Note: Descriptions are shown in the official language in which they were submitted.
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S COATFn AnSORRFl~T FIBF.l~S
I. Field of the Invention
The present invention relates to the use of certain activated m~t~ri~lx to absorb
noxious substances in gaseous and liquid waste streams.
II. Government Rights
This invention was made with Governm.ont support under awarded Contract DMR-
9208545 by the National Science Foundation. The Government has certain rights in this
mventlon.
III. Background of the Invention
Concern for the environment has caused governm~nt, science and industry to seek out
new solutions for removing toxic and noxious m~teri~lx ("co.~ tx") from air and water,
and especially from waste streams. In particular, cont~min~tion of air with pollutants such as
C10 (which depletes the ozone layer), CO2, NOX, SOx, CO, CH4, and loc~li7.e~1 O3 has
become a problem near industrial sites, in large urban centers, and in areas down wind of
such places, where pollutants are carried by weather patterns and returned to earth as, for
example, acid rain. Water pollution, including soil and groundwater co~ tion, also
presents a serious environmental ha_ard.
Science has advanced in its ability to detect increasingly small quantities of
c~ ntx Thus, we now are able to detect the adverse consequences resulting from the
presence of even minute quantities of certain such co~ x For example, small
amounts of cont~min~nt.~ such as PCBs and dioxin are known to cause adverse health effects
in ~nim~lx and in humans. Other cont~min~ntx, such as CO2 and methane, have been held
responsible for global warming. Still other co.lt~ x, like CO, are of concern to people
in their homes, as well as more generally, because CO is present in the emission streams of
automobiles and cigarettes. And chlorofluorocarbons, used as refrigerants and in the
production of certain types of foams, have been found partly responsible for depleting the
ozone layer that protects the earth and its inhabitants from the effects of ultraviolet radiation.
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WO 96138232 PCT/US96/06561
Governm~nt regulation of c~ emission has resulted in m~n~l~tes for the
virlual elimin~tion of certain co.~ .llx, and in limitations upon the emission of other
co..l~...il.:l..l~, in order to protect the environment in general, and human health in particular.
For example, int~rn~tional accords have been reached to m~ntl~te the elimin~tion of
chlorofluorocarbons from industry because of their adverse effects on the environment and, in
10 turn, human health.
Science and industry, too, have proposed solutions to the problems posed by
cont~min~nt~, in order to permit valuable technologies to be utilized despite the generation of
co.~t~"il~nt~ by those technologies. Where the col~ ,lL~ are present in waste streams,
these solutions principally are directed to the creation of mech~ni~m~ to remove the
15 cont~min~nt~. These mech~ni~m~ include devices such as scrubbers, filters, and other
mechanical and chemical systems for removing cont~min~nt~ from waste streams. Such
devices have been responsible for great decreases in the level of cont~min~nt~ found in waste
streams. Such devices may not, however, be cost effective to limit the amount of a noxious
substance found in a particular waste stream. Indeed, the utility of such devices depends
20 upon the particular co~ n~in~ involved, the amount of such material present in the waste
stream, the acceptable level of such materials in the environment (which frequently is
determined by governm~nt regulation), and the costs and benefits achieved from the various
options available to ~limini~h the quantity of col ~1~l " i ~ to acceptable levels while still
permitting the commercial utilization of the underlying technology responsible for the waste
25 stream.
Such solutions have not, however, been as effective as needed to remove minute
quantities of cont~min~nt.~. Typically, activated carbon granules (produced from organic
precursors such as coal, wood, almond shells, coconut shells, etc.) or fibers (produced from
organic and synthetic fiber precursors) have been employed to create sites where such
30 substances can be absorbed as a waste stream passes through a filter made from such
activated carbon materials. Such activated carbon materials are frequently produced from
CA 02236204 l997-ll-26
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S synthetic fibers of m~t~ri~lc such as phenolic resins, such as resols and novolacs, which are
treated with supe.rhe~ted CO2 or steam to carbonize and activate the m~teri~l and to increase
the surface area of the m~teri~l by creating pores in the m~tPriQl~.
Activated carbon m~tPri~l~ t.,vpically have been most effective in removing undesired
substances having a pH above 7.0, because the activation process in carbon tends to create
10 sites that are slightly acidic (pH < 7.0). Thus, activated carbon materials have not been
particularly effective in removing such hnpolL~ll, acidic pollutants as NOX and SOx.
However, a variety of chemical tre~tmPnt~ of such fibers have been proposed to create
dirrel~ tPcl sllrf~ces capable of absorbing many dirrel~ co,ll;~ nt~.
The processes for producing activated carbon fibers also have been limited as a result
15 of the extreme weight losses realized in the production of such fibers. Weight loss is an
hllpol l~ t limitation on the cost-effectiveness of such fibers because it correlates inversely
with the amount of col-~...in~.L that can be absorbed upon the surface of the fiber. The
technique of carbonizing the synthetic precursors to such fibers also produces materials that
are brittle or frangible, limiting their utility to systems in which some type of structural
20 support or co..~ ....Pnt for such fibers is permitted. This tends to increase the cost of using
the activated carbon fibers. Moreover, conventional activated carbon fibers exhibit poor
mechanical plupcllies, and are unavailable or ~A~nsive to produce in forms such as woven
fabrics, felts, or papers.
IV. Summar~ of the Invention
The present invention provides a fiber material for absorbing col.l;1.. i.. ~.,L~ that
overcomes the problems described above, and that offers greater flexibility in applications.
The fiber m~teri~l can be made by coating a fiber substrate with a resin, cross-linking the
resin, heating the etchant and resin to carbonize the resin, and exposing the coated fiber
substrate to an etchant to activate the resin.
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S Thus, it is an object of the invention to provide a method of making an adsorbent
material suitable for use in a wider variety of application than convention fibers because of
superior mechanical plop~.lies.
Another object of the invention is to provide a lower cost method of producing such
adsorbent m~t~-.ri~l~
Still another object ofthe invention is to provide such materials in forms suitable for
use as extremely high efficiency filters.
These and other objects of the invention are described in greater detail below, with
refele.lce to specific examples and embolliment~ of the invention.
V. Brief Description of the Drawings
Figures 1-5 are adsorption isotherms illustrating the efficiency of the materials made
according to the invention for adsorption of CO2, ethane, acetone, butane and HCl.
VI. Detailed Description of the Invention
The invention employs a phenolic resin in the form of a low viscosity melt, or in a
solution (such as an ethanol solution). The resin may be a novolac resin, a resol, or a low
viscosity pitch, but other resins that will produce a reasonable concentration of chars (as low
as 10% by weight) also may be used in order to achieve certain desirable characteristics in the
final product. The resin or resin solution is exposed to a suitable cross-linking agent (which
are generally known in the art to include resols, hexamethylen~t~ e, and ~ Lules of
hydrochloric acid with formaldehyde, but also may be air (depending upon the resin
employed)). Thus, for example, one may employ an ethanol solution of a novolac resin and
5-14% by weight of a cross-linking agent such as hexamethylenetetramine.
The resin (and, where combined in solution or mixture, the cross-linking agent) is
applied to coat an inert fiber substrate (such as a substrate made from glass fibers or mineral
fibers), which may take the form of a woven or nonwoven fabric, a felt, or even paper, with
the dissolved resin. The fiber substrate material is coated preferably by dip coating, vacuum
impregnation, or spraying. The coated fiber then is cured in a conventional manner to trigger
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S the cross-linking reaction, as for example, by heating to about 165~C if the cross-linking
agent is h~mP,thylenetcll~l~ille. (If a phenolic resin solution is employed, the solvent must
first be removed by heating the coated substrate to a tclll~,dlule sufficiently high to
volatilize the solvent.) Curing may take place in one or more steps over a succes~ion of
tempcl~ cs, in order to hlclcase the concellLIdlion of chars in the coating and minimi7e the
10 amount of coating that is vol~tili7~1
The cured, coated m~teri~l then is activated to produce an activated carbon-coated
assembly by proce~ing the m~t~.ri~l in a heated environment co~ g an etçh~nt
according to any of several techniques tli~cucsecl below. The specific technique employed
will vary, depending upon the desired pore size and surface chemistry in the final material to
15 be produced. The activation temperature and time, along with the etchant, will determine the
specific pore size and surface c~l~onni.~try. In general, increase in activation temperature and/or
time will produce a surface having a larger pore size than a correspondingly lesser activation
lclllpeldlule and/or time. Likewise, if the etchant is selected to produce a chemically active
fiber (~le~igntocl to selectively adsorb acidic or basic cont~min~ntc, for exarnple), increasing
20 the flow rate of the etchant increases the concentration of chemically active species in the
fiber.
Thus, to produce a fiber assembly having basic surface chemistry (which is desirable
for adsorption of acidic cont~min~nt~), the coated material is activated in ammonia (or a
llliXLulc of nitrogen and hydrogen) at 400-900~C for a period of time ranging from minlltes to
25 hours, depending upon the desired pore size. The resulting assemblies have B.E.T. surface
areas ranging from 400-1600 m2/g calculated based upon the weight percent of resin coating.
The use of ammonia as an etchant produces a material with a coating having a nitrogen
content from about 1-10% by weight.
Similarly, to produce materials having an acidic surface chemi.ctry, and a pore size of
30 less than about 7A, the coated material is activated in air at 300-450~C for several minutes to
several hours. Once again, the pore size increases with increased activation tc,l,pe,dlu,e
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W 096/38232 PCT/U~ Cl
and/or time. The reslllting fibers have B.E.T. surface areas ranging from 50-650 m2/g. The
use of air as the etchant produces a fiber with a coating that is 20-30% oxygen by weight.
Other oxidants (such as ll~ixLu-es of sulfuric and nitric acids, or mixtures such as hydrogen
peroxide and acetic acid) also may be used as elcl~ll~ to create acidic surface çh~mi.ctries.
Weakly acidic chPmictries also can be produced using the present invention by using
steam or CO2 as the etchant. The coated fibers are activated in steam or carbon dioxide at
600-900~C for several "~ es to several hours, and, as above, the pore size will increase in
relation to increased activation time and/or tellll)~.dlule. Fibers produced using this
embodiment of the invention are believed to exhibit B.E.T. surface areas ranging from about
600-2800 m2/g and an oxygen content of 0-5% by weight.
Inert gases, such as argon or nitrogen, also may be used to activate the coating applied
to the fiber substrate. It is believed that such inert gases activate the coating by causing the
coating to tent over the fiber matrix (which prevents the coating from shrinking upon
heating). In this embodiment, the coated fibers are placed in a high lelllp~ .dLule environment
(above 600DC) under a flow of inert gas, and held for a period of time sufficient to create
pores of a desired size.
Pores also may be created in the coating by other techniques. For example, soluble
inorganic compounds, in the form fine particulates (on the order of l~lm), may be dispersed
within the resin prior to coating. Such inorganic m~tçri~lc must, in order to be used in the
invention, remain stable through at least the curing step of the process for making the coated
fibers, and if an etching step also is used for the particular embodiment, must be stable
through the etching step. Thereafter, the coated materials co~ g the soluble inorganic
matter is placed in a solvent to dissolve the soluble inorganic material. The solvent must be
selected to avoid degrading or dissolving the phenolic resin coating.
Likewise, one or more polymers in addition to the resin that is used for coating the
fibers may be added to the coating. These polymers are selected because they degrade or
volatilize at lower temperatures (such as poly(ethylene oxide)) than the resin coating.
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S Following the curing step, these m~t~ri~lc are vol~tili7e-1, usually by degradation processes,
leaving behind pores of a size ~letermin~cl by the particular polymer employed and the
concentration of the polymer.
Finally, the resin coating may include one or more metals (in trace amounts) that are
used to catalyze chemical reactions. For example, such metals as chro~ l, copper,
10 lil~~ n, or nickel may be included in the resin prior to coating, to catalyze the
decomposition of toxic gases. The metals also may be added after activation by coating the
fibers with a l~ ulc of catalyst and a solvent, and then vaporizing the solvent. This would
be particularly advantageous where the coated fibers are used in devices such as gas masks to
adsorb gases employed in chemical w~ur~c.
Thus, in general, it may be desirable to produce a fiber having a coating with small
pore sizes for use in adsorbing extremely small molecules, such as (for example) CH4, Rn,
NH3, SOx, and HCl. The surface ch~rnictry variations permitted by the present invention can
also be manipulated, along with the pore size, to produce fibers tailored to adsorb specific
co~ i "~ . " .~.
Examples illustrating the method of making fibers according to the invention, and the
benefits obtained thereby, are set forth below.
EXAMPLE I
52.95 g of novolac (GP2006) were mixed with 5.92 g h~x~methylenetetramine, and
then dissolved in 73.11 mL ethanol to make a solution of 48/50 weight percent resin.
25 Preweighed samples of fiberglass lcil~lcelllent (plast #257) made of woven S2 glass fibers
were dipped into the resin solution and cured in a tube furnace under argon at a flow rate of
130-200mL/min. The coated fiberglass was heated in the furnace to 100~C for 20 min to
remove the solvent. The coated fiberglass then was cured, first by heating the coated
fiberglass to 150~C for 20 min, and then at 170~C for an additional 20 min. Descriptions of
30 the samples so produced are set forth in Table I.
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TABLE I
Weight of Coated
Sample No. Weight of Uncoated Fibc. glass, After Weight % Resin
Fiberglass Curing
0.7313 1.0501 30.4
2 0.623 0.3085 33.09
3 0.7948 1.2129 34.47
4 0.8819 1.3295 33.67
0.6923 0.8899 22.2
6 0.7904 1.0403 23.4
7 0.7098 0.9269 23.4
8 0.8400 1.1429 26.5
9 0.7648 1.0879 29.6
0.5588 0.8190 31.77
11 0.4898 0.7571 35.31
It has been found that impregnation of more than 35% resin by weight limits the flexibility of
woven coated material; however, for nonwoven materials, substantially more resin (up to
about 60% by weight resin) may desirably be employed.
The coated fiberglass then is activated by exposing the fiber to ammonia at a high
temperature to produce a very high surface area carbon fiber with basic surface chemistry
permitting adsorption of acidic co~ ."il-~nt~. Activation is accomplished by placing the
coated fiberglass in a tube furnace or other similar heating device and flowing ammonia over
the material while heating it. This has the effect of etching the coating to produce a basic,
15 microporous coated assembly which retains most of its weight, and exhibits a marked
increase in surface area and nitrogen content, revealing the basic surface chemistry of the
fiber. An example of the etching process and its results follows.
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EXAMPLE II
Samples of phenolic-coated glass fiber m~t~ri~l, ranging in mass from 0.6-0.9 g, were
placed in a tube furnace and heated in amrnonia at a flow rate of 200mL/min until reac~ing
600-800~C and held at that ~e~ e~ e for a predelf ~ d time. The samples were then
cooled, and held at 200~C for two hours. The char~ctçri~tics of the samples are set forth in
Table II.
TABLE II
Original
Sample No. Reaction Reaction Weight Loss Resin Activation
Temp. (~C) Time(hrs) (g) Weight(g) Yield (~/O)
600 1 0.0929 0.2262 58.9
2 700 1 0.0929 0.2079 55.32
3 800 0.5 0.0766 0.1573 51.3
4 800 1 0.1248 0.2166 42.38
800 2 0.1523 0.2330 34.64
The surface area of each of these samples was measured using nitrogen adsorption techniques
at 77~K with a Micromeritrics ASAP 2400 and elemental analysis techniques using a Control
15 Eq~ ment Corp. 240XA elem~nt~l analyzer. The results of those analyses is set forth in
Table III.
TABLE III
B.E.T.
Sample No. Surface Nitrogen Hydrogen
Area Carbon (~/O) (~/O) Oxygen (%) (%)
(m2lg)
710 69.95+/- 1.373+/-0.44 26.804 1.873+/-
13.74 0.0907
2 853.38 92.64+/-9.55 5.36+/-0.846 0.63 1.37+/-0.081
3 807.39 65.37+/-7.19 3.48+/-0.415 30.241 0.909+/-0.2
4 1107.48 80.25+/-2.46 4.67+/-0.433 13.94 1.14+/-0.086
1245 87.61+/-7.28 4.86+/-1.12 5.76 1.77+/-0.484
The coated fibers may be used for adsorption of acidic, polar, and nonpolar gases. As
20 shown in the adsorption isotherms of Figures 1-5, CO2, ethane, acetone, HCl and butane all
may be efficaciously adsorbed upon the coated fibers of the present invention. The isotherms
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S of Figures 1~5 were prepared by mP~cll-ing adsorption at room t~ eldl~e using volurnetric
techniques (with a Coulter Omnisorb 100) and gravimetrically (using a TGA 951 connected
to a TA Instrurnents 2100 system C~ ULC1 and three Tylan General FC-280 mass flow
controllers used to dilute standard concentration gas to lower concentration). These results
reveal that the activated carbon-coated fibers of the present invention are much more cost-
10 effective per unit weight than conventional activated carbon fibers for removing
Col~t~ ntc.
In another embodiment of the invention, the coated fibers are activated in heated air toproduce activated carbon coated fibers have acidic surface chemistry in order to permit the
desirable adsorption of small molecules having a basic chemistry. The coated fibers also may
15 be activated by other heated gases, such as argon and carbon dioxide, to produce desirable
surface chemistries. These techniques are described more fully in the examples set forth
below.
EXAMPLE III
Samples of phenolic-coated glass fiber material, ranging in mass from 0.6-0.9 g, were
20 placed in a tube furnace and heated in air at a flow rate of 210 mL/min, where they were held
for five ~inl~lec at 400~C, and then heated to 450~C and held for a predetermined time, as
specified in Table IV. The samples then were cooled in argon and held at 200~C for two
hours. The characteristics of the sarnples are set forth in Table IV.
TABLE IV
Reaction Original B.E.T.
Sample Time Weight Resin Activation Surface
No. (minutes) Loss Weight Yield (%) Area(m2/g)
(g) (g)
0.0294 0.1023 71.3 196
2 20 0.0375 0.0974 61.5 230
3 25 0.0365 0.0894 59.2 252
4 37 0.0531 0.080064 33.7 452
31 0.0436 0.078672 44.6 407
6 45 0.0559 0.066384 15.8 318
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The surface area of each of these samples was measured using nitrogen adsorption techniques
at 77~K with a Micromeritrics ASAP 2400 and elem~nt~l analysis techniques using a Control
Equipment Corp. 240XA elem~?nt~l analyzer.
EXAMPLE IV
In another embodiment of the invention, the coated glass fibers were activated in
10 argon. A coated fiber assembly was placed in a tube furnace that was purged with argon for
ten ~ es at a flow rate of 4800 mL/min. The sample was held in the furnace at 200~C for
ten lllhlules in argon at the same flow rate, and then heated to 600~C and held at that
le,l,pe,a~ule for 25 mimltes The sample was allowed to cool to room temperature in argon,
and then degassed in argon at 200~C for two hours, producing a coated carbon fiber with a
15 B.E.T. surface area of 641m2/g.
EXAMPLE V
Phenolic resin-coated glass fiber assemblies were activated in carbon dioxide byplacing the fibers in a tube furnace under argon for 50 minlltes at a flow rate of 2990 mL/min.
The samples then were heated in carbon dioxide at a flow rate of 503 mL/min. to the desired
20 telllpeldlure (ranging from 600-800~C) for a predet~rmin~ length of time. The sample then
was cooled in argon and held at 200~C for approximately 2 hours at a flow rate of
2990 mL/min. This creafed a porous carbon-coated fiber having the characteristics set forth
below in Table V.
TABLE V
B.E.T.
Sample No. Reaction Reaction Acli~alion Surface
From Temperature Time (hrs) Yield Area (m2/g)
Example I (~C)
600 1.5 62.87 744
700 1.5 61.6 600
11 800 1.5 48.47 800
3 800 3 43.2 900
11
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Fibers made according to the present invention may be regenerated using any of the
collv~lllional regen. .d~ion techniques, including heating in nitrogen, electrical resistance
he~ting, or other conventional regeneration techniques that do not produce degradation. Low
telllpel~lul~ heating of the fiber assembly following co~ adsorption will return the
fiber to its original working capacity so that the fiber assembly may be used again. Purging
10 techniques, by which the col~ is chemically displaced from the fiber, also may be
used to remove co"~ "il-~-t~ from the samples, but less effectively than the heating
techniques mentioned above.
EXAMPLE VI
Novolac solutions were prepared with varying ratios of novolac to
h~x~methylenetetramine concentrations, to alter the final cross-link density upon curing.
These solutions also contained dir~ quantities of solvent (ethanol) to vary the viscosity.
Nonwoven fiberglass reinforcement m~tPri~l from Fibre Glast Developments Corp. were
impregn~te~l under vacuum using standard vacuum impregnation techniques, to produce
coated fibers up to 45% by weight resin. These samples were heated in dirr~lelll air/nitrogen
mixtures and reacted at telllpelaLul~ s from 350-750~C for a period ranging from several
hlu~es up to one hour.
Accordingly, the activated, coated fiber assemblies of the present invention exhibit
notable advantages over conventional activated carbon fibers. First, the wide range of fiber
substrate materials that may be used in the present invention offers greater versatility than
activated carbon fibers. Second, the starting materials are of far lower cost than conventional
materials, and exhibit better mechanical integrity and wear resistance than conventional
activated carbon materials. The coated fibers of the invention also exhibit resi.~t~nce to
shrinkage, resulting in higher surface areas and higher yields than conventional activated
carbon fibers under similar activation conditions. And the techniques of the present invention
make it far easier to less expensive to m~nllf~rture and process materials for cont~min~nt
removal.
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Moreover, the invention is able to produce both coated materials on woven and
nollwo\~en substrates, useful to adsorb co~ and, as noted above, to catalyze their
decomposition. Nonwoven m~ri~l~ having a high weight percentage (45-60%) of resin may
also be used as filters for extremely fine particulates, germs and molecules, because of their
extremely fine pore structure.
Finally, the coated fibers may be used as ion exchange systems, by further processing
of the coated fibers. For example, it is believed that the fibers made according to Example I
may be heated at about 100~C in concellll~led sulfuric acid under inert conditions to produce
a sulfonated coated fabric. Likewise, a fiber assembly could be impregnated with a
melamine, oligomeric resin lllixtule to create an ion exchange coated assembly.
The present invention has been described with respect to certain embodiments andconditions, which are not meant to and should not be construed to limit the invention. Those
skilled in the art will understand that variations from the embodiments and conditions
described herein may be made without departing from the invention as claimed in the
appended claims.
