DE2305548B2 - High strength polycrystalline refractory oxide fiber and method of making the same - Google Patents

Description

The present invention relates to high strength refractory fibers and methods of making them the same.

Polycrystalline, heat-resistant oxide fibers, especially made of aluminum oxide, of great length are in great demand because they have a combination of high theoretical tensile strength and high modulus as well as chemical indifference at room temperature and largely retain ^{these properties at temperatures above 1000 ° C.} Many attempts have been made to produce such fibers, as disclosed in US Pat. Nos. 3,311,689 and 3,327,865. However, the fibers known up to now only had a small fraction of the theoretical strength.

In publication AD 6 49 537 of U.S. Department of Commerce is suggested to have one Polycrystalline oxide core and a glass jacket to be extruded at the same time. Using this method, they were able to small amounts of fibers of increased tensile strength but low modulus can be produced.

However, there is a need for refractory fibers that have both high tensile strength and have a high modulus.

The invention relates to fibers of high strength, consisting μιη of a polycrystalline refractory Oxidfaserkern having a fiber diameter of 3 to 250, consisting of grains having a mean grain diameter of (1) is less than S and further (2) less than 10% of the Faserkerndurehmessers is composed, and a vitrified coating adhering to the core of a glass-forming compound containing a glass-forming oxide in the form of an optically uniform layer with an apparent thickness of (1) less than 1 μm and (2) less than 5% of the fiber core diameter.

The invention also encompasses a process for making these high strength fibers by Application of a coating agent from molecular to colloidal particles of a glass-forming oxide or Oxide former on the surface of the above-described polycrystalline, heat-resistant oxide fiber in sufficient Amounts to (after subsequent heating) a glazed layer with an apparent To obtain thickness of less than 1 μm and less than 5% of the fiber diameter, and then Heating the coated fiber to a temperature sufficient to cause the To glaze the coating into an adherent, optically even layer.

To further explain the invention, reference is made to the drawings.

F i g. 1 illustrates a fiber coated in accordance with the invention.

F i g. Figure 2 shows a series of photomicrographs of polycrystalline refractory fibers before and after coating.

F i g. 3 shows the process stages of the treatment

of polycrystalline, heat-resistant oxide fibers with a liquid coating agent and the vitrification of the fibers treated in this way.

Both the fibers according to the invention and the fibers used as carriers are referred to as "fibers" the term refers to a shaped product, the length of which is greater than its diameter (e.g. with a ratio of length to Diameters greater than 4: 1, preferably greater than 10: 1). This includes both continuous threads and in Staple fibers, all of which are preferably substantially circular in cross-section.

The term "polycrystalline" means that the fiber does not consist of a single crystal, but of numerous refractory oxide crystals. r> The term "heat-resistant oxide" refers to oxides that melt at a temperature of ^{at least 1000 ° C.} Such refractory oxides are e.g. B.

AI ₂ O ₃ , MgO, ThO ₂ , ZrO ₂ , Cr ₂ Oi, Fe ₂ O ₃ , NiO,
CoO, Ce ₂ O ₃ , UO ₂ , BeO, HfO ₂ , TiO ₂ , La ₂ Oj x \

as well as oxide mixtures, such as

3 Al ₂ O ₃ - 2 SiO ₂₁ Al ₂ O ₃ AIPO ₄₁ AI ₂ O ₃ MgO,
ZrO ₂ · SsO ₂₁ ZrO ₂ + CaO and ZrO ₂ + MgO.

The preferred fiber consists of at least 60 percent by weight of a single, simple, heat-resistant fiber permanent oxide, especially from aluminum oxide. The remaining 0 to 40 wt% is other refractory Oxides that acidify as separate phases, as part of a compound or as a solid solution i: ...

heat-resistant oxide may be present. Other oxides that are not to be regarded as heat-resistant, such as B ₂ O ₃ , P ₂ Oa, As ₂ O ₃ , TeO ₂ and SiO ₂ . may be contained in the fiber in such amounts that they do not reduce the melting point of the finished fiber at 100O ⁰ C. Aluminum oxide fibers which contain one or more oxides of cobalt, magnesium, lanthanum, nickel, copper or cadmium in amounts of only up to 5% by weight are particularly preferred.

The fibers consist of grains with an average grain diameter of less than 3 μm and less than m 10% of the fiber diameter. Grains falling under this definition give the fiber a high grade internal strength that is necessary to obtain the maximum strength advantage from the fibers.

The porosity of the fibers is preferably below 4 " 20%, especially below 10%. Such low porosity fibers are recommended for use in accordance with FIG Invention preferred because it can be obtained by coating according to the invention to a higher absolute Solidify strength than fibers of higher> o Porosity. Fibers that give higher absolute strengths apparently have a relatively lower one Number of internal defects (pores and weak grain boundaries) on what can be seen from the extent of the Grain splitting results in breakage. In contrast, fibers of lower strength suffer upon breakage Cleavage along grain boundaries (suggesting the presence of more pores and more weak grain boundaries suggests).

A preferred fiber for the purposes of the invention has a porosity of less than 10%, a Degree of crystallinity of more than 85 percent by weight and such a particle size distribution that practical there are no grains larger than 3 μΐπ and at least 30 percent by weight of the grains are smaller than 0.5 μm.

The fibers preferably have a diameter between 3 and 250 microns and, wei ¹ then can be produced easier μπι range 5 to 125. Fibers with diameters between 6 and 50 μm are particularly preferred.

A "glazed coating" is used here to refer to a coating that is heated for melting or sintering and then cooled to a vitreous coating.

The vitrified coating can be made from a coating agent which is the oxide itself Oxide formers or intermediates and modifiers as well as their starting materials.

"Glass-forming oxides" are those oxides or combinations of oxides that are used in Cooling from the liquid state to form a glass. Suitable oxides are those that stand on their own Forming glasses, d. H. "Glass formers" (especially the oxides of silicon, boron, germanium and phosphorus), and Combinations of one or more "glass formers" with one or more oxides, which are used as "Glass converter" are called. The oxides should preferably have a melting point above 800 ° C.

Suitable »glass converters« are titanium dioxide, zinc oxide, lead oxide (PbO) and beryllium oxide, as well as lithium oxide, Magnesium oxide, calcium oxide, cadmium oxide, barium oxide and strontium oxide.

As is known per se, belong to the oxide formers (i.e. the starting materials that are converted into the oxides be) z. B. silicon tetrachloride (pure or partial hydrolyzed) and organic silicon compounds that can be converted into silicon dioxide, such as Organopolysiloxanes. The glass-forming oxides can be represented by the following repeating unit will:

-O - Si - O - Si

Herein R means an organic group, e.g. B. an aliphatic or aromatic group (preferably with up to 6 or 9 carbon atoms) which is bonded to the silicon atom via a carbon atom, and A can represent hydrogen, a group R or an oxygen atom bonded to another silicon atom. R is preferably the methyl group, and compounds in which the radicals R and A are all methyl groups are particularly preferred (dimethylsiloxane).

Other suitable oxide formers are boron trichloride, boron tribromide, phosphorus trichloride, phosphorus oxychloride, Germanium tetrachloride and similar arsenic compounds.

Coating agents which consist of at least 50% silicon dioxide formers are preferred, with it it is particularly advantageous to produce a vitrified coating consisting practically entirely of silicon dioxide.

The coating agent can be in solid, liquid or vapor form and should consist of molecular to colloidal particles. Since the majority of the suitable oxides and any glass converters that may still be added are insoluble in water, they can easily be '' V. Form of colloidal particles of the oxide itself occurred from an aqueous dispersion. So form z. B. silicon dioxide, aluminum oxide, titanium dioxide, tin (IV) oxide, germanium dioxide, zirconium dioxide, magnesium oxide and

Lead oxide (PbO) under suitable conditions (e.g. concentration, temperature and particle size) is proportionate stable dispersions.

Oxide formers are generally easier to apply to the fibers than liquids or vapors. This method is particularly preferred because bundles (e.g. a cable of closely spaced Continuous filaments) of uncoated fibers can be coated in this way without the fibers after stick together after coating or baking. The fact that the fibers in this case are not stick together is surprising because methods of coating with colloidal dispersions in the generally only suitable for monofilaments (i.e. the fibers must usually be kept separate from one another so that they don't stick together). A suitable method for coating fibers with a water-reactive silicon dioxide former, such as silicon tetrachloride, consists in that on the Fibers initially have an atmosphere with a relative humidity of more than 50% (e.g. water vapor) can act. The wet fibers are then through the liquid or vaporous silicon tetrachloride passed, which with the water on the fiber surface to form an even layer hydrated silica reacts.

The preferred method of coating the fibers is by painting the uncoated fibers immediately after firing through a solution or dispersion of the oxide former.

The coating agent can be applied to individual staple fibers or continuous filaments or to groups of the same (e.g. Yarns or rovings) are applied.

It is preferable to apply relatively small amounts of the coating agent to the fiber, which is just the case sufficient to obtain a thin, even coating.

To coat a multifilament yarn with an organopolysiloxane, one is preferably used Solution containing a silicone in concentrations of 0.2 to 40 percent by volume. The is particularly preferred Use a concentration of about 10 volume percent or less of a silicone oil lower than that Viscosity (about 5 to 50 cSt at 25 ° C). One can use conventional solvents or diluents such as Methyl chloroform and carbon tetrachloride, but also other compatible solvents or diluents apply that is slightly different from the coated fiber let it evaporate.

The silicone coating agent can be applied to staple fibers or continuous filaments, which may be present individually or in groups (as yarns or rovings), by passing them through a bath, passing over a surface wetted with the coating agent, spraying or in some other way. Preferably the fiber is immersed in a bath of the silicone-containing liquid.

Excess coating agent can be removed with the help of wipers, blow-off jets or a simple air flow etc. or by evaporation with subsequent heating to convert the coating into a adhering silicon dioxide coating can be removed.

Any solvent or diluent contained in the coating agent can evaporate on the Air or in the air stream and / or by heating.

Those applied using the procedures outlined above Oxides or oxide formers are through

Heating the fiber to a temperature above the melting or sintering temperature (for S1O2, for example, more than UOO ⁰ C) vitrified to form an adherent, optically uniform coating. The term "vitrify" means that the conditions under which the coated fiber is heated must be sufficient for the oxide to sinter or melt. The temperature and duration sufficient for vitrification of a given coating agent can be selected from fairly wide ranges, the shorter times being used at higher temperatures. Amorphous silica can e.g. B. be vitrified through many hours of heating at about 1100 ⁰ C, by 30 seconds heating to about 1350 ⁰ C for 5 seconds, or by heating to about 1500 ° C. Similar areas can also be determined for the glazing of other oxides. A preferred method of heating a fiber coated with silicon dioxide is that the coated fiber is passed through the flame of a propane air blower (the temperature of which is generally 1500 to 1900 ^° C., depending on the propane to air ratio), the residence time in the flame is about 0.1 to 5.0 seconds.

When heated, the coating agent is vitrified, the oxide formers are in the appropriate Oxides are converted, and the water of hydration (if it is a matter of hydrated substances) is under Driven off formation of a practically anhydrous coating. In addition, any carrier liquids that may be present evaporated, and some volatile oxide former may also evaporate.

If the heating process step is omitted, the coating composition is not vitrified and fibers are obtained whose tensile strength is not higher than that of uncoated control samples. Even if the coating material consists of vitrified particles, glazing in place is still necessary to achieve the required adhesion of the coating (described below).

The heating can be carried out with various combinations of heating times and temperatures (as described above); however, it has been found that excessively long heating times and excessively high heating temperatures lead to a decrease in strength. This loss of strength can be due to evaporation of part of the coating or to the diffusion of the coating into the carrier fiber.

A thinner coating layer (apparent thickness less than 1 μm, preferably less than 0.1 μm), which is also less than about 5% of the fiber diameter, leads to considerable improvements in the tensile strength properties. Since it is difficult to directly determine the thickness of these thin layers on the small diameter carrier fibers, the apparent thickness can be calculated from the amount of coating material on a large group of fibers and the density of the coating material, as described below

For a number of coatings on the same carrier fibers it was observed that there is a substantial increase in the tensile strength of the coated fibers compared to the uncoated carrier fibers when the coating is at least 0.01 µm thick and that the tensile strength increases with the thickness of the coating up to a thickness of 0.1 μπι increases

however, a further increase in thickness decreases.

The preferred products according to the invention have a tensile strength of at least 7,000 and preferably of at least 14,000 kg / cm ² . Preferred embodiments of the invention are further characterized by a flexural modulus of elasticity of at least 2,800,000 kg / cm ² .

It is believed that the coating "heals" small imperfections in the fiber surface, and it is these parts of the Fiber surface that must be coated. The type of coating required is determined by the thin, ensures an optically uniform layer, which is described below. It can also be assumed that the Surface defects are at least partly due to imperfect alignment of the grains in the polycrystalline fibers are due to the fiber surface, which leads to the formation of unevenness. Therefore, it is believed that the apparent coating thickness required to cure these defects is the Half of the mean grain diameter or less should be in order to completely remove the unevenness or to be filled in partially. Preferably the apparent coating thickness is less than half the average Grain diameter. Trying to apply larger amounts of coating will result at least two difficulties: first, there is chipping; the coating reaches one Thickness that it is no longer optically uniform, which is partly due to the fact that it absorbs the tensions (e.g. due to different thermal expansion) can no longer withstand, and breaks off from the core. Secondly takes the modulus of the heat-resistant oxide carrier fiber, which should be as high as possible, in the case of one relatively thick coating (beyond the "zone of chipping"), because the coating itself has a has a lower module (the more the volume or the thickness of a relatively low module increasing coating, the more the modulus of the coated fiber decreases). Hence the Identification of the apparent coating thickness as "less than 1 μm" and as "less than about 5% of the Fiber diameter «important; for the former value ensures the required uniformity of the Coating to achieve high tensile strength, while the latter value is the volume of the coating <r> limited upwards to the maximum that is necessary to completely remove the bumps to be filled (because the mean grain size is less than 10% of the fiber diameter), and further ensures that the coated fiber maintains the desired high modulus (from this upper Limit results in a maximum coating volume of 17.4O / o).

The coating must adhere to the carrier fiber, i.e. H. it must be visually uniform after following the cleaning treatments described below The on-site glazing (described above) leads to the formation of the desired adhesion. It can be assumed that the coating layer is formed by an interface (a «1 Product of a reaction between the coating and the carrier fiber) is bound to the carrier fiber. the However, the interface is difficult to detect because she is so small

F i g. 1 explains a coated fiber 1 according to FIG. B5 Invention. The coating 2 is in the form of an optically uniform layer of the desired thickness, which adhered to the surface of the carrier fiber 3 The coating 2 does not have a constant thickness (it can, however have a constant thickness), and the fiber may under certain circumstances have small uncoated surfaces on its surface Parts 4 have, which are generally in raised places (usually on protruding grains) can appear. The section through the uncoated part of the fiber shows the grains 5, from which the Fiber is made.

F i g. 3 illustrates a method of practicing the invention. A multi-filament yarn 10 made from heat-resistant Oxide fibers are held by a tension coupling 12 coil 11 by a force fit driven roller 13 withdrawn via guide rollers 14 and 14 'and to an annular burner 15, through a chimney 16, over a guide roller 14 ″ and one in a bath of a coating agent 18 rotating finishing roller 17 past the air jet nozzles 19, which blow off excess liquid, underneath the guide 20 through a flame 21 and from there via tensioning devices 22 and 22 'to a winding device 23 funded

The high strength (i.e., high tensile strength and high modulus) fibers of the invention are particularly suitable as reinforcing agents for plastics, metals, ceramics and other products. These fibers can be used in various end uses instead of uncoated, heat resistant Fibers are used especially when it comes to high tensile strength and high modulus arrives, e.g. B. for thread-wound radar housings and sonar device housings, high-temperature jet motor blades and support structures as well as for struts in aircraft frames.

Measurement method a

A substantially straight, 2.5 cm long sample of fiber is placed in a liquid which is the same Has refractive index like the carrier fiber (e.g. 1.760 for a-aluminum oxide). The sample will be in linear more polarized white light using a microscope with a reasonably high numerical aperture (Na about 0.85) and a magnification of 600x. The fiber will be in such a location brought so that its longitudinal axis is essentially parallel to the linearly polarized light The interface between fiber and liquid is adjusted so that the best resolution is obtained. if a line is observed which is substantially parallel to the fiber axis and has the same extent like the fiber-liquid interface, this means that the fiber has a coating if the observed line is essentially uninterrupted so it means that the coating becomes one Extent "optically uniform" is as necessary for the results according to the invention regardless of the fact that small portions of the fiber surface may be uncoated and / or that the coating may not have a constant thickness Examination according to this method shows no optically uniform coating, it should according to the method b to be examined.

Measurement method b

A single fiber is placed on a microscope slide attached and immersed in a liquid that has the same refractive index as the carrier fiber. The fiber and the liquid come with one

Cover slip covered. The fiber is magnified 100 times (objective magnifying 10 times and 1X magnifying eyepiece) in oil immersion (cedar oil between the cover slip and the objective to improve resolution) in a phase contrast microscope considered.

The phase contrast microscope converts the optical path difference, which is the product of the thickness and the Fluctuation in the refractive index is converted into an intensity difference in black and white, which is considered by the eye Contrast is perceived in the image. Because the immersion liquid has the same refractive index as the carrier fiber, the image is based on fluctuations in the refractive index of those exhibiting the contrast Place.

The fiber is examined for phase contrast along its length, and if any is selected Area in the entire field of view (about 0.1 mm at 1000x magnification) is a continuous one Exhibits phase contrast along both edges, the fiber has an optically uniform coating. These Method b is more sensitive and precise than method a and is preferably used.

The composition of the coating is determined by dissolving the coating material and the Solution analyzed for the various elements using conventional chemical methods.

The amount of coating material present on the carrier fiber is determined by removing the coating from a fiber sample weighing 0.5 to 1.0 g with a suitable etchant which dissolves the coating without attacking the carrier fiber. B. has been shown to be a satisfactory etchant for silica coated alumina fibers. When the etching is complete, the excess etchant is driven off by heating the sample to 900 ⁰ C. This etching process is repeated until after two consecutive treatments there is no longer any difference in weight. A weight correction equivalent is applied to the observed weight loss to compensate for any weight change observed when the uncoated fibers were treated with the etchant.

As an example of another method, the fibers of Examples 2 and 5 are mixed with sodium carbonate melted, the melt is dissolved in hydrochloric acid and the solution diluted to a known volume Silicon concentration in the solution is determined with the atomic absorption spectrophotometer and that Weight of the coating calculated; see "Analytical Methods for Atomic Absorption Spectrophotometry", published by Perkin-Elmer, Norwalk, Conn, 1971.

Other methods that have the required sensitivity and accuracy are well known to those skilled in the art known

The apparent coating thickness Ü (in μΐη) for a fiber with a round cross-section is calculated from the amount of the upper covering material per m ^{2 of} carrier fiber surface and the density (de) of the upper covering material in g / cm ³ according to the following equation:

Ü = g / m ² x -
d

The density (d _c ) is determined in a conventional manner (the value 2.19 g / cm ^{3 is} used for silicon dioxide).

The size g / m ² is obtained from the equation

DW 4V

calculated. The diameter of the carrier fiber (D), expressed in meters, is marked with a with a

ίο Measured with a thread micrometer eyepiece equipped microscope. V means the volume (in m ³ ) of the carrier fiber sample and W the weight (: ng) of the coating.
The apparent coating thickness of non-circular fibers can be calculated in an analogous manner from photomicrographs of the coated fiber or the carrier fiber for the purpose of determining the dimensions of the carrier fiber.
The glazed nature of the cover is

proved by examining the solubility of the coating in a liquid that is used as a solvent is known for the coating material in unglazed form. If the cover is glazed, it stays under the conditions under which the unglazed material would go into solution, essentially unaffected (determined according to the optical method described above). When the coating material is e.g. B. consists of silicon dioxide, unglazed silicon dioxide is made by immersing the fiber in 20 percent aqueous sodium hydroxide solution removed at room temperature. Remains within the same period of time vitrified silica is virtually unaffected by this reagent.

The adherence of the coating on the carrier fiber is detected by a fiber having a substantially uniform coating (what is detected after the above-described optical method) of 10 minutes at 50 ⁰ C with gentle agitation by hand with a O.lprozentigen aqueous solution then the fibers are rinsed and dried and again examined by the same optical methods to determine whether the coating is still present

The porosity of the starting fiber is determined according to DE-OS 19 13 663. The percentage of crystallinity the output fiber can be determined according to the same laid-open specification. The grain size and the grain size distribution are also according to

so the method described in the aforementioned laid-open specification determined with the difference that the Size distribution E values as cumulative percentage values depending on the average grain diameter on a probability paper with a logarithmic scale and a Probability scales are plotted, the latter of which is based on the error rate The best fitting straight line is drawn through the data points between 10 and 98%. The dem The average grain diameter corresponding to the cumulative value of 50% on this line is the mean grain diameter. A typical coated alumina fiber according to the examples below has an average grain diameter of 0.43 μπτ.

The tensile strengths are determined under room conditions using the method of R. D. Schile and co-workers in "Review of Scientific Instruments", Volume 38, No. 8, August 1967, pages 1103-1104, determines the caliber

length is 0.1 cm and the sliding jaw speed 0.025 to 0.10 mm / min.

The modulus of elasticity (the flexural modulus of elasticity) is determined by the vibroscope method according to the »Journal of Applied Physics ", Vol. 26, No. 7, July 1955, pages 786 and 792.

A preferred method, which is described in DE-OS 19 13 663, makes of a two-phase Spin mix use which contains small particles of a refractory oxide, such as alumina, zirconium in dioxide, etc., in an aqueous solution of a salt, which when heated into a heat-resistant oxide passes over (and is therefore referred to as an oxide former). The fibers of Examples 1 to 4 are made according to this Method made.

US Pat. No. 3,322,865 describes the extrusion of viscous aqueous solutions Metal salt mixtures and the subsequent firing to heat-resistant oxide fibers. According to this method the starting fibers of Example 5 are produced.

Suitable fine particles of -alumina (used in Examples 1 to 4) are prepared by Classification of an aqueous dispersion adjusted to a pH of about 4.0 with hydrochloric acid, which contains 20 percent by weight of finely divided aluminum oxide. All particles are settled with sizes over 2 μιη removed. The dispersion will be on an alumina content of 40 to 70 percent by weight concentrated by the method of G. A. Loomis (Journal of the American Ceramics Society, Volume 21, 1938, page 393) it is determined that about 100% of the Particles in a typical classified product have an equivalent spherical diameter of less than 2 μm and 89% have a diameter of less than 0.5 μm.

Examples

In the examples below and in the description above, part and number refer to Percentages, unless otherwise specified, are based on weight

The alumina fibers used as a carrier in the examples have a degree of crystallinity from 85 to 100%.

Experience with many fibers has shown that the calcined alumina fibers used in the examples average grain diameter due to the composition of the spinning masses and the firing conditions of less than 3 μm and less than 10% of the fiber diameter and a degree of crystallinity of have at least 85%.

example 1

This example illustrates fibers coated according to the invention and the influence of the concentration of Coating.

In this example, a polycrystalline, heat-resistant oxide carrier fiber (! 00% aluminum oxide) with a tensile strength of 138 10 ⁷ g / cm ² , a modulus of elasticity of 3.5 χ 10 ⁹ g / cm ² , a porosity of less than 5 %, a degree of crystallinity of more than 85%, an average grain diameter of about 0.5 μm and a diameter of 20 μm.

The fibers are heated to 1500 ° C for 2 minutes, then placed in a 5 cm wide glass tube and once or several times according to the following procedure treated: (1) The fibers are exposed to a humid atmosphere for 5 minutes by bubbling nitrogen at a rate of 0.163 mVh through a water bath of 50 ° C is generated; (2) the fibers are exposed to a silicon tetrachloride vapor atmosphere for 5 minutes exposed by passing nitrogen over the surface at a speed of 0.168 mVh is generated by liquid silicon tetrachloride at room temperature. To increase the evaporation rate increase, the liquid is stirred. The coated fiber sample is distilled for 30 minutes Washed with water and then dried at 150 ° C.

The fiber is then placed in the tube furnace for 12 seconds heated to 1500 ° C to vitrify the coating. the Analysis values and tensile strengths of representative fiber samples are summarized in Table I.

Table I. tensile strenght Elasticity Amount of Apparent Coating thickness number of module SiO ₂ % of the fiber Procedures diameter (g / cm ² ) (g / cm ² ) (g / m ² ) Ü ( _t im) (μπι) 1.38XlO ⁷ 3.5X10 ' 0 0 0 0 1.84XlO ⁷ 3,5XiO ⁹ 0.05 0.025 0.1 1 2.15X10 ⁷ - 0.19 0.1 0.5 4th 2.17XlO ⁷ - 0.20 0.1 0.5 7th l, 85X10 ⁷ - 0.71 0.3 1.5 10 1.47XlO ⁷ - 0.80 0.4 2.0 13th

All coated fibers (with the exception of the fiber coated in thirteen operations) are characteristic of the invention; the coatings are glazed, optically uniform layers of the required apparent thickness adhering to the alumina carrier fiber. The above values show that the coating according to the invention increases the tensile strength of the uncoated fiber at very high low coating thickness (0.025 μm) improves that one but in the case of coating thicknesses of more than 0.1 μm, none further improvement achieved. When the coating thickness reaches 0.3 µm, about 10% of the fibers do not have the

uniformity of the coating required according to the invention. However, with those fibers that are coated in accordance with the invention, substantial improvements are achieved, as the table above shows. With this coating amount, the apparent coating thickness of 03 μm is somewhat less than the mean grain diameter of the fired coated fibers in this example. (A fiber produced with four process steps has an average grain diameter of 0.5 μm). At approximately this thickness, the splintering zone is reached for the first time, which results from the fact that 10% of the fibers are unevenly coated. The fiber with the 0.4 µm thick outer layer is an example of a particular fiber of the neck which does not have the uniformity of the outer layer as the products according to the invention. The tensile strength of this fiber is significantly lower than that of the other coated fibers.

This influence of the amount of coating on the uniformity of the coating is also illustrated by FIG Fi g. Fig. 2 shows a series of photomicrographs according to method (a) for Examination of the coating for irregularities. F i g. 2-A is a photomicrograph of an uncoated sample; this sample does not show any lines parallel to the fiber axis which are the same Like the interface between fiber and oil, and the presence of a Would indicate coating. Such lines are shown in FIG. See 2-B, 2-C, and 2-D (the photomicrograph Recordings of those obtained after one process, after four or after seven processes Represent fiber samples); these lines are practically unbroken lines, from which it follows that the Coating has the required optical uniformity. Fig. 2-E is a photomicrograph Recording of two different types of fiber samples from the same experiment described above, the is carried out with ten procedural steps. The fiber in the upper part of the figure is satisfactory and is similar to the fibers of Figures 2-B, 2-C, and 2-D; however, the fiber is in the lower part of FIG. 2-E not satisfactory, since the observed line has clear interruptions. These interruptions are flaws in the coating (e.g. places where the coating has started to splinter or fall off Has).

Example 2

This example illustrates other types of silica formers as a coating material.

In this example, aluminum oxide milling cutters are used with a tensile strength of 1.40 10 ⁷ g / cm ² , a module of 3.5 10 ⁹ g / cm ² , a porosity of less than 5%, an average grain diameter of 0.5 μπι and a diameter of IW μπι.

Individual fibers are heated for 2 to 4 seconds in the flame of a propane air blower, then immersed in an aqueous waterglass solution at 40 ° Be (Na ₂ O: SiO ₂ = 1: 3.25, which theoretically amounts to 76 percent by weight SiO ₂ and 24 percent by weight Na ₂ O), and heated in a propane air flame for 1 second to vitrify the coating. The tensile strength of a representative coated fiber sample is 2.02 10 ⁷ g / cm ² and the apparent thickness is 0.55 μm. The coating is optically even.

The process for coating the fiber is repeated with an aqueous solution of guanidinium silicate (prepared according to Example 1 of US Pat. No. 3,475,375 _{) and lithium chloride (ratio Li 2 OiSiO 2} = 1:19), which theoretically contains 95 percent by weight SiO _{2 after firing} and 5 percent by weight Li ₂ O yields After heating the coated fibers in an air-propane flame for a period of 1 Secunda purpose glazing of the coating shows a representative sample has a tensile strength of 2.14 χ 10 ⁷ g / cm ^2. The coating is optically even.

Example 3

This example illustrates a continuous process for making the coated fibers according to FIG Invention.

The starting fibers consist of 99.5% Al ₂ Oi which contains less than 0.5% MgO and have an average tensile strength of 1.60 χ 10 ⁷ g / cm ² . The mean grain diameter is 0.47 μm, the porosity is less than 10% and the fiber diameter is 20.4 μm.

Coating and firing the fibers

The fibers (in the form of filaments) are continuously coated and burned by using them at a speed of 4.55 m / min through the following zones: (1) the approximately 3.8 cm long flame zone of a Meker burner (methane air); (2) a 5 cm long water vapor zone; (3) a 3 '' long bath of liquid silicon tetrachloride and

jo (4) the flame zone of a surface burner with a diameter of 3.8 cm, which is fed through a stainless steel wire netting with 20 meshes per cm with a mixture of propane and air, with a contact time of 0.5 seconds and one with an optical pyrometer determined apparent fiber temperature of 1060 ⁰ C. No correction is made for the emissivity, so that the actual temperature (depending on the type of fiber and the coating agent) is about 400 to 600 ⁰ C higher than

4 (i the displayed temperature.

According to the invention, the fibers obtained in this way are coated with silicon dioxide and have an average tensile strength of 1.85 × 10 ⁷ g / cm ² . The coating density is about 0.2 g / cm ² , which is one

■ π apparent coating thickness of about 0.1 μπι corresponds. The coating is optically even.

Example 4

This example explains a continuous process

> i> ren and the influence of the coating thickness.

The carrier fiber consists of aluminum oxide (which contains 0.2% MgO) and has an average diameter of 22 μ, an average coriid diameter of 0.5 μm, a porosity of less than 10% and a tensile strength of 1.49 χ 10 ⁷ g / cm ² and is in the form of a thread spool (735 denier) made of 60 continuous threads.

For coating, the spool of thread is stored horizontally on a spindle and the thread is exposed under one rotating polytetrafluoroethylene disc in one

«I immerse a 20 cm long bath in the coating agent, pulled through and then through five nitrogen nozzles (which are set so that the majority of the in the Bath absorbed solvent and diluent evaporates without breaking the threads in the yarn)

hi and then spaced 6.4mm across a surface burner 3.8 cm in diameter threaded through a stainless steel wire mesh with 20 meshes per cm with a mixture of gas and air

is fed, carried away and finally with a Wound onto a spool at a speed of 4.55 m / min. In some cases outlined in Table III by If several temperature values are marked, the yarn will be at the same without going through a bath Speed heated again.

The various listed in Table II are used Silicone oils used in the form of solutions in methyl chloroform or in undiluted form:

Table Il

Bc / ciihn ii η u

Viscositul. cSl at 25 ° C

Λ
H
C "
D.

100

2000

500

A, B and C are dimethylpolysiloxane, D is one Chlorophenyl methyl silicone fluid.

Table III gives the bath compositions (as percent by volume in methyl chloroform) and temperatures to which the yarn is heated (determined with an optical pyrometer). For the emissivity will no correction attached, so the actual temperature will vary depending on the type of fiber and the Coating agent is about 400 to 600 ° C higher than the indicated temperature. Also in Table III are the Tensile strength of the coated yarns after all specified treatments as well as the apparent Called the coating thickness of the coated yarns.

When examined by method b, samples a to e show an optically uniform coating. Sample f has an uneven coating, which is apparently due to the excessive amount of coating. The poor results obtained with sample f are likely to be ascribed to the relatively large uptake of viscous silicone fluid by the yarn bundle. If a single thread of the yarn is dipped in the same liquid, then allowed to run off and fired, a tensile strength of 2.3 × 10 ⁷ g / cm ^{2 is obtained} .

Table III

(iiiriUcnip.

0.25% Λ

3% Λ

10% Λ

20% Λ

40% Λ

100% Λ

3% B

3% C

.1% D

1085, 1100 1105, 1110 1165, 1140 HW. 1210, 1180 1070.1100 1085

tensile strenght

(ü / envxKI I

. Apparent (lbcmigstlieke

0 (VJIl)

1.97

2.04

2.04

2.11

2.11

0.915

1.76

2.04

2.25

0.015

0.035

0.05

0.09

0.2

0.7

0.03

0.04

0.025

Example 5

The carrier fibers consist of aluminum oxide (which contains 1.2% MgO) and have a diameter of 25 μm, an average grain diameter of 0.64 μm, a tensile strength of 0.98 χ 10 ⁷ g / cm ^: and a porosity of less than 10%.

The individual fibers are immersed three times in a dispersion of colloidal silica and then heated in a propane air flanime for 2 seconds. The fibers have an optically uniform coating. The fibers have an apparent coating thickness of 0.06 μm and a tensile strength of 1.82 χ 10 ⁷ g / cm-.

For this purpose 3 sheets

Claims (15)

Patent claims: 1. Polycrystalline, heat-resistant oxide fiber of high strength with a diameter between 3 · -, and 250 μηι, consisting of grains with a mean grain diameter of less than 3 μm and less than 10% of the fiber diameter, characterized by a vitrified cover made of a glass-forming oxide containing n> the glass-forming mass in the form of an optically adhering to the heat-resistant oxide fiber uniform layer with an apparent thickness of less than 1 μm and less than 5% of the Diameter of the heat-resistant oxide fiber. ι - ■ 2. Fiber according to claim 1, characterized in that the glazed coating to at least 50 Weight percent consists of S1O2. 3. Fiber according to claim 2, characterized in that the apparent thickness of the layer 0.01 to jii 0.1 μΐη. 4. Fiber according to claim 1, characterized in that the apparent thickness of the layer is less than half the mean grain diameter. 2> 5. Fiber according to claim 4, characterized in that the apparent thickness of the layer 0.01 to 1 μπι is. 6. Fiber according to claim 1, characterized in that the glazed coating is practically complete consists of S1O2. 7. Fiber according to claim 6, characterized in that the apparent thickness of the layer 0.01 to 1 μηι is. 8. Process for the production of fibers of high strength by applying a coating on the surface of polycrystalline, heat-resistant oxide fibers with a diameter between 3 and 250μ made from grains with a mean diameter of (i) less than 3μ and (ii) less in exist as 10% of the fiber diameter, characterized in that a coating agent is made molecular to colloidal particles of a glass-forming oxide or oxide-forming agent in sufficient quantities Applies quantities to a layer with a <r> apparent thickness of less than 1 μπι and to produce less than 5% of the fiber diameter, and the fibers coated in this way for sufficient Heated for a period of time to a temperature sufficient to form an adherent, w to glaze optically uniform layer. 9. The method according to claim 8, characterized in that that a coating agent which contains a silicon dioxide former in liquid or applied in a vaporous state. -r> 10. The method according to claim 9, characterized in that the silicon dioxide former is silicon tetrachloride used. 11. The method according to claim 10, characterized in that the coated fibers less than 30 seconds to at least 1350 ° C heated 12. The method according to claim 11, characterized in that the coated fibers Heated for 0.1 to 5.0 seconds to temperatures between 1500 and 1900 ° C. 13. The method according to claim 8, characterized in that the coating agent used is an organi cal silicon compound is used. 14. The method according to claim 13, characterized in that the organic silicon compound an organopolysiloxane is used. 15. The method according to claim 14, characterized in that the organopolysiloxane is used Dimethylsiloxane used.