GB2094363A - Method of making silica yarn - Google Patents

Abstract

Leachable glass yarn is formed into a loose fabric to facilitate conversion thereof into silica yarn by acid leaching, washing and firing. A leachable glass yarn is knitted into a tubular fabric which is wound around a skeiner and severed to form skeins of the fabric. The skeins of fabric are tied and placed in porous bags which are in turn placed in a leaching tank. A heated acid solution is recirculated through the leaching tank to effect leaching of the skeins of fabric, following which water is circulated through the tank to wash the fabric. The skeins of leached and washed fabric are removed from the bags, dried in an oven, placed on a firing rack and fired in a furnace. Following firing, the skeins of fabric are cooled, immersed in a solution to form an abrasion resistant coating thereon, dried and then deknit into an essentially silica yarn which is wound onto a form.

Description

SPECIFICATION Method of making silica yarn It is well known to make silica yarn from a leachable glass yarn by leaching the glass yarn with acid to extract flux metals from the yarns, washing the leached yarn to remove residual acid and acid-metal salts and then firing the yarn at an elevated temperature to drive off water of hydration and allow at least some shrinkage to occur which is going to occur at the temperature of ultimate use. The resulting yarns which are essentially of silica composition are frequently coated with an abrasion resistant material to facilitate subsequent handling and to make the yarns suitable for typical end uses thereof.

A typical method used in making silica yarns involves a "pump leaching" process. A leachable glass yarn is wound onto a perforated hollow roll or mandrel to a desired thickness, with the yarn thereafter being leached by pumping acid through the perforations in the roll and then washed by pumping water through the perforations into the mass of yarn. The yarn is then unwound from the roll, advanced through a firing furnace and then dipped in a solution to form an abrasion resistant coating thereon.

Another typical example of a "pump leaching" process in which glass cloth is converted to silica cloth is provided by U.S. Patent 3,125,476 of Anderson, issued March 17, 1964, METHOD OF EXTRACTING GLASS FIBERS. In the Anderson patent leachable glass cloth is wrapped in several layers around a hollow foraminous roll or mandrel having 1 0-50% perforations in its surface area.

Leaching acid is then introduced in the hollow interior of the roll and forced through the perforations in the roll and through the layers of glass cloth from which the acid is recirculated to the hollow interior of the roll. Washing of the cloth is accomplished by circulating a wash water into the hollow interior of the roll and then through the perforations in the roll and through the glass cloth.

Such conventional processes of making silica material as described above suffer from a number of limitations which result in a final silica material of varying uniformity and. having some portions thereof which are unusable. During leaching of glass yarn, the yarn loses approximately 40% of its weight and shrinks approximately 34%. These changes do not appear to occur simultaneously.

Rather, the yarn loses roughly 1020% of its weight before it begins to shrink. This weight loss occurs during the first 5-1 5 minutes of the leach in an exothermic chemical reaction. During this stage the yarn becomes relatively soft and its strength relatively weak, and a 1 650 denier yarn by way of example breaks with a static load of 1-2 pounds. Later in the leach process, the same yarn regains enough strength so that after firing and coating it has a break strength of 2-5 pounds. Therefore, the controlling of both yarn tension and the exothermic chemical reaction during leaching are critical to the final product quality, uniformity and strength. In an ideal leaching system, yarn tension should be well below 1 pound and preferably zero.Such low tension would allow shrinkage to occur without filament breakage by reducing the shear and abrasion caused by one filament rubbing on another.

In the "pump leaching" process of making silica yarn, the yarns closest to the perforated roll are subjected to the greatest amount of tension as shrinkage occurs during leaching. The result is that those yarns in contact or relatively close to the perforated roll fracture or otherwise become unusable and must be discarded at the end of the process. In order to hold the yarn on the roll during the initial weight loss stage of leaching and to resist pump pressure from blowing the yarn off the roll, the yarn must be tightly wound around the perforated roll. A tight wrap keeps the yarn package intact. However, during the shrinkage phase of the leach process, the yarns closest to the roll are over stressed, filaments are broken, and up to 2 inch of the wrap thickness adjacent the roll must then be scrapped. A looser wrap minimizes the amount of core scrap, but contributes to slump.When the yarn package is wound loosely, pump pressure or gravity can cause yarns to shift, one on top of another. The most common form of shifting is called slump, which is a telescoping of the annular yarn package wound around the leach roll. Standard practice in "pump leaching" is to orientate the yarn packages in a vertical direction perpendicular to the ground.

When slump occurs, the base of the package is concave and the top of the package is convex, the yarn having shifted in the direction of gravity.

Because weight and volume losses occur prior to yarn shrinkage, perfect wrap tightness is impractical to maintain and some slump will occur.

In addition to low yarn tension, the leaching technique should provide maximum acid-yarn contact to minimize leach nonuniformities and reduce the chance of "hot spots" due to the exothermic reaction. Such "hot spots" produce weak, brittle material when later fired. The leach nonuniformities cause sections of the yarn to be under-leached with low silica content (normal silica levels are 98% or greater) and which break when fired at temperatures on the order of 1200-1 80O0F or higher. In typical "pump leaching" systems, the forcing of the leaching acid through the perforated roll causes greater exposure to the leaching acid of those portions of the yarn adjacent the perforations as contrasted with the portions of the yarn which do not reside over the perforations.The practical result is nonuniformities in silica content which may render the resulting silica yarn unsuitable or undesirable for many applications thereof. In addition, the "hot spots" mentioned above typically occur with unwanted yarn breakage and other undesirable problems.

While leaching determines breaking strength and high temperature durability of the yarn, washing after leaching is essential to produce a material free of residual acid and acid-metal salts.

Currently low acid salts such as chloride salts are required for many end uses of the silica yarn. For example, where the silica yarn is used in the manufacture of high temperature thermocouple covers and wire insulation, a low chloride content is essential to avoid corrosion of the thermocouples and wires. In an ideal washing system, the water-yarn contact should be maximized in order to minimize chloride levels and the levels of other residual acid and acid metal salts in the final yarn product. Yet in "pump leaching" processes, the washing tends to be nonuniform at the very least and in some cases simply inadequate for certain end uses of the silica yarn.This again results from the fact that the wash water is circulated through the perforations in the roll, tending to wash those portions of the yarn adjacent the perforations more thoroughly than other portions of the yarn which are not adjacent the perforations in the roll.

Processes for making silica yarn in accordance with the invention allow for the relatively uniform leaching of large quantities of leachable glass yarns under little or no tension, with little or no formation of "hot spots" and with little or no breakage of the yarns. Preferred processes in accordance with the invention further allow for washing of the leached yarns in such fashion that there is a relatively thorough and uniform removal of unwanted residual acid and acid-metal salts from the yarns. Following washing the yarns may be fired using a batch process which greatly facilitates the firing step.

In accordance with the invention leachable glass yarns are formed into a loose fabric to facilitate leaching, washing and firing thereof.

Preferably the fabric is first severed into lengths thereof The fabric can be immersed in a recirculating bath of acid solution to favour yarnacid contact and uniformity of leach and to disencourage the formation of unwanted "hot spots". Washing is suitably accomplished by circulating a wash water through the loose fabric immersed in the wash water in the container. This allows for relatively thorough and uniform removal of unwanted residual acid and metal salts from the yarn. The use of lengths of loose fabric facilitates a batch firing process, e.g. in which the lengths of washed fabric are placed loosely on firing trays which are then placed in a firing furnace and fired at a selected temperature for a selected period of time.

The loose fabric is preferably tubular. The loose fabric of glass yarn is preferably knitted, e.g. into a tubular sock. The glass yarn fabric may be wound around a skeiner and periodically severed to form the plurality of lengths or skeins of fabric; each skein is then tied and removed from the skeiner.

The fabric is preferably leached in a porous bag, usually being removed therefrom before firing.

Thus porous bags, each preferably containing one skein of fabric, may be placed in a leaching tank and a heated acid solution recirculated through the tank to effect leaching of the skeins of fabric.

The acid solution can then be removed from the tank, and washing water is circulated through the tank to wash the skeins of fabric. The washed fabric is preferably dried before firing; e.g. the skeins of fabric are removed from the bags and placed in an oven for drying thereof. Following drying, the fabric can be placed on firing racks which in turn are placed in a firing furnace. After firing the fabric is preferably allowed to cool at room temperature. The first fabric may be given an abrasion resistant coating, e.g. by immersion in a solution such as a "Teflon" (Trade Mark) solution.

The fabric, if knitted and after drying if necessary, is deknitted by pulling a loose end of yarn therefrom. The product yarn is best wound onto a form of desired shape as disassembly of the treated fabric occurs.

The invention is illustrated, by way of example only, by the following more particular description of a preferred embodiment, to be taken in conjunction with the accompanying drawings, in which: Fig. 1 is a block diagram of the successive steps in a preferred basic method of making silica yarns in accordance with the invention; Fig. 2 is a block diagram of the successive steps in a detailed example of the method of Fig. 1; Fig. 3 is a block diagram of the basic apparatus used in carrying out the method of Figs. 1 and 2; Fig. 4 is a simplified perspective view of apparatus used to form a plurality of leachable glass yarns into a knitted tubular fabric in accordance with the method of Figs. 1 and 2; Fig. 5 is a plan view of a portion of a knit fabric formed by the apparatus of Fig. 4; and Fig. 6 is an enlarged plan view of a portion of the fabric of Fig. 5 illustrating the manner in which the fabric is easily deknitted.

Fig. 1 depicts the successive steps in a preferred method of making silica yarns in accordance with the invention. In a first step 10 a leachable glass yarn is provided. The leachable glass yarn may be comprised of a borosilicate glass such as an E glass or any other appropriate type of leachable glass in yarn form. In a next step 12 the yarn is formed into a loose fabric. A preferred form of carrying out this step is by knitting the yarn, although other techniques could be used so long as the resulting fabric is a loose, porous one so as to optimize yarn-acid contact during leaching and yarn-water contact during washing.

In a next step 14 the loose fabric is leached. In accordance with the invention this is done in such a way that the fabric is under little or no tension and the yarn-acid contact is optimized. As described hereafter a preferred form of this step is to place lengths of the loose fabric in porous bags and submerge the bags in a recirculating heated acid solution. Alternatively, the loose fabric can be advanced under very low tension through a succession of acid baths at a speed selected to provide complete leaching, which technique readily lends itself to a continuous leaching process.

In a next step 1 6 of the method depicted in Fig. 1 the loose fabric is washed with an appropriate cieansing agent such as water to remove unwanted residual acid and acid-metal salts. In accordance with the invention this step is accomplished in such a way that the yarn-water contact is relatively thorough and uniform. In the case where the loose fabric is severed and placed in porous bags to carry out the leaching step, the washing step 1 6 is carried out by replacing the recirculating heated acid solution with water. A continuous supply of fresh water is provided so that the water may be circulated through the porous bags and the included lengths of loose fabric and then ultimately disposed of.Where the loose fabric is run through a succession of acid baths to effect leaching, the fabric is subsequently run through a succession of water baths to effect washing.

In a next step 1 8 of Fig. 1 the washed fabric is fired. The firing step 1 8 subjects the fabric to a temperature approaching the service temperature which is the temperature that the silica yarns must be capable of withstanding. Firing removes the chemically bound water (water of hydration) and eliminates most shrinkage at the service or end use temperature by causing most shrinkage which will occur at that temperature to occur during the firing step 18.

During a following step 20 of the method of Fig. 1, the fired fabric is immersed in a solution to apply an abrasion resistant coating thereto. Teflon or other suitable material can be used in the solution to provide the yarn of the fabric with an acceptable coating.

Following the application of the abrasion resistant coating of the step 20, in a following step 22 the loose fabric is disassembled into a yarn of essentially silica composition. Where the yarn is knitted in the step 12 to form the loose fabric, the fabric is disassembled by pulling an end of the yarn therefrom so as to deknit the fabric as the loose end of yarn is wound onto a bobbin or other form of desired shape.

The successive steps of a detailed form of the method of Fig. 1 are depicted in Fig. 2. In a first step 26 corresponding to the step 10 of Fig. 1 one or more lengths of leachable glass yarn of desired ply are provided. In a following step 28 corresponding to the step 12 of Fig. 1 the glass yarn is knit into a tubular fabric as described hereafter in connection with Fig. 4.

In a following step 30 the tubular fabric is wound around a skeiner. When a sufficient quantity of the tubular fabric is wound upon the skeiner, the fabric is severed. The length or skein of the tubular fabric remaining on the skeiner is tied with a separate piece of fabric to prevent entanglement with itself, and is then removed from the skein and placed in a porous bag of polypropylene, Dynell or other appropriate material which is resistant to leaching acid. Each time the length of tubular fabric is severed, tied and removed, the fabric is again wound onto the skeiner until the desired quantity has been wound thereon, following which the fabric is severed. The length of fabric which has been wound on the skeiner is then tied, removed from the skein and placed in its own porous bag.

Following the skeining, severing and bagging of the fabric, the porous bags containing lengths or skeins of the knitted fabric are placed in a leaching tank in a next step 32. A solution of hydrochloric or other appropriate leaching acid which has been heated is recirculated through the leaching tank and its included bags of the knitted fabric for an appropriate period of time such as 8-1 6 hrs. The acid solution is heated in a heater located outside of the leaching tank and is then pumped into the tank for exposure to the lengths of knitted fabric in the bags. The acid solution has a typical starting temperature of 1 4O0F when leaching is first begun and is raised to a final leaching temperature of about 2200F, usually within an hour or two after leaching is begun.The temperature is maintained at this level for the remainder of the leaching process. After exposure of the lengths of fabric to the heated acid solution, the solution is returned to the heater, reheated and then recirculated back into the tank. When the leaching step 32 is completed, the acid solution is removed from the leaching tank.

The bags of fabric are then washed in water in a next step 34 by admitting water from a continuous supply thereof into the leaching tank.

The wash water flows into the tank where it contacts the bags of knitted fabric to remove residual acid and acid-salts therefrom prior to exiting from the tank. Fresh water is continuously circulated through the tank as the washing step 34 proceeds. Adequate washing of the fabric is usually accomplished in about 4 hours.

Following washing of the lengths of knitted fabric, the lengths of fabric are removed from the bags and dried in a next step 36. The lengths of fabric are placed in an oven at an appropriate temperature and for an appropriate period of time to accomplish drying of the fabric.

In a firing step 38 the dried lengths of fabric are placed on a rack which in turn is placed in a firing furnace. The fabric is fired at a selected temperature, typically in the range of 1 2000F-1 8000 F, for a selected period of time, typically about 1 hour. At the end of the firing step 38, the rack is removed from the furnace and the lengths of fabric are allowed to coll at room temperature in a following cooling step 40.

Following the firing and cooling of the lengths of fabric in the steps 38 and 40, the fabric lengths are immersed in a solution of Teflon or similar material to provide the fabric with an abrasion resistant coating in a next step 42. After immersion for a suitable period of time, the lengths of fabric are removed from the solution and are dried in an oven in a following step 44.

Following the coating and drying of the lengths of skeins of fabric in the steps 42 and 44, the lengths of fabric are deknitted in a following step 46. Each length of fabric is deknitted by pulling a loose end of the yarn therefrom and then unraveling or deknitting the length of fabric as the yarn is wound onto a bobbin or other form of suitable shape.

Fig. 3 depicts one example of a combination of equipment which can be used to carry out the process of Figs. 1 and 2. In Fig. 3 the leachable glass yarn which has been previously wound onto a spool 50 is unwound from the spool 50 and pulled into a knitting machine 52. The knitting machine 52 knits the yarn into an elongated fabric which is wrapped around a skeiner 54 and then severed as previously described. The length of the fabric which is wrapped around the skeiner 54 is then tied with a separate piece of fabric, removed from the skeiner and placed in a different one of a plurality of porous bags 56.

The porous bags 56 containing the skeins of knitted fabric are placed in a leaching tank 68 where they are subjected firstly to a recirculating heated acid solution to effect leaching and secondly to circulating water to effect washing.

Upon completion of the water wash, the porous bags 56 are removed from the leaching tank 58 and the skeins of knitted fabric are removed from the bags 56 and placed in an oven 60 to dry the skeins of fabric.

The dried skeins of fabric are placed on a rack 62 which in turn is placed in a firing furnace 64 to fire the skeins of fabric. At the end of the firing step,the rack 62 containing the skeins of fabric is removed from the furnace 64 and the skeins of fabric are allowed to cool to room temperature.

The fired skeins of fabric are then placed in a coating tank 66 where they are immersed in a Teflon or similar solution to form an abrasion resistant coating thereon. The coated skeins of fabric are then placed in an oven 68 which can be the same oven as the oven 60 used to dry the skeins of fabric following leaching and washing.

Following coating and drying of the skeins of fabric, each of the skeins of fabric is deknitted by pulling a loose end therefrom and feeding it into rewinding machines 70 which wind the yarn onto a bobbin or other form of appropriate shape.

Fig. 4 depicts an arrangement for knitting the leachable glass yarn into the loose fabric. The arrangement of Fig. 4 includes the spool of yarn 50 and the knitting machine 52. The spool 50 is positioned so that the yarn can be fed therefrom into the top of the knitting machine 52. The knitting machine 52, which is of the circular type knits the yarn into a continuous, hollow, cylindrical sock 74. The knitted sock 74 advances to the bottom of the knitting machine 52 from which it is collapsed flat by a pair of rollers 76 and deposited in a container 77. As so folded, the sock 74 forms a flat, double-ply, elongated fabric of generally uniform width and thickness which is advantageously used for processing in accordance with the invention.

Fig. 5 depicts a portion of the knitted sock 74.

The knit design is itself a conventional one in which a single length of yarn forms a continuous series of loops that interlock with one another to form the assembled fabric. In the present example the single length of yarn is formed by twisting together strands which are themselves made up of twisted glass filaments.

While the yarn can be formed into an elongated length of fabric using any one of a variety of different techniques such as weaving, knitting is preferred because of the ease and economy of formation and disassembly. Where weaving is used, for example, not only does fabric formation occur at a slower rate, but the fill yarns must usually be discarded upon disassembly. The advantages of knitting on disassembly of the fabric can be seen with reference to Fig. 6 which is a closeup view of the knitted sock 74 similar to that of Fig. 5 but illustrating the ease with which the knitted sock 74 is deknitted simply by pulling on an end 78 of the yarn bundle. The knitted sock 74 offers very little resistance to deknitting, enabling the sock 74 to be deknitted by feeding the end 78 of the yarn bundle into the next piece of apparatus in the process such as the rewinding machines 70 noted in Fig. 3.The knitting and deknitting of a tubular sock is shown and described herein for purposes of illustration only, and the knitting and deknitting of a flat, single ply fabric can just as easily be used.

EXAMPLE The invention may be better understood by considering a specific example of making silica yarns using the process of Figs. 1 and 2. In the example the leachable glass yarn was 1 650 denier, E glass (borosilicate glass) yarn having a twist of 3.8 Z.

The glass yarn was drawn into a Leighton circular knitting machine. The knitting machine knits a tube approximately 5" in diameter.

Approximately 5 pounds of the knitted fabric was then wound onto a skeiner and the fabric was severed. The resulting length of fabric on the skeiner was tied with a separate piece of fabric and removed from the skeiner and placed in a bag of porous polypropylene material.

The knitting, skeining, severing, tying and bagging operation was continued until at least 50 bags had been filled with skeins of the knitted fabric. The bags were then placed in a leaching tank. A solution of hydrochloric acid having a concentration by weight of about 1 7% was heated to an initial temperature of 1 6O0F by an external heater and then circulated into the tank, through the bags and then back to the heater. The temperature of the acid solution was raised from 1 600F to 22O0F in 1-2 hours and was thereafter maintained at about 2200 F. Leaching was carried out for approximately 1 6 hours. At the end of the leaching process, the hydrochloric acid solution was removed from the tank and fresh water was circulated through the tank for approximately 4 hours to effect washing of the skeins of fabric.

The water used for the wash was tap water at room temperature.

Following washing of the skeins of fabric, the skeins were removed from the bags, placed on a rack and dried in an oven.

Following drying of the skeins of fabric, the skeins were placed on a firing rack which itself was placed inside of a firing furnace. There, the skeins of fabric were fired at approximately 1 20O0F for about 1 hour. Approximately 18 skeins of the fabric were placed on each rack.

Following firing, the racks were removed from the firing furnace and the skeins of fabric were allowed to cool at room temperature for 1-2 hours.

When the skeins of material were cool, they were placed in a solution of 36% Teflon solids by weight at room temperature for about 30 minutes. Following that, the skeins of fabric were set on bars, allowed to drip dry and were then put in the oven to dry.

Following drying, each skein of fabric was then deknit and the resulting yarn of essentially silica composition therefrom rewound using rewinding machines.

Processes in accordance with the invention have been found to provide on the order of a 5152% yield from starting material in the form of leachable glass yarns which are typically about 53% silica. This is far better than the 45% yield which is a typical figure for "pump leaching" processes. Sixty-three pounds of leachable glass yarn processed in accordance with the example described above produced 34.6 pounds of usable silica yarn. The average Teflon content of the end product was 6.5%. Wastage was limited to approximately .15 pounds of the yarn. In addition, the uniformity of the silica yarns produced by such method was found to be very good, and the material was found to be substantially free of undesirable "hot spots".

Further processing in accordance with the invention of 1072 pounds of glass yarn produced 543.5 pounds of usable silica yarn which had 32 pounds of Teflon coating applied thereto. One hundred test samples showed an average break strength of the silica yarns of 2.5 pounds. Ten test samples showed an average chloride content of less than 30 parts per million.

Claims (25)

1. A method of making silica yarn from leachable glass yarn by leaching the glass yarn and washing and firing the leached yarn, in which the glass yarn before leaching is formed into a loose fabric which after firing is disassembled into the resulting silica yarn.

2. A method according to claim 1 wherein the hot fired loose fabric is cooled at room temperature.

3. A method according to claim 1 or 2 including applying an abrasion resistant coating to the fired loose fabric.

4. A method according to claim 3 wherein the abrasion resistant coating is applied by immersing the fired loose fabric in a solution.

5. A method according to claim 4 including drying the loose fabric after the coating immersion.

6. A method according to any preceding claim wherein the loose fabric of glass yarn is leached in recirculating acid.

7. A method according to claim 6 wherein the loose fabric of glass yarn is leached in heated recirculating acid.

8. A method according to any preceding claim wherein the loose fabric of glass yarn is leached in a tank.

9. A method according to any preceding claim wherein the leached loose fabric is washed in water.

10. A method according to claims 8 and 9 wherein the leaching and washing are conducted in the same tank, the acid being removed from the tank after leaching and washing water then being circulated through the tank.

11. A method according to any preceding claim wherein the washed loose fabric is dried before firing.

12. A method according to any preceding claim wherein the firing of the loose fabric is conducted on a rack in a firing furnace.

13. A method according to any preceding claim wherein the glass yarn is knitted into the loose fabric.

14. A method according to claim 13 wherein the glass yarn is knitted into a tubular loose fabric.

1 5. A method according to claim 13 or 14 wherein the loose fabric is disassembled into the product yarn by pulling an end of the yarn from the knitted loose fabric to deknit it.

16. A method according to any preceding claim wherein the loose fabric of glass yarn is severed into a plurality of individual lengths for the leaching and subsequent steps from which a corresponding plurality of individual lengths of silica yarn is obtained.

17. A method according to claim 16 wherein the loose fabric before severing is wound on a skeiner and wherein following severing the loose fabric is tied with another piece of fabric and then removed from the skeiner.

18. A method according to any preceding claim wherein the loose fabric of glass yarn is leached in a porous bag.

19. A method according to claim 1 8 together with claim 1 6 or claim 17, wherein each individual length of loose fabric is placed in a separate porous bag for leaching.

20. A method according to claim 1 8 or 1 9 wherein the leached loose fabric is washed in the porous bag or bags.

21. A method according to claim 20 wherein the washed loose fabric is removed from the porous bag or bags and dried before firing.

22. A method according to any preceding claim wherein the silica yarn as it is disassembled from the loose fabric is wound around a form.

23. A method of making silica yarn, the method being substantially as hereinbefore described in the Example.

24. A method of making silica yarn, the method being substantially as hereinbefore described with reference to Fig. 1 of the accompanying drawings.

25. A method of making silica yarn, the method being substantially as herein before described with reference to Fig. 2 of the accomoanying drawings.