supply material is reduced from 199 to 161, then to 109, 17, 7, 4 and 0.6 ppm. It is also worth noting that the maintenance of the quemadoree that ee requires greatly reduces as the concentration of high boiling impurities is reduced. For example, with the supply material 1, which contains 199 ppm of high boiling impurities, only two templates were produced before brushing the burners was required. With the supply material 4, whose concentration of high boiling impurities was 17 ppm, ten templates were made before it was necessary to brush the quemadoree. The use of the supply material 7, with a concentration of high boiling impurities of only 0.6 ppm, allowed to make 22 templates before maintenance was required for the burners. The supply materials 1 - 3 contain 0.05 to 0.2 weight percent of hexamethylcyclotrisiloxane and silanols of boiling point; the supply material 4 contains 0.11% by weight; the supply material 5 contains 0.27% by weight; the supply material 6 contains 0.01% by weight; and the supply material 7 contains 0.19% by weight. Thus, the method of the present invention provides a surprisingly large improvement in the efficiency of the production of fused silica glass from purified polyalkylsiloxane supply materials. The ignition of the smoke tubes of the burners after the combustion of several supply materials showed that with the supply material 4 a moderate residue of gel is observed in all of them. With the supply materials 5 and 7 only a small amount of gel was deposited and not in all the smoke tubes. No gel was detected when the supply material 6 was used in the production of glass templates.

Claims (21)

NOVELTY OF THE INVENTION CLAIMS

1. A purified polyalkylsiloxane composition characterized by having a boiling point, under atmospheric conditions, of rnenoe of 250 ° C, and because it contains high boiling impurezae including siloxanes and silanol-terminated siloxanes having boiling points under atmospheric conditions of more than 250 ° C in a total concentration of less than 14 ppm.

2. A purified polyalkylsiloxane composition according to claim 1, further characterized in that the purified polyalkyl-siloxane composition has a total concentration of high boiling impurities of less than 6 ppm or, less than 2 ppm, less than 0.25 ppm

3 A purified polyalkylsiloxane composition according to claim 1, further characterized in that it comprises no more than about 100 ppm of hexamethylcyclotrisiloxane and silanols having a molecular weight less than about 250 g / mol.

4. A purified polyalkylsiloxane composition according to claim 1 further characterized in that the purified polyalkylsiloxane composition comprises polyethylsilocylosiloxane selected from the group consisting of hexamethylcyclotrieiloxane, octamethylcyclotetrasiloxane, deca ethylcyclopentasiloxane, dodecamethylcyclohexasiloxane and mixtures thereof.

5. A purified polyalkyl-siloxane composition according to claim 4, further characterized in that the polyalkyl-siloxane composition comprises hexamethyldisiloxane, or a polyethylohydrocyclosiloxane or that the polymethylcyclosiloxane comprises octamethylcyclotetrasiloxane.

6. A purified polyalkyl-siloxane composition according to claim 5, further characterized in that the high-boiling impurities include a bimodal distribution of components having a median molecular weight greater than 900 gram / mole.

A method for making the purified polyalkylsiloxanes of any of claims 1-6 characterized in that it provides a starting material of polyalkylsiloxane containing high boiling impurities including siloxanes and silanones terminated in eilanol, having boiling points under atmospheric conditions greater than 250 ° C, in a total concentration of at least 14 ppm; and distilling the pclialqyrosiloxane starting material under effective conditions to produce a purified polyalkylsiloxane composition, having a boiling point under atmospheric conditions, less than 250 ° C and a total impurity concentration of less than 14 ppm.

8. A method according to claim 7, further characterized in that the total concentration of the high boiling impurities in the polyalkylsiloxane starting material is up to 200 ppm.

9. A method according to claim 7 or 8, further characterized in that the polyalkylsiloxane starting material contains a certain concentration of low boiling impurities which have molecular weights of less than 250 grams / mol comprising the silanols; and said concentration of low boiling impurities is reduced during said distillation.

A method according to claim 7, 8 or 9 further characterized in that said distillation is carried out at substantially atmospheric pressure or under vacuum at reduced pressure or at a temperature where the vapor pressure of the polyalkylsiloxane starting material exceeds the total pressure for said distillation.

11. A method in accordance with the claim 7, further characterized in that providing a polyalkylsiloxane starting material comprises: distilling a mixture containing polyalkylsiloxane and silanols to remove most of the eilanols from the mixture and forming a polyalkylsiloxane distillate; and passing the polyalkylsiloxane distillate through an activated carbon filter and a molecular sieve bed, thereby producing said polyalkylsiloxane starting material.

12. The further processing of the polyalkyl-siloxane made by the method of any of claims 7 to 11, characterized in that it comprises the step of converting the purified polyalkylsiloxane composition into molten glass of silica.

13. A method according to claim 12, further characterized in that said polyalkyl-siloxane composition comprises not more than 100 ppm of hexamethylcyclotrisiloxane and silanols having a molecular weight of less than 250 grams per mole.

A method according to claim 12, further characterized in that said provision of a polyalkylsiloxane starting material comprises: distilling a mixture containing polyalkylsiloxanes and silanols to remove most of the silanols from the mixture and forming a polyalkylsiloxane distillate; and passing the polyalkylsiloxane distillate through an activated carbon filter and a bed of molecular sieves, thereby producing said polyalkylsiloxane starting material.

15. A method according to claim 12, further characterized in that said conversion comprises: providing a gas stream containing the purified polyalkylsiloxane composition; oxidizing the gas stream to convert the composition polialq? ilsiloxano purified amorphous soot finely divided, optionally passing current gae through the flame combustion of a burner in the presence of oxygen, depositing the soot on or in a substrate for producing a porous mass, optionally collecting the soot on a rotating mandrel; and heat treating the porous mass under effective conditions to form a consolidated mass, optionally in a medium containing helium and chlorine.

16. A method according to claim 15, further characterized in that providing a gas stream comprises: nebulizing or vaporizing the purified polyalkylsiloxane compositions in a carrier gas, and / or said carrier gas is selected from the group consisting of of an oxidizing gas, a combustible gas, an inert gas and mixtures thereof, and / or from the group consisting of hydrogen, nitrogen, oxygen and mixtures thereof.

17. A method according to claim 15, further characterized in that the consolidated mass is used to form an optical fiber by means of a process that further comprises: heating the consolidated mass and extracting an optical fiber from the consolidated hot line.

18. A method according to claim 15, 16 or 17 further characterized in that the purified polyalkylsiloxane composition is mixed with a composition capable of being converted to Pa03 and / or an oxide of method with a metal component selected from the group consisting of Group IA, IB, HA, IIB, IIIA, IIIB, IVA, IVB, VA, the series of rare earths and mixtures thereof.

19. A method in accordance with the claim 15, 16, 17 or 18 further characterized in that said oxidation, deposition and heat treatment are carried out if a solid line.

A method according to any of claims 15-19 further characterized in that it comprises: applying additional soot to the consolidated mass to form another porous mass: heat treating the other porous mass under effective conditions to convert it into a second consolidated mass; and treating the second layer under effective conditions to produce an optical waveguide component.

21. A method according to claim 15 further characterized in that the consolidated mass is used to form an optical fiber by means of a process that also comprises: heating the consolidated mass and extracting an optical fiber from the hot consolidated mass . SUMMARY OF THE DESCRIPTION The present invention is directed to? Na composition polyalkylsiloxane purified having? N boiling point, under atmospheric conditions, of less than 250 ° C and containing impurities of high boiling, including siloxanes and siloxanes silanol terminated, having points boiling, under atheropherical conditions, greater than 250 ° C in a total concentration of less than 14 ppm. The present invention is also directed to a method for producing a composition polyalkylsiloxane purified, q? E has a boiling point, under atmospheric condicionee, less than 250 ° C, distilling a starting material polyalkylsiloxane containing impurities of higher boiling , of more than 250 ° C in a total concentration of at least 14 ppm, under conditions effective to produce a purified polyalkylsiloxane composition which has a boiling point under atmospheric conditions of less than 250 ° C and which contains impurities of high boiling which have a boiling point, under atmospheric conditions, of more than 250 ° C in a total concentration of less than 14 ppm. In the preferred embodiments, the low boiling components (including silanols and preferably also hexamethylcyclotrisiloxane) are reduced to less than 100 ppm. The present invention is further directed to a method for producing fused silica glass by conversion of the purified polyalkylsiloxane composition, RB P96 / 420