U.S. patent application number 10/822440 was filed with the patent office on 2004-11-11 for low density nonwoven glass fiber web.
Invention is credited to Choi, Wai Ming, Lifshutz, Norman.
Application Number | 20040224594 10/822440 |
Document ID | / |
Family ID | 33313459 |
Filed Date | 2004-11-11 |
United States Patent
Application |
20040224594 |
Kind Code |
A1 |
Choi, Wai Ming ; et
al. |
November 11, 2004 |
Low density nonwoven glass fiber web
Abstract
The present invention relates to wet laid nonwoven glass fiber
webs, filter media formed of or containing wet laid nonwoven glass
fibers webs, and methods of making the same using a wet laid
processing technique. The filter media are particularly
advantageous in that it has been discovered that adjusting the pH
during wet laid processing will produce a glass fiber web having
improved filtration properties. In particular, neutralizing the pH
of a slurry containing mainly glass wool fibers unexpectedly yields
a non-electret, filter media that has a gamma value of at least
about 14, which is a significant improvement over non-electret, wet
laid glass filter media currently on the market which have been
shown to have a gamma value that does not exceed 13. The nonwoven
glass webs prepared according to the present invention preferably
contain a combination of glass wool fibers and chopped glass
fibers. The resulting glass fiber web can be used alone, or can be
combined with additional fiber webs, to form a filter media which
can be used in a variety of filtering applications.
Inventors: |
Choi, Wai Ming; (West
Newton, MA) ; Lifshutz, Norman; (Nashua, NH) |
Correspondence
Address: |
NUTTER MCCLENNEN & FISH LLP
WORLD TRADE CENTER WEST
155 SEAPORT BOULEVARD
BOSTON
MA
02210-2604
US
|
Family ID: |
33313459 |
Appl. No.: |
10/822440 |
Filed: |
April 12, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60463870 |
Apr 18, 2003 |
|
|
|
Current U.S.
Class: |
442/348 ;
442/415 |
Current CPC
Class: |
B01D 39/2024 20130101;
Y10T 442/697 20150401; Y10T 442/623 20150401 |
Class at
Publication: |
442/348 ;
442/415 |
International
Class: |
D04H 001/00 |
Claims
What is claimed is:
1. A nonwoven filter media, comprising at least one glass wool
fiber web having a gamma value of at least about 14, and a surface
area of at least about 1.2 m.sup.2/g.
2. The nonwoven filter media of claim 1, wherein the glass wool
fiber web is formed from glass wool fibers having a diameter in the
range of about 0.1.mu. to 4.5.mu..
3. The nonwoven filter media of claim 2, wherein the glass wool
fibers have a diameter selected from the group consisting of about
0.69.mu. and about 4.5.mu..
4. The nonwoven filter media of claim 1, further comprising chopped
glass fibers combined with the glass wool fibers.
5. The nonwoven filter media of claim 4, wherein the glass wool
fibers and the chopped glass fibers form a filtration layer.
6. The nonwoven filter media of claim 5, wherein glass wool fibers
are present in the filtration layer in the range of about 70% to
99% by weight and the chopped glass fibers are present in the
filtration layer in the range of about 1% to 30% by weight.
7. The nonwoven filter media of claim 1, wherein the filter media
is a wet laid filter media.
8. A nonwoven filter media, comprising at least one glass fiber web
having a gamma value of at least about 14, and an apparent density
of at least about 0.15 g/cc.
9. The nonwoven filter media of claim 8, wherein the at least one
glass fiber web includes glass wool fibers having a diameter in the
range of about 0.1.mu. to 4.5.mu..
10. The nonwoven filter media of claim 9, wherein the glass wool
fibers have a diameter of about 0.69.mu..
11. The nonwoven filter media of claim 8, wherein the filter media
is a wet laid filter media.
12. The nonwoven filter media of claim 8, wherein the apparent
density is in the range of about 0.15 g/cc to 0.21 g/cc.
13. A filter media, comprising: a support layer; and a filtration
layer including glass wool fibers having a diameter in the range of
about 0.1 to 4.5.mu.; wherein the filter media has a gamma value of
at least about 14.
14. The filter media of claim 13, wherein the support layer
includes glass fibers having a diameter in the range of about 4.mu.
to 30.mu..
15. The filter media of claim 13, wherein the filter media has a
surface area of at least about 1.2 m.sup.2/g.
16. The filter media of claim 13, wherein the filter media has an
apparent density of at least about 0.15 g/cc.
17. The filter media of claim 16, wherein the filter media has an
apparent density in the range of about 0.15 g/cc to 0.21 g/cc.
18. The filter media of claim 14, wherein the glass fibers in the
support layer have a fiber diameter of about 4.2.mu. and the glass
wool fibers that form the filtration layer have a fiber diameter of
about 0.69.mu..
19. The filter media of claim 13, wherein the filtration layer
further includes chopped glass fibers combined with the glass wool
fibers.
20. The filter media of claim 19, wherein the glass wool fibers are
present in the filtration layer in the range of about 70% to 99% by
weight and the chopped glass fibers are present in the filtration
layer in the range of about 1% to 30% by weight.
21. A method of making a filter media, comprising the steps of:
preparing a slurry containing glass wool fibers, chopped glass
fibers, water, and an acidic agent, the slurry having a pH in the
range of about 1 to 12; subsequently adding a pH adjusting agent to
the slurry to adjust the pH to within the range of about 6 to 10;
and removing the water from the slurry to form a wet laid glass
fiber web having a gamma value of at least about 14.
22. The method of claim 21, wherein the slurry is prepared having a
pH in the range of about 2 to 4.
23. The method of claim 21, wherein the pH adjusting agent is an
alkaline pH adjusting agent.
24. The method of claim 23, wherein the alkaline pH adjusting agent
is selected from the group consisting of a metal hydroxide, a
bicarbonate, a carbonate, an amine, and ammonium hydroxide.
25. The method of claim 23, wherein the alkaline pH adjusting agent
is ammonium hydroxide.
26. The method of claim 21, wherein the acidic agent is selected
from the group consisting of mineral acids and organic acids.
27. The method of claim 21, wherein the acidic agent is selected
from the group consisting of sulfuric acid, hydrochloric acid,
formic acid, and citric acid.
28. The method of claim 21, wherein the glass wool fibers are
present in the web in the range of about 50% to 99% by weight, and
the chopped glass fibers are present in the web in the range of
about 1% to 50% by weight.
29. The method of claim 21, wherein the glass wool fibers have a
fiber diameter in the range of about 0.1.mu. to 4.5.mu..
30. The method of claim 21, wherein the chopped glass fibers have a
fiber diameter in the range of about 4.mu. to 30.mu..
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority from U.S. Provisional
Patent Application Serial No. 60/463,870, filed on Apr. 18, 2003,
entitled "Low Density Nonwoven Glass Fiber Web," which is expressly
incorporated by reference herein.
FIELD OF THE INVENTION
[0002] The present invention relates to filter media formed of or
containing nonwoven glass fiber webs, and in particular to methods
of forming nonwoven glass fiber webs having enhanced filtration
performance characteristics.
BACKGROUND OF THE INVENTION
[0003] Glass fiber mats are used for a variety of purposes,
including, for example, in battery separators, air and water
filters, vacuum bags, automobile air conditioning filters, and
indoor air cleaner filters. The two most common methods for
producing glass fiber mats are dry laid processing and wet laid
processing. In a dry laid process the glass fibers are chopped and
dispersed in air that is blown onto a conveyor, and a binder is
then applied to form a mat. Such a process is typically more
suitable for the production of highly porous mats having bundles of
glass fibers. In a wet laid process, a slurry is formed containing
the glass fibers, and optionally other chemical agents such as
dispersants, viscosity modifiers, and defoaming agents. Glass
fibers are anionic by nature, and thus the acidic slurry is used to
remove any charge present on the fibers to disperse the fibers from
the glass wool conglomeration. As a result, the frictional contact
between the fibers is increased and the processability of the
fibers is improved. The fibers from the slurry are then collected
on a screen which allows a substantial portion of the water from
the slurry to be drained. The resulting mat is then dried to yield
a nonwoven mat formed of glass fibers.
[0004] A major objective of wet laid nonwoven manufacturing is to
produce materials with textile-fabric characteristics, primarily
flexibility and strength, at speeds approaching those associated
with papermaking. Current processes, however, tend to affect
filtration properties, producing nonwoven webs that have a high
density with high resistance.
[0005] Accordingly, there is a need for improved glass fiber webs,
filter media containing glass fiber webs, and methods for making
the same.
SUMMARY OF THE INVENTION
[0006] In general, the present invention provides nonwoven filter
media formed of or containing glass fiber webs, and methods for
making the same. In one embodiment, a nonwoven filter media is
formed of or contains at least one glass wool fiber web and has a
gamma value of at least about 14, a surface area of at least about
1.2 m.sup.2/g, and/or an apparent density of at least about 0.15
g/cc. The glass wool fibers that form the glass wool fiber web
preferably have a diameter in the range of about 0.1.mu. to
4.5.mu., and more preferably the diameter of the fibers is in the
range of about 0.3.mu. to 4.5.mu.. In an exemplary embodiment, the
glass wool fiber web is formed from glass wool fibers having a
diameter of about 0.69.mu. and/or 4.5.mu.. The glass wool fibers
can also optionally be combined with chopped wool fibers.
[0007] In another embodiment, a filter media is provided having a
support layer, and a filtration layer including glass wool fibers
having a diameter in the range of about 0.1.mu. to 3.5.mu.. The
filter media preferably has a gamma value of at least about 14, a
surface area of at least about 1.2 m.sup.2/g, and/or an apparent
density of at least about 0.15 g/cc. The support layer preferably
includes glass fibers having a diameter in the range of about
0.1.mu. to 30.mu.. More preferably, the support layer includes
chopped glass fibers that have a fiber diameter in the range of
about 4.mu. to 30.mu., and more preferably in the range of about
5.mu. to 12.mu., and the filtration layer includes glass wool
fibers that have a fiber diameter in the range of about 0.1.mu. to
4.5.mu., and more preferably the diameter of the fibers is in the
range of about 0.3.mu. to 4.5.mu.. In an exemplary embodiment, the
support layer includes glass wool fibers having a diameter in the
range of about 3.mu. to 7.mu., and the filtration layer includes
glass wool fibers that have a fiber diameter of about 0.69.mu.
and/or 4.5.mu.. The filtration layer can also optionally include
chopped glass fibers, preferably organic fibers, combined with the
glass wool fibers. The glass wool fibers are preferably present in
the filtration layer in the range of about 50% to 99% by weight and
the chopped glass fibers are preferably present in the filtration
layer in the range of about 1% to 50% by weight.
[0008] In other aspects of the present invention, a method of
making a filter media formed of or containing one or more nonwoven
glass fiber webs is provided. The method includes the steps of
preparing a slurry having a pH in the range of about 1 to 12, and
more preferably in the range of about 2 to 4, which contains glass
wool fibers, chopped glass fibers, water, and an acidic agent,
subsequently adding a neutral or alkaline pH adjusting agent to the
slurry to adjust the pH to the range of about 6 to 10, and removing
the water from the slurry to form a wet laid glass fiber web having
a gamma value of at least about 14.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] The invention will be more fully understood from the
following detailed description taken in conjunction with the
accompanying drawings, in which:
[0010] FIG. 1 is a flow chart illustrating an exemplary method of
making a filter media according to the present invention;
[0011] FIG. 2A is a photomicrograph at 370.times. of the filtration
layer of a filter media according to the present invention;
[0012] FIG. 2B is a photomicrograph at 370.times. of the support
layer of the filter media shown in FIG. 2A;
[0013] FIG. 3 is a graph illustrating the effect of pH on apparent
density;
[0014] FIG. 4 is a graph illustrating the effect of pH on specific
resistance;
[0015] FIG. 5 is a graph illustrating the effect of pH on surface
area;
[0016] FIG. 6 is a graph illustrating the effect of pH on DOP
penetration;
[0017] FIG. 7 is a graph illustrating the effect of pH on Gamma
value; and
[0018] FIG. 8 is a photomicrograph at 710.times. of the filtration
layer of a filter media shown in FIGS. 2A and 2B.
DETAILED DESCRIPTION OF THE INVENTION
[0019] The present invention relates to nonwoven glass fiber webs,
filter media formed of or containing nonwoven glass fibers webs,
and methods of making the same using a wet laid processing
technique. The filter media can be used in a variety of
applications, including as battery separators, air and water
filters, vacuum bag filters, cabin air filters, and indoor air
cleaner filters. The filter media are particularly effective,
however, for use as pleated filters in clean room environments. The
present invention is particularly advantageous in that it has been
discovered that adjusting the pH during wet laid processing will
produce a glass fiber web having improved filtration properties. In
particular, neutralizing the pH of a slurry containing mainly glass
wool fibers unexpectedly yields a non-electret, glass filter media
that has a gamma value of at least about 14, which is a significant
improvement over non-electret, glass filter media currently on the
market which have been shown to have a gamma value that does not
exceed 13. Moreover, the adjusted pH unexpectedly produces a filter
media having an improved surface area, which is preferably at least
about 1.2 m.sup.2/g, and an improved apparent density, which is
preferably at least about 0.15 g/cc. In an exemplary embodiment,
the apparent density of the filter media is in the range of about
0.15 g/cc to 0.21 g/cc.
[0020] The nonwoven glass fiber webs prepared according to the
present invention can contain glass wool fibers, or more preferably
a combination of glass wool fibers, chopped glass fibers, and
optionally other polymeric fibers. The resulting glass fiber web
can be used alone, or it can be combined with additional fiber
webs, to form a filter media. By way of non-limiting example,
suitable fiber webs that can be combined with the glass fiber
include polymeric and/or metallic expanded mesh.
[0021] Glass wool fibers are a specific type of fiber that are
prepared by blowing or spinning molten glass through small holes.
Unlike typical chopped glass fibers, glass wool fibers have a very
small diameter, which typically ranges from about 0.1.mu. up to
4.mu. or 5.mu.. In an exemplary embodiment, the nonwoven glass webs
are formed from glass wool fibers having a fiber diameter in the
range of about 0.1.mu. to 4.5.mu., and more preferably in the range
of about 0.3.mu. to 4.5.mu.. In an exemplary embodiment, the glass
wool fibers have a fiber diameter of about 0.69.mu. and/or 4.5.mu..
The glass wool fibers tend to vary significantly in length, and
thus no specific length is required. In an exemplary embodiment,
however, the aspect ratio (length to diameter ratio) (l/d) of the
glass wool fibers is preferably generally in the range of about 100
to 10,000, more preferably in the range of about 200 to 2500, and
most preferably in the range of about 300 to 600. A person having
ordinary skill in the art will appreciate that a variety of
different glass wool fibers can be used to form a nonwoven glass
fiber web according to the present invention.
[0022] As indicated above, the nonwoven glass webs can also include
chopped glass fibers that can be combined with the glass wool
fibers during processing. The chopped glass fibers can be present
in any amount, but are preferably present at about 1% to 30% by
weight in a web containing about 70% to 99% by weight glass wool
fibers. The chopped glass fibers preferably have a fiber diameter
in the range of about 4.mu. to 30.mu., and more preferably about
5.mu. to 12.mu., and a length in the range of about 0.125 inch to 1
inch. In an exemplary embodiment, the nonwoven glass webs contain
glass wool fibers having a diameter of about 0.69.mu. and/or
4.5.mu., and chopped glass fibers having a diameter in the range of
about 5.mu. to 7.mu..
[0023] The nonwoven glass fiber webs and filter media containing
the nonwoven glass fiber webs are formed using a wet laid
processing technique, which involves preparing a slurry containing
glass wool fibers, chopped glass fibers, and water. The fibers are
suspended uniformly in the slurry at very low concentrations in the
range of about 0.01 to 0.5% by weight of fiber. Since glass wool
fibers are anionic by nature, an acidic agent can be added to the
slurry to form a slurry having a pH in the range of about 2 to 4,
and most preferably about 3. The pH can, however, be adjusted to a
range of about 1 to 12. The acidic agent is also effective to
remove the charge on the fibers, thereby improving dispersion of
the fibers and facilitating processing of the web. While virtually
any acidic agent can be used, sulfuric acid is an exemplary pH
reducing agent. Other suitable acidic agents include, for example,
hydrochloric acid, formic acid, citric acid, and other mineral and
organic acids.
[0024] Once the slurry is prepared and the pH is adjusted to about
3, a neutral or alkaline pH adjusting agent is added to the slurry
to adjust the pH to a pH in the range of about 6 to 12, and more
preferably in the range of about 7 to 10. It has been discovered
that this additional step of adding a neutral or alkaline pH
adjusting agent to the slurry unexpectedly produces a nonwoven
glass web having improved filtration properties, as will be
discussed in more detail below. Virtually any neutral or alkaline
agent can be used to adjust the pH of the slurry. In an exemplary
embodiment, ammonium hydroxide is added to the slurry to adjust the
pH to about 7. Other suitable alkaline pH adjusting agents include,
for example, metal hydroxides, such as potassium hydroxide and
sodium hydroxide, calcium bicarbonate, and buffer solutions. After
adjusting the pH, the fibers can be collected on a screen and dried
to form a nonwoven glass web having one or more layers of glass
fibers.
[0025] Wet laid nonwoven glass webs are typically prepared using a
papermaking process, which includes a hydropulper, a former or
headbox, a dryer, and optionally a converter. As shown in FIG. 1,
the original slurry, which contains the glass wool fibers, the
chopped glass fibers, an acidic agent, and water, is prepared in
the hydropulper (1). The temperature of the slurry is preferably
maintained in the range of about 40.degree. F. to 100.degree. F.,
and more preferably in the range of about 50.degree. F. to
85.degree. F. After the slurry has been mixed in the hydropulper
(1) for about 3-10 minutes, it is pumped into the former or headbox
(2), where the neutral or alkaline pH adjusting agent is preferably
added. The slurry is also diluted with additional water such that
the final concentration of fiber is in the range of about 0.1% to
0.5% by weight. The fibers are then collected on a screen (3)
preferably at a rate of in the range of about 20 g/m.sup.2 to 200
g/m.sup.2. Before the slurry is sent to headbox, the slurry is
passed through centrifugal cleaners to remove unfiberized glass or
shot. The slurry may or may not be passed through additional
equipment such as refiners or deflakers to further enhance the
dispersion of the fibers. Care must be taken to minimize the work
done to the fiber. Glass fibers tend to be very brittle and excess
fiber shortening should be avoided. Excess water is removed by
gravity and vacuum assisted drainage. A binder can be added to the
fiber in the wet web or green state. The wet formed web is then
passed over a series of drum dryers (4) to dry at a temperature in
the range of about 250.degree. F. to 350.degree. F., preferably in
the range of about 275.degree. F. to 325.degree. F. Typical drying
times vary until the moisture content of the composite fiber is
less than about 6%.
[0026] In another embodiment, the nonwoven glass webs can be
combined with one or more additional fiber layers to form a filter
media. The filter media can include any number of layers, can be
formed from a variety of fibers, and can be prepared using a
variety of manufacturing methods. By way of non-limiting example,
the filter media can be laminated or otherwise attached to an
organic or metal backing.
[0027] In an exemplary embodiment, the filter media includes a
support layer and one or more layers of a nonwoven glass web
deposited onto the support layer to form a filtration layer. The
filtration layer(s) are preferably prepared as described above at a
pH of at least about 6. The support layer is effective to provide
structural integrity to the filter media, and is preferably a wet
laid glass fiber web. In an exemplary embodiment, the support layer
is formed from chopped glass fibers having a fiber diameter in the
range of about 4.mu. to 30.mu., and more preferably about 5.mu. to
12.mu., and the filtration layer(s) are formed from a combination
of glass wool fibers having a diameter in the range of about
0.1.mu. to 4.5.mu., and more preferably about 0.3.mu. to 4.5.mu.,
and chopped glass fibers having a diameter in the range of about
4.mu. to 30.mu., and more preferably about 5.mu. to 12.mu.. In an
exemplary embodiment, the support layer is formed from chopped
glass fibers having a fiber diameter in the range of about 3.mu. to
7.mu., and the filtration layer(s) are formed from a combination of
glass wool fibers having a diameter in the range of about 0.69.mu.
and/or 4.5.mu. and chopped glass fibers having a diameter in the
range of about 5.mu. to 7.mu.. By way of non-limiting example,
FIGS. 2A and 2B are photomicrographs, at 370.times., of a filter
media according to the invention having a support layer (FIG. 2B)
and a filtration layer (FIG. 2A) deposited on the support
layer.
[0028] The filter media can be prepared using a variety of
techniques, but preferably the support layer is prepared in a
slurry and passed through a first headbox where the fibers are
collected on a screen. The fibers then travel on the screen to a
second headbox, which contains the glass wool/chopped glass fiber
combination having a pH of at least about 6. The glass wool/chopped
glass fiber mixture is deposited onto the support layer, and the
two-layer structure is then dried to form a filter media. The web
can optionally be passed through one or more headboxes to add
additional layers to the web.
[0029] A person having ordinary skill in the art will appreciate
that the glass fibers used according to the present invention, as
well as the compositions of these glass components, can be varied
to achieve optimal performance depending on the intended use. The
nonwoven glass webs are not intended to be limited to webs formed
from only glass wool fibers, but can include a variety of other
fiber types in addition to the glass wool fibers and the chopped
glass fibers disclosed herein. Preferably, however, the nonwoven
glass webs contain a majority of glass wool fibers. The nonwoven
glass webs can also include a variety of other ingredients, such as
additives, surfactants, coupling agents, crosslinking agents, etc.
In one embodiment, the nonwoven glass webs contain a binding agent.
The binder coats the fibers and is used to adhere the fibers to
each other to facilitate adhesion between the fibers. In general,
the binder, if present in the nonwoven web, is in the range of
about 2% to 10% by weight, preferably in the range of about 3% to
9% by weight, and most preferably in the range of about 4% to 7% of
the total composite weight.
[0030] As previously stated, it has been discovered that
neutralizing the pH of the glass fiber slurry during the wet laid
process unexpectedly improves filtration efficiency of the
resulting filter media. In general, filter performance is evaluated
by different criteria. It is desirable that filters, or filter
media, be characterized by low penetration across the filter of
contaminants to be filtered. At the same time, however, there
should exist a relatively low pressure drop, or resistance, across
the filter. Penetration, often expressed as a percentage, is
defined as follows:
Pen=C/C.sub.0
[0031] where C is the particle concentration after passage through
the filter and C.sub.0 is the particle concentration before passage
through the filter. Filter efficiency is defined as
100-% Penetration.
[0032] Because it is desirable for effective filters to maintain
values as low as possible for both penetration and pressure drop
across the filter, filters are rated according to a value termed
gamma value (.gamma.). Steeper slopes, or higher gamma values, are
indicative of better filter performance. Gamma value is expressed
according to the following formula
.gamma.=(-log(DOP penetration %/100)/pressure
drop,mm).times.100
[0033] The pressure drop across the filter is typically a few mm of
H.sub.2O.
[0034] FIGS. 3-7, which will be discussed in more detail below,
illustrate the effect of pH on the filtration efficiency. In
particular, by neutralizing the pH of the glass fiber slurry to a
value in the range of about 6 to 12, and more preferably in the
range of about 7 to 10, the resulting filter media has an increased
surface area. This increase in the surface area eliminates the need
to add additional microfibers having a diameter in the range of
about 0.1.mu. to 0.5.mu. during manufacturing of the filter media.
Sub-micron fibers having a very small diameter, e.g., around
0.5.mu., typically tend to wash through the screen during the wet
laid process. Thus, some conventional techniques attempt to
compensate for this loss by adding additional sub-micron fibers to
the slurry during the wet laid process to compensate for the loss.
Microfibers with a sub-micron fiber diameter are very expensive,
however, and thus their excessive use is undesirable. By
neutralizing the pH of the slurry to at least about 6, the
sub-micron sized fibers appear to remain in the resulting filter
media. This is illustrated in FIG. 8, which shows a
photomicrograph, at 710.times., of a filter media according to the
invention having submicron sized fibers therein. An additional
advantage of retaining sub-micron fibers in the resulting filter
media is that the surface area of the filter media is significantly
increased.
[0035] FIGS. 3-7 show graphs illustrating the effect of pH on
filtration properties. The graphs were prepared using data was
obtained through test run on samples prepared as follows:
EXAMPLE 1
[0036] A slurry was prepared containing 50 lbs. of Evanite
706.times. fiber having an average fiber diameter of about
0.69.mu., 30 lbs. of Evanite 712.times. fiber having an average
fiber diameter of about 4.2.mu., 3 lbs. of Owens-Corning Chopped
Glass fiber DE having an average fiber length of about 0.25 inches,
and 3 lbs. of Owens-Corning Chopped Glass fiber DE having an
average fiber length of about 0.5 inches. The slurry contained
water and sulfuric acid sufficient to yield a fiber concentration
of 0.75% by weight. The headbox pH was varied between 2.3 and 3.8,
and the samples of web were collected at a headbox pH of 3.8, 3.6
and 2.3. The experiment was repeated containing the same fiber
formulation combined with water and ammonium hydroxide to vary the
headbox pH between 4.3 and 10.3. Samples were collected at a
headbox pH of 10.4, 9.6, 9.2, 8.4, 7.0, 6.0 and 4.2. The properties
of each sample were tested and are shown in the chart below. All
tests were conducted at an air velocity of 5.33 cm/sec with a DOP
particle size of 0.3 microns.
1TABLE 1 Ream Specific Caliper Apparent Surface DOP Resistance
Weight Resistance (mm @ Density Area Gamma pH (%) (mmH.sub.2O)
(lbs) (mm/lb) 10 kpa) (g/cc) (sq.m/g) (TDA100P) 2.3 0.297088 22.31
42.8 0.5213 0.331 0.21 n/a 13.115 3.6 0.4116 20.88 44.4 0.4703
0.358 0.202 n/a 13.57 3.8 0.173885 24.26 50 0.4853 0.403 0.202 n/a
13.283 4.2 0.181495 23.54 47.1 0.4997 0.380 0.202 1.2517 13.42 6.0
0.02003 29.39 47.9 0.6135 0.425 0.184 1.4488 14.475 7.0 0.004853
32.64 48.8 0.6689 0.433 0.1845 1.5174 15.083 8.0 0.001248 37.23
49.9 0.7461 0.462 0.176 n/a 15.03 8.4 0.001883 35.90 49.7 0.7224
0.461 0.176 1.5596 16.037 9.2 0.002668 34.74 48.8 0.7119 0.473
0.168 n/a 16.055 9.6 0.001552 36.79 51.5 0.7143 0.510 0.164 1.9726
15.615 10.4 0.00291 34.36 48.2 0.7128 0.451 0.174 n/a 16.76
[0037] As shown in Table 1 and in FIGS. 3-7, filter media prepared
from a slurry having a pH, adjusted at the headbox, of at least
about 6, advantageously show improved filtration properties. FIG. 3
illustrates the effect of pH on the apparent density of the filter
media. A lower density is desirable since the filter media will
have more loft and thus a longer path for the dust particle to
travel through the filter media. This resulted in increasing
probability of the dust particles being intercepted by a fiber. As
shown, samples having a pH in the range of about 1 to 5 have an
apparent density greater than 0.202 g/cc, while filter media having
a pH above 6 show a drop in the apparent density to 0.184 g/cc or
less.
[0038] Adjusting the pH during processing of glass fibers webs also
significantly improves the specific resistance of the web. Specific
resistance is tested by blowing air through the web at a particular
velocity and measuring the pressure drop across the web. A fiber
web having a high specific resistance is preferred, as the filter
media is more effective to capture small particles. FIG. 4
illustrates the effect of pH on specific resistance. As shown, webs
produced with a pH between 2 and 5 yield a filter media have a
specific resistance around 0.5 mm/lb, while webs produced having a
pH of at least about 6 have a specific resistance of at least about
0.6 mm/lb or greater. Fiber webs produced at a pH in the range of
about 8 to 10 yield filter media which show an even greater
increase in specific resistance to about 0.7 mm/lb or greater.
[0039] FIG. 5 illustrates the effect of pH on surface area, and in
particular that a change in pH from about 4 to about 6 or more
significantly increases the surface area of the resulting filter
media. In particular, as shown, the surface area of filter media
produced from a slurry having a pH of about 4.2 is about 1.25,
while the surface area of filter media produced from a slurry
having a pH of about 6 or more significantly increases to at least
1.44 or greater.
[0040] FIG. 6 illustrates the effect of pH on DOP penetration,
which is tested by blowing DOP particles through a filter media and
measuring the percentage of particles that penetrate through the
filter. A low DOP penetration is desirable since more particles are
captured by the filter. As shown in FIG. 6, webs produced at a pH
around 2 or 3 yield filter media having a DOP penetration in the
range of about 0.3% to 0.4%, and webs produced at a pH around 4
have a DOP penetration of about 0.18%. Conversely, webs produced at
a pH of 6 or more yield filter media that show a significant
decrease in the DOP penetration to about 0.03% or less. Filter
media having a DOP penetration of 0.02% or less are effective for
use in HEPA filters, and filter media having a DOP penetration of
0.001 or less are effective for use in ULPA filters. Thus, by
increasing the pH to at least about 6, filter media prepared
according to the present invention can advantageously be effective
for use in HEPA and ULPA filters.
[0041] FIG. 7 illustrates the effect of pH on Gamma value, which is
a measure of filtration efficiency, as previously discussed. The
Gamma value can be tested using a TDA 100P. As used herein, the
Gamma value refers to the value tested using Aerosol Penetrometer,
Model TDA-100P manufactured by Ait Techniques, Owings Mills, Md.
Again, webs produced at a pH of 6 or greater yield filter media
that show a significant increase in Gamma value. In particular,
webs produced at a pH in the range of about 2 and 4 yield filter
media that show a TDA 100P tested Gamma value of about 13, while
webs produced at a pH greater than about 6, and more preferably
greater than about 7, yield filter media that show an Aerosol
Penetrometer tested Gamma value of at least about 14.
[0042] Accordingly, FIGS. 3-7 illustrate a significant, unexpected
increase in filtration properties as a direct result of controlling
pH during web processing. Nonwoven glass fiber webs produced at a
pH of at least about 6, and more preferably in the range of about 7
to 10, yield filter media that show a significant increase in
apparent density, specific resistance, surface area, DOP
penetration, and Gamma value.
EXAMPLE 2
[0043] A slurry was prepared containing 50 lbs. of Evanite
706.times. fiber having an average fiber diameter of about
0.69.mu., 30 lbs. of Evanite 712.times. fiber having an average
fiber diameter of about 4.2.mu., 6 lbs. of Owens-Corning Chopped
Glass fiber DE having an average fiber length of about 0.25 inches,
and 1.7 lbs. of 1.7 denier Polyester fiber having an average fiber
length of about 0.25 inches. The slurry was prepared in a
hydropulper with a fiber concentration of about 2.7% and a pH at
about 9. The slurry was then transferred to a storage tank
containing additional water. The fiber concentration in the storage
tank was about 0.7%, and the pH was about 9.0. The slurry was then
fed from the storage tank into the headbox to yield a fiber
concentration less than about 0.2%. The headbox was maintained at a
pH of 9.2 and a sample of web was collected. The headbox pH was
then adjusted to 3.6 and 3.4 by the addition of sulfuric acid, and
samples of web were collected at a headbox pH of 3.6 and 3.4. The
experiment was repeated containing the same fiber formulation
combined with water and ammonium hydroxide to vary the headbox pH
between 4.1 and 8.2. Samples were collected at a headbox pH of 4.1,
6.6, 7.4, and 8.2. The properties of each sample were tested and
are shown in the chart below. The apparent density was determined
according to the following formula: 1 basis wt . ( g / m 2 ) /
caliper @ 10 Kpa ( mm ) 1000
2TABLE 2 Basis Caliper Apparent DOP Resistance Gamma Weight (mm @
Density pH (%) (mmH.sub.2O) (TDA 100P) (lbs) 10 Kpa) (g/cc) 9
0.0003 34 16.24 47.28 0.4708 0.162 9 0.0003 32.9 16.79 n/a n/a n/a
9.1 0.0026 29 15.84 45.59 0.4295 0.172 9.1 0.0023 29.5 15.72 n/a
n/a n/a 9.2 0 36.4 n/a 50.58 0.4748 0.173 9.2 0 36.1 n/a n/a n/a
n/a 9.3 0.0078 25.5 16.1 n/a n/a n/a 9.3 0.0079 25.8 15.9 n/a n/a
n/a 3.6 0.38 17.2 14.07 36.53 0.3 0.197 3.6 0.4 17.2 13.94 n/a n/a
n/a 3.4 0.33 17.3 14.34 34.11 0.274 0.202 3.4 0.36 17.4 14.07 n/a
n/a n/a 4.1 0.17 19 14.57 37.60 0.3133 0.194 4.1 0.16 19.2 14.56
n/a n/a n/a 5 0.085 20.4 15.05 36.63 0.3283 0.181 5 0.078 20.7
15.01 n/a n/a n/a 6 0.012 25.4 15.44 37.89 0.355 0.173 6 0.011 25.3
15.64 n/a n/a n/a 6.9 0.0022 29.2 15.95 39.81 0.3715 0.174 6.9
0.0017 29.2 16.33 n/a n/a n/a 7.4 0.0013 29.1 16.79 39.18 0.3735
0.17 7.4 0.002 29.1 16.15 n/a n/a n/a 8.2 0.0027 28.8 15.86 41.12
0.3965 0.168 8.2 0.0027 28.6 15.97 n/a n/a n/a
EXAMPLE 3
[0044] A two layer fiber web was prepared. The first layer was
formed from a slurry containing 10 lbs. of Evanite 706.times. fiber
having an average fiber diameter of about 0.69%, 50 lbs. of Evanite
712.times. fiber having an average fiber diameter of about 4.2.mu.,
12.5 lbs. of Owens-Corning Chopped Glass fiber DE having an average
fiber length of about 0.25 inches, and 2.1 lbs. of cellulose pulp.
The cellulose pulp was refined to a Canadian standard freeness
number of less than 200. The first layer was collected from a
headbox at a pH of about 3.2 to 3.3. The second layer was formed
from a slurry containing 50 lbs. of Evanite 706.times. fiber having
an average fiber diameter of about 0.69.mu., 6.25 lbs. of
Owens-Corning Chopped Glass fiber DE having an average fiber length
of about 0.25 inches, and 0.56 lbs. of cellulose pulp. The second
layer was collected on the first layer from a headbox at a pH of
about 7.0. Each layer had a basis weight in the range of about 34
to 37 g/m.sup.2 by weight. The properties of each sample were
tested and are shown in the chart below.
3 TABLE 3 Basis Weight 74 g/m.sup.2 (TAPPI T-410 om-98) Thickness
0.371 mm Apparent Density 0.199 g/c.c. DOP penetration 0.010%
Resistance 26.1 mm Gamma(TDA 100P) 15.3 Stiffness - Machine
Direction 286 mg (TAPPI T-410 om-98)
[0045] One of ordinary skill in the art will appreciate further
features and advantages of the invention based on the
above-described embodiments. Accordingly, the invention is not to
be limited by what has been particularly shown and described,
except as indicated by the appended claims. All publications and
references cited herein are expressly incorporated herein by
reference in their entirety.
* * * * *