Note: Descriptions are shown in the official language in which they were submitted.
l~)S'7G7S
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
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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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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;
.
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,
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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
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~ 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.
~ .
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,
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~ 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
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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
~ /
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.~ 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
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'''-'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 ~ .
"., /
;: 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'~
" ~ ', ~!e~ `''~
~ ' 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
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-. 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~
:, :
: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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TABLE
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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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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~ 2) a trademark of Philadelphia Quartz Company ~ ~
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TABLE I I
Tube Properties
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Tube Code Collapse Strength Air Flow Resistance
PS I Inches _Water
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AS 4 3.1
BS 7
BF 7
CS 7 3 0
. 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 ~
,~ .
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.
.
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~; ~! 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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~35~75 ~
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
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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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