CA1057675A - Filter tube and method of making same - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

The invention relates to a filter tube and a method for making the filter tube. The filter tube consists of a plurality of randomly disposed interrelated inorganic fibers having a diameter of from about 0.001 to 10 microns. The tube forms a semirigid, self-supporting, porous, filter tube of desired filtering porosity and efficiency, and the fibers of the filter are bonded at the junction of the fiber crossover points with a bonding amount of a substantially nonhydroscopic pyrolyzed quaternary ammonium silicate as a binding agent. The filter tube of the invention is character-ized by a high collapse strength and substantially nondusting properties during handling. The method for making the tube con-sists of impregnating a filter tube composed of interrelated, non-woven negatively charged inorganic glass fibers, having a diameter from about 0.001 to 10 micorns, with a liquid dispersion of positively charged particles of a nitrogen substitute silicate compound. The impregnated filter tube is then heated to a tempera-ture of about 800 to 1100°F to provide an inorganic pyrolyzed silica binder, thereby forming a self-supporting porous filter tube subject tot use at high temperature.

Description

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In various processes requiring the Eiltration of a gas or liquicl stream, filters composed of woven or nonwoven fibers are often employed, either alone in flat sheet or tubular form or supported by a suitable porous support. In the selection of such filters ~or a particular application, one characteristic relates to the efficierlcy, flow rate and life of the filter~
These criteria typically depend upon khe particular properties of the material of which the fiber is composed, but more particularly, are related to the particular diameter of the : , .
fibers used. Another important criteria is an environmental one as to whether the filters will withstand the particular pressures, :
~ temperatures and physical nature of the gas or liquid material to -` be filtered.
- As to this latter criteria, filters made from organic material are quite susceptible to pressure, temperature and the ;~
chemical nature of the gas or liquid to be filtered. Filters made from glass fibers have been employed where the glass fibers are ;1 formed into an interlaced mass of the glass fibers without any physical coherency or strength; that is, without a binder to re-~1 2a tain the glass fibers in a coherent manner. Although such glass - fibers have good temperature and chemical resistance, the physical . ,;
strength of such fibers is quite low, and such fibers may only be used in most undemanding applications. The filters must, there~
fore, be treated with a great deal of care by the user in order t~at they remain as useful filters. -... 1 . .
The lack of strength of such glass fibers has been ;
~¦ overcome by bonding the glass fibers wlth suitable organic bonding agents~ The bonding agents may typically be p~enol-formaldehyde ~ ~`
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' or epoxy resins or other thermosetting-type resins with which the -3Q mass of typically interrelated nonwoven glass fibers are impreg~
nated during the formation process ofthe fibers into their

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particular form or thereai~t~:r. ~Iowev~3r, the chemical and temperature resistance of the filter so prepared is modified by the employment of such bonding agents. Typically an epoxy resin is used which provides good strength and fairly high-temperature and chemical resistance to a self~supporting nonwoven tubular fiber filter. Such filter tubes are described more particularly in U.S. Patent 3,767,054, issued October 23, 19730 Although such epoxy resin glass fiber filter tube~ are suitable for many uses, the organic bonding agent restricts the scope of the filter applications available, particularly since the maximum temperature of use of such tubes is no-t over 200C. In addition, such filter tubes are not very resistant to degradation in use ~-~
with many liquids, such as concen-trated acids. F~lrthermore, such filter tubes, due to the presence of the organic resinous bonding agent, often have an off-white-to-light brown color which darkens with age and sunlight due to the presence of such bonding agent. `
Thus, such filters may not be employed where a color indicator is employed with the filter, or where a white color is desirable.
SUMM~RY OF T~E INVENTION
My inve~tion concerns an improved fil~er and the .; .- .
; method of prepariny such filter~ In particular, my invention `~
relates to an improved nonwoven fibrous filter tube composed of :; ;: . ., inorganic fibers, particularly glass fibers with a pyrolyzed ~
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silica as a bonding agent. The method of prepariny such filter tubes comprises impregnating the porous fiber mass with electro-positive silicate particles and heating the impregnated mass to a temperature to Eorm a silica bondiny agent. My improved filter ~ :
tubes may be used at high temperatures; e.g., in excess of 200C., . . . .
have improved resistance to acids, are characterized by a light ~ ~
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white color and have improved collapse strength.

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.: ' ;75 My inventiorl provides a fi:Lter tube composed of glass fibers, which filter tube is similar or better in filtration pro-perties, e.g., flow rate and eEficiency, as prior art filters with epoxy resin as a bonding agen-t, but have improved physical properties which permit my filter tubes to be employed in demanding uses not possible by such epoxy resin tubes. My filters are com-posed of inorganic fibers, particularly glass fibers such as boro~
silicate glass fibers, interrelated in a nonwoven, randomly dis-posed pattern defining interstices of desired porosity therebetween, O and are bonded together into a self-supporting filter tube with a .
pyrolyzed silica binder.

My invention provides for a mat or mass in desired -~orm of interlaced, overlapping or interrelated fibers having diameters, for example, of from about 0.001 to about 10 microns, . .~ ,.
such as 0.03 to 8 microns, particularly 0.1 to 3.0 microns, bonded together by pyrolyzed silica as a binding agent. ~;
A wide variety of fibers may be employed in preparing -the self-supporting filters of my invention. Preferably, the -fibers are glass fibers, and more particularly borosilicate glass -~ibers, which fibers have an electronegative charge~ `

My improved filter tubes are prepared by impregnating the filter tubes composed of electronegatively charged glass fibers .;~. :
with a solution contain;ng electropositively charged silicate ;~
particles, and then heating the impregnated filter tubes to a temperature sufficient to pyrolyze the silicate and form a silica bonding agent.

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I have found that while th~ impre~nation of glass fiber filter -tubes with colloidal silicate solutions and drying the impregnated tubes provide tubes of improved color which are sultable for high--temperature use, such tubes and the method of preparing such tubes have certain disadvantages~ Such tubes tend to be dusty and difficult to handle, and often e~hibit an undesirable degree of hygroscopicity. In addition, such tubes, are not wholly satisfactory on impregna-tion of the tubes with the silica sol where the glass fiber diameters are less than about 1~ 2.5 to 3.0 microns, resul-ting in lower bond str0ngth for the filter tubes where the fiber diameters range less than ~.0 microns;

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for exampLe, 0.5 to 2.0 microns. Impregnation oE the glass fiber tubes where the surface-volume ratio is high is difficult to achieve with the silica sol particles. Typicall.y, filter tubes are manufactured and sold in various grades of porosity and filtering efficiency related to the fiber diameter ranges employed in the tube manufacture. Thus, it is important to be able to provide good bond strength at all filter tube grade levels~ My method permits the preparation of glass fiber filter tubes of all grades, with excellent bond and collapse strengths, white color, nondusty and nonhygroscopic in character.
M~ method comprises impregnating, such as by dipping, , .~
s the filter tubes preferably in a dry state with an electropositive . . , ¦ silicate colloidal solution~ I have discovered that electro-positive nitrogen--substituted silicate compound solutions, particularly quaternary ammonium silicate solutions, permit i, , .
~ the preparation of my impxoved filter tubes. Such quaternary ''.'J ammonium silicate solutions are commercially available as milky, ~-,;
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opalescent, colloidal-type dlspersions of the silicate particles in water. Although not wishing to be bound by any particular theory of operation or explanation, I believe that the high electropositive nature of nitrogen-substit~lted silicates permits the excellent impregnation of the solution into intimate contact with all grades of the filter tubes composed of the electro-negatively charged glass fibers.
; Nitrogen-substituted, and particularly qua-ternary, silicates exhibit electropositive properties re~uired for my invention. Various organic substituent groups may be employed on the nitrogen atom as desired, since such substituents are removed during the pyrolysis step. The quaternary ammonium may comprise -from one to four groups of hydrogen or organic substituents alone or in com~ination, such as alkyl, benzyl and halo and alkyl-benzyl groups. Such quaternary silicate compounds include, but are not limited to mono and dialkyl quaternary silicates, mono and dialkyl in combination with one or two methyl groups or benzyl groups. The alkyl groups include Cl-C20 carbon atoms and Z particular Cl-C4 groups, e~g , methyl and ethyl groups, and C12-ClZ3 fatty-acid groups, such as stearyl, cetyl, oleyl, myristate and mixed CL~-C14-C16-C18 groups. Useful silicates would include ; n-alkyl dimethyl benzyl ammonium silicate, stearyl dimethyl benzyl 1 ammonium silicate, C12 C18 di n-alkyl methyl benzyl ammonium ! silicate, trimethyl ammonium silicate, n-alkyl dimethyl ethyl benzyl ammonium silicate, etc. The preferred quaternary . .~
, silicates are those wherein the organic substituent i5 easily ;~
removed by pyrolysis, such as the methyl-substituted silicates~
After impregnation, the impregnated tube is heated .: .
to a high temperature to pyrolyze the silicate which drives off

3~ any organic substituents and leaves behind pyrolyzed silica. The .

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quaternary ammonium or nitrogen atoms may have hydrogen or oxganic pyrolyzable substituents so long as the silicate is electropositively charged, since the substituent groups are removed in the heating step.
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The filter tubes should be heated to a temperature of greater than 800 F, and preferably 900 to 1000F1 but less than the softening temperature of the fibers which, with borosilicate glass fibers, is about 1100F. Filter tubes impregnated with a ; quaternary ammonium silicate and heated to 500F have a brown ~, 10 color which becomes lighter at 600 F, and generally at 800F and above, the tubes are substantially white in color.
~;~ In the preferred embod:iment, the filter tubes are first dried prior to impregnation, for example, at a temperature of `~
. .
80 to 150~C. Impregnation may be accomplished in a variety of ways, however, dipping the dried tube into a solution of the silicate is typically employed. The concentration of the silicate ;~ solution may vary as desired, depending on the bond strength and ``~ the weight of the silica desired, for example, from 1 to 25% by ;~ weight based on the silicate as silica; e.g., 2 to 10%. The pre-. ~ ~
' 2Q ferred solution to be used ranges from 4 to 8% by weight to ::. . . .
obtain optimum tube collapse strength.
~`1 It is also important to heat the tubes directly after impregnation with the silicate, such as within 30 minutes to 1 hour, or preferably less than 4 hours after impregnation to obtain goodtube collapse strength.
, ~ My Eilter tubes ater pyrolysis may contain varying amounts of silica as the bonding agent as desired, which amounts , l~ may range from 5 to 40% by weight of the tube, for example, 10 ::1 -;' to 30% by weight.
The filter tubes of my invention are prepared in !, the same general manner as epoxy resin filter tubes~ For example, , - 7 -~, , , .:

~ 57~'75 glass fibers, such as borosilicate glass flbers, are dispersed in an aq~teous furnish solution of 0.1 to 2~0% by welght of the fibers.
A wet ma-t-ter tube of glass fibers is formed onto a tubular porous mandrel, such as by immersing the mandrel in-to the glass fiber dispersion, and, for example, removing water by draining or pre-ferably with the aid of a vacuum attached to the interior of the mandrel. Typically, the mandrel may be of stainless steel mesh having holes covered with a woven stainless steel wire~
If desired, a binder may be introduced in small amounts into the aqueous dispersion to aid in keying the fibers together prior to the impregnating step~ Preferably, the fibers are formed without the use of a bonding agent, and the nonwoven fibers after drying are subsequently :impregnated with the desired quaternary silicate solution.
Once the inorganic fiber mat has been formed as described, the material is then dried in a steam oven or simiLar drying apparatus to provide a dried filter tube. Impregnation of the binder material onto the nonwoven mass of fibers is then : 1i ;l accomplished by treating the dried filter with the impregnation : ~ .
2~ solution. For example, the dried filter is impregnated by immersing, spxaying, coating, or otherwise treating the tube with ~ a solution, dispersion, emulsion, or bu1k liquid silicate material, ; all referred to as sollltions. The impregnated tube is then heated in a circulation hot-air oven to the pyrolysi9 temperature, and then removed.
The filters prepared in accordance with my invention may, of course, take any form, for example, in -the form of flat ; sheets, discs or tubes. In the case of tubes of glass fibers in par- `
ticular, such tubes are self-supporting and self-gasketing, i.e.
~ ~Q the fibers at the end of the tube are axially compressible to form .,' :. ~.. ,. . . . .. , . ~. . . -, .. . . . .

1~i7675 a peripher~l seal/ ~nd often no gasketlny or other sealing is necessary.
In accordance with an embod.iment o~ the invention, a :~ method of preparing a filter tube adapted to withstand high . .
tempera-tures in use, comprises: (a) irnpregnating a filter j tube composed of interrelated nonwoven negatively charged . inorganic glass fibers having a diameter of from about 0~001 :~
to 10 microns with a liquid dispersion of positively charged ~.
. particles of a nitrogen-substituted silicate compound, and (b) heating the impregnated filter tub~ to a temperature of about 800 to 1100F to provide an inorganic W rolized silica -1 binder, thereby forming a self-supporting porous filter tube .
:: subject to use at high temperatures.
.:~ From a di.fferent aspect, in accordance with an embodi-''.'J ,' :1 ment of the invention, a method of preparing a filter tube ~ /
..
.~ adapted to withstand high temperatures in use, comprises~
.-~ (a) dipping a dry filter tube composed of interrelated nonwoven glass fibers having a diameter of from about 0.001 to 10 microns ., into an aqueous dispersion of quaternary ammonium electro-~. ~ ,. .
.. 20 positively charged silicate particles to impregnate the filter :~ tube, and ~b) heating the impregnated filter tube to a tempera- :
:- ture greater than about 800F to pyrolyze the silicate compound and to form a pyrolyzed silica binder for the glass fibers.
In accordance with a still Eurther aspect, and in accordance with an embodiment of the invention, the filter tube comprises a:plu~rality of randomly disposed interrelated . inorganic fibers having a diameter of from about 0.001 to 10 .~ microns, the filter tube forming a semirigid, self~supporting, :~ porous, filter tube of desired filtering porosity and efficiency, ; ~
' 30 the fibers bonded at the junction of the fiber crossover po.ints, .~ with a bonding amount o a substantially non-hydroscopic pyrolyzed quaternary ammonium silicate, including electro-.

~)57~75 positively chargecl sillcate particles as a bi.nding ayent, the filter tube characterized by a high collapse strength and substantially nondusting properties dur.ing handling.
In accordance with a further embodiment, an improved filter tube adapted for use at temperatures of greater khan 200C, comprises a plurality of randomly disposed, interrelated, necJatively charged, ino.rganic borosilicate glass fibers having -a diameter of from about 0.03 to 8 microns to form a semirigid, self-supporting, porous tube of desired filtering porosity and ` 10 efficiency, the tube characterized by a light color, a high ;' .
collapse strength, the bonded fibers at the end of the tube .. axially compressible to form a self-gasketing peripheral seal, the fibers bonded at the junctions of the fiber crossover ..: i '''-'f points with an inorganic bonding agent composed of a pyrolyzed decomposition silica product of a quaternary ammonium silicate ~ compound, comprising electropositively charged silicate ;~

~ particles, and wherein the silica bonding agenk comprises from :, about 5 to 40% by weight of the filter tube.
, t:.i My invention will be described for the purposes of ~ .
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;: 20 illustration only, and in particular in connection with the . preparation of various sel.f-supporting borosilicate glass fiber `~
;: tubes.
~: DESCRIPTION 0~ TI-E EMBODIM~N'~
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~ ' Dry filter kubes having an average pore diameter of '. a~out 2.8 microns (range 0.5 to 3.5 microns) were dipped in '. three different concentrations of a colloidal silica solution t~l of Ludox 1) 130M:
. A - 1.2 parts (volume~ Ludox (30% SiO2)/3 parts water ~ 30 B - 1.0 parts (volume) Ludox (30% SiO2)/3 parts water i C - 0.8 parts (volume) Ludox (30% SiO2)/3 parts water a trademark of E.I. DuPonk de Nemours Company -9a-` 3L~)57~7S
-. The tubes were clriecl under two condi.tion~
S - slowly dxied at room temperature for 48 hours F - fastly dried in a tunnel oven at 200F
` The tubes were limp after d.ippiny, and handling was .` extremely cl.ifficuLt. The filter tube~s so prepared were then tested for collapse strength and air flow resistance~
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:i Collapse strength was determined by enclosing the filter , . , tubes in a rubber bladder vented to the atmosphere, and air .1 pressure supplied externally to the bladder until the tube .~ ~:
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. 10 collapsed. The test resu~ts are illustrated in Table I.
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Tube Pr~ærties , . . ~ .
'rube Code Collapse Strength, Air Flow Resistance :
PS I ~15 cm/sec face veloc Inches water AS 4-5 4 . 8 . AF 4-5 ` BS 10-11 5 . 5-6 ~, 3 `
- BF 11 8 . O--11. 8 ' lQ CS ~ 1~-13 6~5-6,7 ~ ;:
: ~ CF 9-10 11,9-12.7 ~........................................................................ . .
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3Lq~57~75 Example 2 Filter tubes of the same type as Example 1 were dipped in three different concentrations of Quram~) 220, a solution of an organic ~uaternary ammonium silicat~a containing about 45% SiO2 as silicate:
:.~ , , ~A - 1.2 parts (volume) Q220/5 parts water , .~~ ~ loO part~ (volume) Q220/5 parts water C - 0.8 parts (volume) Q220~5 parts water ~;
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:~The tubes were immersed for a period of about 30 seconds. The tubes were dr.ied under two conditions~
S - Dried at room temperature for 4.5 hours, then dried l in tunnel oven at 200F
-~F - Placed immediately in tunnel oven at 200F.
, ;, All tubes w~are then dried for 2.5 hours at 400F, and the tubes tested as before with the results shown in Table II. ~ :
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- ~57~i~5 TABLE I I
Tube Properties : :
Tube Code Collapse Strength Air Flow Resistance PS I Inches _Water : :
AS 4 3.1 . CF 9 ' :
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~ Exam~e 3 ~n57~ 5 Filter tubes o~ the ~ame type as Example 2 were dipped in two different concentrations of quarternary am~tonium silicates identified by the Trade Mark Quram 220:
A - 0.7 parts Q220/5 parts water -. , . . ~ .
B - 0.6 parts Q220/5 parts water ~
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The tubes were dried under the same conditions as Exam-ple 2, except that final drying waq as follows:
1/2 hour ~ 250F
1/2 hour ~ 360P`
2 1/2 hour ~ 400F
The collapse strength o~ the tubes was determined, with the following results; tube AF - 8-9 p.5i, tube AS - 7.S psi;
tube BF - 7.5 p~i; and tube BS - 6.0 psi.
.
,: ', ~; ~! Filter tubes similar to those of Example 1 in 1 and 2 :,-inch diameters were dipped in two differ~nt concentrations of a quaternary ammonium silicate solution:

, A - 0.8 parts (vol) Q220/5 parts w~ter B - 0~7 parts (vol) Q220/5 parts water , :
' ~ All tubes were immediately placed in a tunnel oven at , I~, 210F until dry (about 45 minutes). They were then heated to 360F for 1 hour. Later they were heated in steps to 1000F
over a 3-hour period (pyrolyzed). Smoke evolved as the tubes were pyrolyzed. Smoke evolution had ceased before the temperature ` ;~
~`~ reached 1000F. The tubes were then tested for tube properties, ,~ ~ with the results shown in ~able III.
~ TABLE III

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TABLE III
Tube Properties Code Grade Diam. Air Flow Collapse Collapse StrengthInch Resistance Strength ~ter pyrolysis :
Before Py- before py- PSI
rolysis roylsis _ inch water PSI c A C 1 2.5 lO 13 B C l 2.4 lO-ll 14 l.O B C l 2.6 lO ' -A C l 2.5 ,, . i .1, . ' . ~ ~ ' :i :

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My invention has been described in reference to the preferred embodiment, however, as will be apparent to those per-sons skilled in the art, various modifications, changes and additions may be made without departing from the spirit and scope of my invention as described.

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Claims (23)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:-

1. A method of preparing a filter tube adapted to withstand high temperatures in use, which method comprises:
(a) impregnating a filter tube composed of interrelated nonwoven negatively charged inorganic glass fibers having a diameter of from about 0.001 to 10 microns with a liquid dispersion of positively charged particles of a nitrogen-substituted silicate compound; and (b) heating the impregnated filter tube to a temperature of about 800 to 1100°F to provide an inorganic pyrolized silica binder, thereby forming a self-supporting porous filter tube sub-ject to use at high temperatures.

2. The method of claim 1 wherein the inorganic fibers are borosilicate glass fibers.

3. The method of claim 1 wherein the liquid dispersion is an aqueous or alcohol dispersion of the silicate compound.

4. The method of claim 1 wherein the silicate compound is a quaternary ammonium silicate.

5. The method of claim 1 wherein the filter tube is impregnated by dipping a dry filter tube into an aqueous dis-persion.

6. The method of claim 1 wherein the filter tube is impreg-nated by dipping the filter tube in a dispersion containing from about 2 to 10% of the silicate compound as silica.

7. The method of claim 1 wherein the filter tube is heated within about four hours after impregnation with the silicate com-pound.

8. The method of claim 1 wherein the impregnated filter tube is heated to a temperature of from about 900 to 1000°F.

9. The method of claim 1 wherein the glass fibers have a diameter of from about 0.03 to 3.0 microns.

10. A method of preparing a filter tube adapted to withstand high temperatures in use, which method comprises:
a) dipping a dry filter tube composed of interrelated nonwoven glass fibers having a diameter of from about 0.001 to 10 microns into an aqueous dispersion of quaternary ammonium electropositively charged silicate particles to impregnate the filter tube, and b) heating the impregnated filter tube to a temperature greater than about 800°F to pyrolyze the silicate compound and to form a pyrolyzed silica binder for the glass fibers.

11. The method of claim 10 wherein the fibers are borosilicate glass fibers having a diameter of from about 0.03 to 3.0 microns.

12. The method of claim 10 wherein the impregnated filter tube is heated to a temperature of from about 900 to 1000°F.

13. The method of claim 10 wherein the impregnated filter tube is heated within about one hour after impregnation with the silicate compound.

14. The method of claim 10 wherein the aqueous dispersion comprises from about 5 to 7% by weight of the silicate compound as silica.

15. An improved filter tube adapted for use at temperatures of greater than about 200°C, which filter tube comprises a plurality of randomly disposed, interrelated, negatively charged, inorganic borosilicate glass fibers having a diameter of from about 0.03 to 8 microns to form a semirigid, self-supporting, porous tube of desired filtering porosity and efficiency, the.
tube characterized by a light color, a high collapse strength, the bonded fibers at the end of the tube axially compressible to form a self-gasketing peripheral seal, the fibers bonded at the junctions of the fiber crossover points with an inorganic bonding agent composed of a pyrolyzed decomposition silica product of a quaternary ammonium silicate compound, comprising electropositively charged silicate particles, and wherein the silica bonding agent comprises from about 5 to 40% by weight of the filter tube.

16. A filter tube which comprises a plurality of randomly disposed interrelated inorganic fibers having a diameter of from about 0.001 to 10 microns, the filter tube forming a semirigid, self-supporting, porous, filter tube of desired filtering porosity and efficiency, the fibers bonded at the junction of the fiber crossover points, with a bonding amount of a substantially non-hydroscopic pyrolyzed quaternary ammonium silicate, including electropositively charged silicate particles as a binding agent, the filter tube characterized by a high collapse strength and substantially nondusting properties during handling.

17. The filter tube of claim 16 wherein the organic fibers are borosilicate glass fibers.

18. The filter tube of claim 16 wherein the binding agent comprises from about 5 to 40% by weight of the filter tube.

19. The filter tube of claim 16 wherein the inorganic fibers comprise glass fibers having a diameter of from about 0.1 to 3 microns.

20. The filter tube of claim 16 is further characterized in that the inorganic fibers at each end of the tube are axially compressible to form a self-gasketing peripheral seal.

21. The filter tube of claim 16 wherein the pyrolyzed quaternary ammonium silicate is an organic quaternary ammonium silicate pyrolyzed at a temperature of from about 800 to 1100°F.

22. The filter tube of claim 16 wherein the pyrolyzed quaternary ammonium silicate is the sole binding agent for the inorganic fibers.

23. The filter tube of claim 16 which comprises borosilicate glass fibers having a diameter of less than about 3 microns, and wherein the binding agent is substantially uniformly disposed throughout the depth of the filter tube.