U.S. patent application number 13/123097 was filed with the patent office on 2011-08-18 for high flux high efficiency nanofiber membranes and methods of production thereof.
This patent application is currently assigned to THE RESEARCH FOUNDATION OF STATE UNIVERSITY OF NEW YORK. Invention is credited to Benjamin Chu, Benjamin s. Hsiao, Hongyang Ma.
Application Number | 20110198282 13/123097 |
Document ID | / |
Family ID | 42101191 |
Filed Date | 2011-08-18 |
United States Patent
Application |
20110198282 |
Kind Code |
A1 |
Chu; Benjamin ; et
al. |
August 18, 2011 |
HIGH FLUX HIGH EFFICIENCY NANOFIBER MEMBRANES AND METHODS OF
PRODUCTION THEREOF
Abstract
A membrane is provided including a coating layer having
cellulose nanofibers produced from oxidized cellulose microfibers
and an electrospun substrate upon which the coating layer is
applied. The nanofibers of the electrospun substrate have a
diameter greater than that of the cellulose nanofibers. The
membrane also has non-woven support upon which the electrospun
substrate is disposed. Microfibers of the non-woven support have a
diameter greater than that of the nanofibers of the electrospun
substrate. Application of electrospun membrane is in
microfiltration area, while the cellulose nanofiber membrane serves
in ultra-filtration, nanofiltration, and reverse osmosis after
chemical modification.
Inventors: |
Chu; Benjamin; (Stony Brook,
NY) ; Hsiao; Benjamin s.; (Stony Brook, NY) ;
Ma; Hongyang; (Stony Brook, NY) |
Assignee: |
THE RESEARCH FOUNDATION OF STATE
UNIVERSITY OF NEW YORK
Stony Brook
NY
|
Family ID: |
42101191 |
Appl. No.: |
13/123097 |
Filed: |
October 7, 2009 |
PCT Filed: |
October 7, 2009 |
PCT NO: |
PCT/US2009/059884 |
371 Date: |
April 7, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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61103479 |
Oct 7, 2008 |
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61146939 |
Jan 23, 2009 |
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61153669 |
Feb 19, 2009 |
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61153666 |
Feb 19, 2009 |
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Current U.S.
Class: |
210/500.29 ;
210/500.28; 427/372.2; 427/430.1; 428/213; 428/221 |
Current CPC
Class: |
B01D 2239/0631 20130101;
B01D 2239/1233 20130101; Y10T 428/2495 20150115; B01D 2239/025
20130101; D01F 2/24 20130101; B01D 2239/0478 20130101; D01F 2/00
20130101; B01D 67/0002 20130101; D06M 13/123 20130101; B01D 39/1623
20130101; D01D 5/0084 20130101; B01D 2323/39 20130101; B01D 39/18
20130101; B01D 71/38 20130101; B01D 39/1615 20130101; Y10T
428/249921 20150401; B01D 71/12 20130101; B01D 2239/1208 20130101;
D06M 17/00 20130101; B01D 2239/1216 20130101; B01D 2239/0654
20130101; B01D 2239/10 20130101; D06M 15/263 20130101; B01D 69/10
20130101 |
Class at
Publication: |
210/500.29 ;
427/430.1; 427/372.2; 210/500.28; 428/221; 428/213 |
International
Class: |
B01D 71/08 20060101
B01D071/08; B05D 1/18 20060101 B05D001/18; B05D 3/02 20060101
B05D003/02; B01D 67/00 20060101 B01D067/00; B01D 71/12 20060101
B01D071/12; B01D 69/10 20060101 B01D069/10 |
Goverment Interests
GOVERNMENT RIGHTS
[0002] The invention of the present application was made with
government support under grant number N0014-03-1-0932 awarded by
the Office of Naval Research. The government has certain rights in
the invention.
Claims
1. A membrane comprising: a coating layer having a non-woven
structure of polysaccharide nanofibers, wherein the polysaccharide
nanofibers have a diameter between 5 to 50 nanometers (nm); an
electrospun substrate having a non-woven structure upon which the
coating layer is applied, wherein nanofibers of the electrospun
substrate have a diameter greater than that of the polysaccharide
nanofibers; and a non-woven support upon which the electrospun
substrate is disposed, wherein microfibers of the non-woven support
have a diameter greater than that of the nanofibers of the
electrospun substrate.
2. The membrane of claim 1, wherein the nanofibers of the
electrospun substrate have a diameter between 50 and 300 nm.
3. The membrane of claim 1, wherein the microfibers of the
non-woven support have a diameter between 5 and 20 micrometers
(.mu.m).
4. The membrane of claim 1, wherein the polysaccharide nanofibers
comprise at least one of cellulose nanofibers, oxide cellulose
nanofibers, chitin nanofibers and chitosan nanofibers.
5. The membrane of claim 1, wherein the electrospun substrate
comprises polymers from a group consisting of polyolefins,
polysulfones, fluoropolymers, polyesters, polycarbonates,
polystyrenes, polynitriles, polyacrylates, polyacetates and
copolymers thereof.
6. The membrane of claim 1, wherein a thickness of the coating
layer is smaller than a thickness of the electrospun substrate and
the thickness of the electrospun substrate is smaller than that of
the non-woven support.
7. The membrane of claim 6, wherein the thickness of the coating
layer is between 10 and 500 nm.
8. The membrane of claim 6, wherein the thickness of the
electrospun substrate is between 10 and 50 .mu.m.
9. The membrane of claim 6, wherein the thickness of the non-woven
support is between 100 and 250 .mu.m.
10. The membrane of claim 1, wherein a maximum pore size of the
electrospun substrate is approximately three times a mean pore size
of the electrospun substrate when a porosity of the electrospun
substrate is approximately 80%.
11. The membrane of claim 1, wherein a mean pore size of the
electrospun substrate is approximately three times larger than the
diameter of the nanofibers of the electrospun substrate, when a
porosity of the electrospun substrate is approximately 80%.
12. The membrane of claim 1, wherein the polysaccharide nanofibers
of the coating layer are cross-linked via at least one of a heating
process, immersion in glutaraldehyde (GA), and incorporation of
polyacrylic acid (PAA) in the coating layer.
13. A membrane comprising: a coating layer comprising a mixture of
nanofibers comprising PolyVinyl Alcohol (PVA) and cellulose; an
electrospun substrate upon which the coating layer is applied,
wherein nanofibers of the electrospun substrate have a diameter
greater than that of the nanofibers of the coating layer; and a
non-woven support upon which the electrospun substrate is disposed,
wherein microfibers of the non-woven support have a diameter
greater than that of the nanofibers of the electrospun
substrate.
14. The method of claim 13, wherein a defined amount of the
electrospun substrate is embedded into the coating layer to
reinforce mechanical strength of the coating layer.
15. The membrane of claim 13, wherein a maximum pore size of the
electrospun substrate is approximately three times a mean pore size
of the electrospun substrate when a porosity of the electrospun
substrate is approximately 80%.
16. The membrane of claim 13, wherein a mean pore size of the
electrospun substrate is approximately three times the fiber
diameter of the nanofibers of the electrospun substrate, when a
porosity of the electrospun substrate is approximately 80%.
17. A membrane comprising: a coating layer having a non-woven
structure format of at least one of cellulose nanocrystals and
microcrystals; an electrospun substrate having a non-woven
structure format upon which the coating layer is applied, wherein
nanofibers of the electrospun substrate have a diameter greater
than that of the cellulose nanocrystals and microcrystals; and a
non-woven support upon which the electrospun substrate is disposed,
wherein microfibers of the non-woven support have a diameter
greater than that of the nanofibers of the electrospun
substrate.
18. A method for producing a membrane, the method comprising steps
of: producing a coating layer having a non-woven structure format
from polysaccharide nanofibers, wherein the polysaccharide
nanofibers have a diameter between 5 to 50 nanometers (nm);
immersing an electrospun substrate having a non-woven structure
format into a water-based solution; and applying the coating layer
to the electrospun substrate, wherein a gel barrier is formed at an
interface between the electrospun substrate and the coating layer
to slow diffusion of the coating layer into the electrospun
substrate, and wherein nanofibers of the electrospun substrate have
a diameter greater than that of the polysaccharide nanofibers;
wherein the electrospun substrate is disposed on a non-woven
support having microfibers with a diameter greater than that of the
nanofibers of the electrospun substrate.
19. The method of claim 18, further comprising heating the coating
layer after application to the electrospun substrate to cross-link
the polysaccharide nanofibers.
20. A membrane comprising: an electrospun substrate having a
non-woven structure format; and a non-woven support upon which the
electrospun substrate is disposed, wherein microfibers of the
non-woven support have a diameter greater than that of nanofibers
of the electrospun substrate; wherein a maximum pore size of the
electrospun substrate is approximately three times a mean pore size
of the electrospun substrate when a porosity of the electrospun
substrate is approximately 80%; and wherein a mean pore size of the
electrospun substrate is approximately three times the fiber
diameter of the nanofibers of the electrospun substrate, when a
porosity of the electrospun substrate is approximately 80%.
21. The membrane of claim 20, wherein the nanofibers of the
electrospun substrate have a diameter between 50 and 300 nanometers
(nm).
22. The membrane of claim 20, wherein the microfibers of the
non-woven support have a diameter between 5 and 20 micrometers
(.mu.m).
23. The membrane of claim 20, wherein a thickness of the
electrospun substrate is smaller than that of the non-woven
support.
24. The membrane of claim 23, wherein the thickness of the
electrospun substrate is between 10 and 50 .mu.m.
25. The membrane of claim 23, wherein the thickness of the
non-woven support is between 100 and 250 .mu.m.
Description
PRIORITY
[0001] This application claims priority under 35 U.S.C.
.sctn.119(e) to provisional applications filed in the U.S. Patent
and Trademark Office on Oct. 7, 2008, Feb. 19, 2009, and Feb. 19,
2009, respectively assigned Ser. Nos. 61/103,479, 61/153,666 and
61/153,669, the contents of which are incorporated herein by
reference.
BACKGROUND OF THE INVENTION
[0003] 1. Field of the Invention
[0004] The present invention relates generally to nanofiber
filtration and, more particularly, to a three-layer filter membrane
having a top coating layer with cellulose nanofibers.
[0005] 2. Description of the Related Art
[0006] The present application is related to Publication No. US
2009/0078640 A1 of U.S. patent application Ser. No. 12/126,732,
filed May 23, 2008, and provisional application Nos. 60/931,765 and
60/947,045 filed on May 26, 2007 and Jun. 29, 2007, respectively,
the contents of each of which is incorporated herein by
reference.
[0007] A unique class of nanofibrous membranes with fiber diameters
of approximately 100 nanometers (nm) and lengths on the order of
thousands of meters have been created by precision multi jet
electrospinning technology. This technology takes advantage of a
non-woven nanofibrous structure with uniform distributions of fiber
diameter and membrane pore size that can significantly improve the
flux of water transport at low operating pressures without loss of
selectivity. The diameter of a fiber prepared from an
electrospinning technique ranges from 1 micron to 50 nm. This
implies that a mean pore size of the nanofiber membrane will range
from 3 microns to 150 nm, enabling its use as a microfiltration
membrane. However, it is difficult for the electrospinning process
to prepare a nanofiber having a diameter that is less than 50
nm.
[0008] Methods have also been developed for the fabrication of
cellulose nanofibrous scaffolds from cellulosic biomass. These
nanofibers have diameters of approximately 5 nm and lengths of a
few micrometers (.mu.m). The use of cellulose nanofibers for water
filtration is especially advantageous because the surface of
cellulose nanofibers can be functionalized to guide the flow of
water inside water channels, or to selectively alter the adsorptive
or repulsive properties for particulate separation. The cellulose
nanofibers are mostly crystalline in nature and, unlike amorphous
cellulose, they have shown to be relatively bio-inactive.
[0009] A first type of nanofibrous membrane created through
electrospinning technology utilizes PolyVinyl Alcohol (PVA),
preferably on a non-woven PolyEthylene Terephthalate (PET)
substrate. Different concentrations (such as 6, 8, 10, 12 wt %) of
PVA solutions have different effects on fiber diameter in the
membrane. Due to the fact that electrospun PVA nanofibers can be
dissolved in water, the electrospun PVA membrane is chemically
cross-linked before use with one of many aldehydes, such as
GlutarAldehyde (GA) and glyoxal. A reaction forms acetal bridges
between the hydroxyl groups in PVA and the aldehyde molecules.
[0010] The maximum pore size of the electrospun membrane may be
determined by a bubble-point method, which is based on a pressure
measurement that is necessary to blow air though a liquid-filled
membrane. Water is preferably used as the wetting reagent. The
relationship between maximum pore size (d) and the corresponding
pressure is given by Young-Laplace Equation (1):
d = 4 .gamma.cos.theta. .DELTA. p ( 1 ) ##EQU00001##
[0011] A schematic diagram of the bubble point test set-up is shown
in FIG. 1. An immersed electrospun PVA membrane is placed in a
membrane cell 102 having a diameter of 1.2 inches. A syringe 104 is
connected to one end of the cell to provide the gas pressure, and a
pressure gauge 106 is connected to the other end to monitor the
pressure. A plastic tube is connected to the pressure gauge and
inserted within a water filled beaker 108 to observe the air
bubble. When the membrane is completely wetted by liquid, cos
.theta.=1, and .gamma. is the surface tension of the membrane. The
minimum pressure that blows the first air bubble is recorded, and
related to the maximum pore size of the membrane.
[0012] The pure water flux of the electrospun membranes is
characterized using a dead-end filtration set-up, as shown in FIG.
2. A water tank 202 is placed at the water level located at 1.6
meters higher than a membrane cell 204. Therefore, it provides a
differential pressure of 2.28 psi higher than gravity. The pressure
was kept within 1% deviation by adding water periodically to the
tank for all the measurements. Set-up of a rejection test is
achieved by replacing the pure water in the flux test by a
polycarboxylate feed solution.
[0013] As illustrated in FIG. 3, the average fiber diameter is
reduced to 140 nm at 28 kV and 100 nm at 32 kV in electrospinning.
The decrease in the average fiber diameter could be attributed to
the greater elongation force provided by the increase in the
electric field strength. FIG. 4 illustrates that viscosity values
for PVA solutions were found to increase as the concentration
increased. Specifically, there was a sharp increase in the
viscosity from 50 cp to 669 cp when the concentration was increased
from 10% to 12%.
[0014] FIGS. 5(a)-(d) show a series of Scanning Electron Microscope
(SEM) images in order to illustrate the effect of concentration of
PVA solutions on the morphological appearance of the electrospun
membranes. At a low concentration of 6% or low viscosity of 16 cp,
only a few nanofibers were produced, and a large number of
microdroplets were formed creating a porous film-like structure. As
the concentration was increased to 8% and 10%, beads gradually
became less and were eliminated at 10%, whereby a uniform
fiber-structure with the fiber diameter of 100 nm was formed. With
a further increase in concentration to 12%, beads were formed again
in the structure, and the fiber diameter increased to 150 nm.
[0015] The porosity of electrospun PVA membranes fabricated using
different PVA concentrations is shown FIG. 6. At a concentration of
6%, the porosity of the membrane is quite low, 57%. Other membranes
all exhibited porosity higher than 75%, and the largest reached 83%
at 10% concentration.
[0016] Using a 10% PVA solution, 32 kV for the electrospinning, and
membranes electrospun into sheets of 20 cm (width).times.30 cm
(length) at different thicknesses ranging from 3 .mu.m to 35 .mu.m,
properties of the membranes are listed in Table 1. The pure water
flux of Millex-GS is in the range of 1300-1400 (L/m.sup.2 h), with
average pore size determined by the image analysis of multiple SEM
images, sampled at different membrane locations.
TABLE-US-00001 TABLE 1 Thickness (.mu.m) 3 5 8 15 18 24 35 Porosity
85% 84% 85% 85% 85% 85% 83% Maximum pore size 21.0 14.0 8.4 3.2 2.8
1.4 0.4 (.mu.m) Average pore size* 7.0 4.7 2.8 1.1 0.9 0.5 0.1
(.mu.m) Flux (L/m.sup.2h) 16400 14900 11600 8500 8200 7500 5900
[0017] FIG. 7 illustrates that with a thickness of 8 .mu.m, a
membrane exhibits a rejection of 89% to 0.2 .mu.m microparticles.
With the membrane being thicker, the rejection increased to higher
than 95% and reached 98% as a highest value. Therefore, the
rejection of electrospun PVA membranes is also affected by the
thickness of the membrane.
[0018] A second type of nanofibrous membrane created through
electrospinning technology utilizes PolyAcryloNitrile (PAN)
solutions. Different wt % PAN solutions are prepared by dissolving
PAN powder in DiMethylFormamide (DMF) and stirring the solution at
60.degree. C. for 2 days until homogeneous. PAN/DMF is preferably
electrospun directly onto a PET substrate in an electrospinning
machine.
TABLE-US-00002 TABLE 2 Solution Apparatus Concentration Distance
Voltage E-field strength Sample (wt %) (cm) (kV) (kV/cm) A-1 4 6 18
3.0 .+-. 0.1 A-2 4 11 25 2.3 .+-. 0.1 A-3 4 19 30 1.6 .+-. 0.1 A-4
4 24 29 1.2 .+-. 0.1 A-5 6 6 14 2.3 .+-. 0.1 A-6 6 11 17 1.6 .+-.
0.1 A-7 6 19 26 1.4 .+-. 0.1 A-8 6 24 28 1.2 .+-. 0.1 A-9 8 6 13
2.2 .+-. 0.1 A-10 8 11 16 1.5 .+-. 0.1 A-11 8 19 23 1.2 .+-. 0.1
A-12 10 6 12 2.0 .+-. 0.1 A-13 10 11 14 1.3 .+-. 0.1 A-14 10 19 21
1.1 .+-. 0.1 B-1 4 6 18 3.0 .+-. 0.1 B-2 4 6 25 4.2 .+-. 0.1 B-3 4
6 30 5.0 .+-. 0.1 B-4 6 6 15 2.5 .+-. 0.1 B-5 6 6 20 3.3 .+-. 0.1
B-6 6 6 25 4.2 .+-. 0.1
[0019] Dead-end filtration cells are used for bubble point testing
on the membranes, in order to determine the maximum pore size. The
Young-Laplace Equation shown in Equation (1), after substitution,
is used to determine maximum pore size. Bulk porosity of the
electrospun membrane is calculated by Equation (2), where p is
density of electrospun PAN and .rho..sub.0 is density of PAN
powder:
Bulk porosity=(1-.rho./.rho..sub.0).times.100% (2)
[0020] FIG. 8 illustrates that both dynamic viscosity and
conductivity increase with increased solution concentration.
According to FIGS. 9, 10 and 11, nanofibrous membranes having
higher concentrations have a fairly constant larger fiber diameter,
without beads or melting parts. Higher conductivity increases the
stretching forces of the electric field on the polymer, which
typically results in a decreased fiber diameter. However, the
increasing intermolecular forces of the polymer solution,
represented by increasing viscosity, counteract the stretching of
the polymer. Additionally, at higher concentrations, the polymer
chains entangle to a greater degree and viscoelastic properties of
the solution favor thicker fiber formation, and not beads. 2 wt %
is likely to be close to the critical overlap concentration (1.8%)
for PAN, which prevents the stable formation of fibers.
[0021] With increasing voltage, the variability of the fiber
diameters increases, as shown in FIG. 12. Because increasing the
voltage has the same effect as increasing solution conductivity, it
is believed that the decreasing uniformity is due to electron
repulsion.
[0022] FIG. 13 illustrates the effects of average fiber diameter,
controlled by PAN concentration, on both maximum pore size and pure
water flux. For a given membrane thickness and amount of polymer,
the fiber length decreases with increasing fiber diameter per unit
area, resulting in a lower number of fiber crossings. A decreased
number of fiber crossings reduces the amount of times that pores
are defined per unit area, thereby increasing the pore size. This
is also consistent with the fact that smaller fiber diameter will
provide a smaller pore size. Flux and bubble point data for
electrospun membranes of various thicknesses, prepared from 6 wt %
solution, are listed in Table 3.
TABLE-US-00003 TABLE 3 Pure Water Flux Pure Water after Ethanol
Average Maximum Flux Pre-treatment Fiber Membrane Pore (L/m.sup.2h)
(L/m.sup.2 h) Diameter Thickness Size 1.sup.st 5.sup.th 1.sup.st
5.sup.th Sample # (nm) (.mu.m) (.mu.m) Minute Minute Minute Minute
E-1 110 .+-. 20 10 1.7 7100 2400 10000 6000 E-2 110 .+-. 20 20 0.9
5600 2000 6800 4100 E-3 110 .+-. 30 30 0.7 4400 2800 5700 3400
Millex-GS N/A N/A 0.7 1400 1300 1400 1300
[0023] The smallest maximum pore size attained was 0.7 .mu.m by
sample E-3, which is equal to that of a Millipore Millex-GS.TM.
microfiltration membrane. Pure water flux rates for this sample
were three times higher than those of Millex-GS during the 1st
minute and two times higher in the 5th minute. Sample E-3 exhibited
the highest rejection out of all of the electrospun membranes
produced, which was predicted by bubble point results. Compared to
Millex-GS, it showed significantly higher flux (2800 to 800) at
comparable rejection of 1 .mu.m particles, as shown in FIG. 14.
During filtration of 0.20 .mu.m particles, electrospun PAN
performed significantly better in both flux (2600 to 700) and
rejection (90% to 25%).
[0024] Cellulose nanofibers are new nano-scale materials, which can
be prepared from natural plants after chemical and mechanical
treatments. Nano-scale cellulose-based fibers have many
applications because of their smaller diameters and the ability for
surface modifications. Advantages of cellulose nanofibers over
other nano-scale materials are set forth below.
[0025] (1) The diameter of cellulose nanofiber is very small,
usually only .about.5 nm, implying higher surface area (about 600
m.sup.2/g) and higher slip flow for gas (e.g., air) filtration.
[0026] (2) The surface of cellulose nanofibers is very hydrophilic
since there is one primary hydroxyl group (12 mol % or more can be
transferred into carboxyl groups) and two secondary hydroxyl
groups, which can be utilized to change the hydrophilic nature of
the surfaces and thereby to construct liquid nano-channels.
[0027] (3) Highly functionalized surface of cellulose nanofibers
means that the chemical modification can be performed more easily
to achieve multiple functions, such as charged or chelating
properties.
[0028] (4) Biocompatibility of cellulose nanofibers is very good,
which permits biomedical applications. For long term use, such as
in hemodialysis, the complementary reactions have to be properly
taken into account, e.g., by reducing the active groups on
cellulose.
[0029] (5) Cellulose nanofiber aqueous solutions are pH sensitive
and ionic strength sensitive, permitting the formation of new
gel-like structures.
[0030] (6) The low concentration of cellulose nanofibers in an
aqueous solution can be utilized to prepare membranes with very
thin barrier layers, useful for low-pressure micro-filtration,
ultra-filtration, nano-filtration, and pre-filtration in reverse as
well as forward osmosis.
[0031] (7) Cellulose nanofibers can be fabricated from cellulose
under environmentally benign conditions, including the production
of bacterial cellulose.
[0032] (8) Cellulose nanofibers with oxidized carboxyl groups have
anti-bacterial properties. In addition, the surface property can be
modified to resist interaction with bacteria.
[0033] (9) Initial source materials for the preparation of
cellulose nanofibers are relatively cheap and easily available from
natural plants.
[0034] The conventional preparation of cellulose nanofibers
includes pre-treatment (swelling with alkali aqueous solution) of
cellulose fiber bundles, acid hydrolysis to remove pectin and
hemicellulose, alkali treatment again to remove lignin, high
impacted cryo-crushing to liberate the microfibril from the cell
wall, and high impacted and high sheared defibrillation to obtain
the individual nanofibers, as shown in FIG. 15.
[0035] The diameter of cellulose nanofibers prepared by the above
method is about 10 to 100 nm and having a yield of about 20%.
Moreover, many of the steps often used highly corrosive reagents,
such as strong acids and alkali. The cryo-crushing and
defibrillation processes require special devices, which can
seriously affect the extension of this method for large scale
operations.
[0036] One benign preparation of cellulose nanofibers is the
production of Bacterial Cellulose (BC) nanofibers using acetobacter
xylinum. BC fibers have a network structure with diameters in the
10 to 70 nm range and excellent physical properties.
[0037] Physical preparation of cellulose-based nanofibers can also
be achieved using the electro-spinning technology. A cellulose
solution can be prepared using an ionic liquid, such as
1-butyl-3-methylimidazolium chloride, N-methylmorpholine-N-oxide,
or a mixture of solvents. Alternatively, cellulose acetate
nanofibers are hydrolyzed, as fabricated by the electro-spun
method, by using an alkali aqueous solution. However, such
cellulose nanofibers have higher fiber diameter in an approximate
range of 300 to 1000 nm, and the process includes an additional
post-treatment step using either a toxic or volatile reagent.
[0038] Membranes suitable for filtration which involve one or more
of the above technologies can be found in International Publication
Nos. WO 2005/0049102 and WO 2007/001405.
SUMMARY OF THE INVENTION
[0039] The present invention has been made to address at least the
above problems and/or disadvantages and to provide at least the
advantages described below. Accordingly an aspect of the present
invention provides a high flux high efficiency nanofiber
membrane.
[0040] According to one aspect of the present invention, a membrane
is provided including a coating layer having a non-woven structure
format of polysaccharide nanofibers. the polysaccharide nanofibers
have a diameter between 5 to 50 nanometers (nm). The membrane also
includes an electrospun substrate having a non-woven structure
format upon which the coating layer is applied. Nanofibers of the
electrospun substrate have a diameter greater than that of the
cellulose nanofibers. The membrane further includes a non-woven
support upon which the electrospun substrate is disposed.
Microfibers of the non-woven support have a diameter greater than
that of the nanofibers of the electrospun substrate.
[0041] According to another aspect of the present invention, a
membrane is provided including a coating layer comprising a mixture
of nanofibers comprising PVA and cellulose. The membrane also
includes an electrospun substrate upon which the coating layer is
applied. Nanofibers of the electrospun substrate have a diameter
greater than that of the cellulose nanofibers. The membrane also
includes a non-woven support upon which the electrospun substrate
is disposed. Microfibers of the non-woven support have a diameter
greater than that of the nanofibers of the electrospun
substrate.
[0042] According to an addition aspect of the invention, a membrane
is provided having a coating layer having a non-woven structure
format of at least one of cellulose nanocrystals or microcrystals.
The membrane also includes an electrospun substrate having a
non-woven structure format upon which the coating layer is applied.
Nanofibers of the electrospun substrate have a diameter greater
than that of the cellulose nanocrystals and microcrystals. The
membrane further includes a non-woven support upon which the
electrospun substrate is disposed. Microfibers of the non-woven
support have a diameter greater than that of the nanofibers of the
electrospun substrate.
[0043] According to a further aspect of the present invention, a
method is provided for producing a membrane. A coating layer is
produced having a non-woven structure format from polysaccharide
nanofibers. The polysaccharide nanofibers have a diameter between 5
to 50 nanometers (nm). An electrospun substrate is immersed into a
water-based solution. The coating layer is applied to the
electrospun substrate. A gel barrier is formed at an interface
between the electrospun substrate and the coating layer to slow
diffusion of the coating layer into the electrospun substrate.
Nanofibers of the electrospun substrate have a diameter greater
than that of the cellulose nanofibers. The electrospun substrate is
disposed on a non-woven support having microfibers with a diameter
greater than that of the nanofibers of the electrospun
substrate.
[0044] According to another aspect of the present invention, a
membrane is provided having an electrospun substrate having a
non-woven structure format and a non-woven support upon which the
electrospun substrate is disposed. Microfibers of the non-woven
support have a diameter greater than that of the nanofibers of the
electrospun substrate. A maximum pore size of the electrospun
substrate is approximately three times a mean pore size of the
electrospun substrate when a porosity of the electrospun substrate
is approximately 80%. A mean pore size of the electrospun substrate
is approximately three times the fiber diameter of the nanofibers
of the electrospun substrate, when a porosity of the electrospun
substrate is approximately 80%.
BRIEF DESCRIPTION OF THE DRAWINGS
[0045] The above and other aspects, features and advantages of the
present invention will be more apparent from the following
description when taken in conjunction with the accompanying
drawings, in which:
[0046] FIG. 1 is a diagram illustrating a bubble point test;
[0047] FIG. 2 is a diagram illustrating a dead-end filtration
test;
[0048] FIG. 3 is a chart showing effects of applied voltage on the
diameter of electrospun PVA nanofibers fabricated from 8 wt % PVA
solution;
[0049] FIG. 4 is a chart showing viscosity as a function of
concentration of PVA solutions;
[0050] FIGS. 5(a)-(d) are SEM images of PVA membranes fabricated on
PET non-woven substrate by electrospinning of PVA solution;
[0051] FIG. 6 is a chart showing effects of concentration on fiber
diameter and porosity of electrospun PVA membrane;
[0052] FIG. 7 is a chart showing a rejection ratio of electrospun
PVA membranes of different thicknesses to the polycarboxylate
particles;
[0053] FIG. 8 includes charts showing viscosity and conductivity of
PAN solutions with different concentrations;
[0054] FIG. 9 is a chart showing average fiber diameters of
electrospun samples with error bars, grouped by solution
concentration and arranged by increasing tip-to-collector
distance;
[0055] FIG. 10 includes SEM images (.times.5K) of nanofibrous
membrane from solution A9, A5 and A1;
[0056] FIG. 11 includes SEM images (.times.20K) of nanofibrous
membrane from solution A9, A5 and A1;
[0057] FIG. 12 includes charts showing a relationship between
applied voltage and average fiber diameter of electrospun membranes
for PAN (A) and (B) solutions;
[0058] FIG. 13 is a chart showing an effect of PAN concentration on
maximum pore size and pure water flux;
[0059] FIG. 14 includes charts showing flux and rejection during
filtration of 0.20 .mu.m and 1 .mu.m particles for electrospun PAN
and Millipore Millex-GS microfiltration membranes;
[0060] FIG. 15 is a schematic representation of the preparation of
cellulose nanofibers;
[0061] FIG. 16 is a diagram illustrating a three-tier structure of
different fiber diameters in a nanofibrous membrane for water
purification, according to an embodiment of the present
invention;
[0062] FIG. 17 is a diagram illustrating a nanofibrous composite
barrier layer in thin film composite membranes, according to an
embodiment of the present invention;
[0063] FIG. 18 is a chart showing relationships between mean pore
size/maximum pore size and mean fiber diameter in nano-woven
nanofibrous membranes, according to an embodiment of the present
invention;
[0064] FIG. 19 includes SEM images illustrating preparation of
cellulose nanofibers from wood bleached pulps, according to an
embodiment of the present invention;
[0065] FIG. 20 is a chart showing yield and morphology of the
oxidized cellulose microfibers, according to an embodiment of the
present invention;
[0066] FIG. 21 is a chart showing contents of carboxylate group of
cellulose microfibers at different amounts of oxidizing reagents,
according to an embodiment of the present invention;
[0067] FIG. 22 includes SEM images of cellulose nanofibers prepared
by using different concentrations of the solution, according to an
embodiment of the present invention;
[0068] FIG. 23 includes Transmission Electron Microscope (TEM)
images of cellulose nanofibers at different cellulose
concentrations, according to an embodiment of the present
invention;
[0069] FIG. 24 is a chart showing viscosity of cellulose nanofiber
aqueous solutions at different concentrations, according to an
embodiment of the present invention;
[0070] FIG. 25(a)-(b) include charts showing viscosity of cellulose
nanofiber aqueous solutions at different pH values, and
reversibility of cellulose nanofiber gels at different pH values,
according to an embodiment of the present invention;
[0071] FIG. 26 is a chart showing viscosity of cellulose nanofiber
aqueous solutions at different ionic strengths;
[0072] FIG. 27 is a chart showing viscosity of cellulose nanofibers
aqueous solution prepared with oxidized cellulose microfibers at
different degrees of oxidation, according to an embodiment of the
present invention;
[0073] FIG. 28 is a TEM image of cellulose nanofibers (0.01 wt %)
dispersed in DMF, according to an embodiment of the present
invention;
[0074] FIG. 29 is a chart showing thermal stability (Thermal
Gravimetric Analysis (TGA)) of oxidized cellulose microfibers,
according to an embodiment of the present invention;
[0075] FIG. 30 is a chart showing thermal stability (TGA) of
cellulose nanofibers prepared at different % cellulose
concentrations after freeze drying, according to an embodiment of
the present invention;
[0076] FIG. 31 is a chart showing crystallinity of wood pulp,
oxidized cellulose microfibers, and cellulose nanofibers by using
freeze drying, according to an embodiment of the present
invention;
[0077] FIG. 32 shows preparation of cellulose nanofibers by
2,2,6,6-Tetramethylpiperidine-1-oxyl (TEMPO)/NaBr/NaClO oxidation,
according to an embodiment of the present invention;
[0078] FIG. 33 shows chemical modification of cellulose nanofibers,
according to an embodiment of the present invention;
[0079] FIG. 34 includes charts showing thermal stability ((a) TGA
and (b) DSC) of cellulose nanofibers and cationic cellulose
nanofibers, according to an embodiment of the present
invention;
[0080] FIG. 35 is a chart showing crystallinity (Wide Angle X-ray
Diffraction (WAXD)) of cellulose nanofibers, cationic cellulose
nanofibers, according to an embodiment of the present
invention;
[0081] FIG. 36 shows esterification of the cellulose nanofibers,
according to an embodiment of the present invention;
[0082] FIG. 37 includes charts showing thermal stability of
cellulose nanofibers, acetyl cellulose nanofibers, cellulose
triacetate, and cellulose acetate, according to an embodiment of
the present invention;
[0083] FIG. 38 is a chart showing crystallinity (WAXD) of cellulose
nanofibers, acetyl cellulose nanofibers, cellulose triacetate, and
cellulose acetate, according to an embodiment of the present
invention;
[0084] FIG. 39 is a TEM image of cationic cellulose nanofibers,
according to an embodiment of the present invention;
[0085] FIG. 40 is a chart showing viscosities of cellulose
nanofibers, acetyl cellulose nanofibers, and cellulose triacetate
in DMF with 0.05 wt % of concentration, according to an embodiment
of the present invention;
[0086] FIG. 41 is a flow chart illustrating preparation of
cellulose nanofiber membrane, according to an embodiment of the
present invention, according to an embodiment of the present
invention;
[0087] FIGS. 42(a)-(c) are SEM images of PET, according to an
embodiment of the present invention;
[0088] FIG. 43 is a chart showing thermal stability of cellulose
nanofiber membranes prepared from different concentration of
solutions, according to an embodiment of the present invention;
[0089] FIG. 44 includes SEM images of cellulose nanofiber
membranes, according to an embodiment of the present invention;
[0090] FIG. 45 is a chart showing pure water fluxes of cellulose
nanofiber membrane based on PAN and PES E-spun membranes, according
to an embodiment of the present invention;
[0091] FIG. 46 includes SEM images of cellulose nanofiber membrane
based on PAN (cross-sectional and top views) with 0.50 .mu.m
thickness of top layer, according to an embodiment of the present
invention;
[0092] FIG. 47 includes SEM images of cellulose nanofiber membrane
based on PAN (cross-sectional and top views) with 1.0 .mu.m
thickness of top layer, according to an embodiment of the present
invention;
[0093] FIG. 48 includes SEM images of cellulose nanofiber membrane
based on PAN (cross-sectional and top views) with 1.0 .mu.m
thickness of top layer, according to an embodiment of the present
invention;
[0094] FIG. 49 includes SEM images of cellulose nanofiber
membranes, according to an embodiment of the present invention;
[0095] FIG. 50 includes SEM images of PAN10 (A) and PAN400 (B)
commercial membranes with top view, according to an embodiment of
the present invention;
[0096] FIG. 51 is a chart showing permeation flux and rejection
ratio of cellulose nanofiber membrane as a function of Molecular
Weight Cut-Off (MWCO), according to an embodiment of the present
invention;
[0097] FIG. 52 is a chart showing permeation flux and rejection
ratio of cellulose nanofiber (prepared from 0.10% and 0.05% aqueous
solutions, respectively)/PAN membranes for filtration of oil/water
emulsion at different pressures, according to an embodiment of the
present invention;
[0098] FIG. 53 is a chart showing permeation flux and rejection (%)
of cellulose nanofiber (0.10 wt % aqueous solution)/PAN and PAN 10
(30 psi) membranes during 24 hours for filtration of oil/water
emulsion, according to an embodiment of the present invention;
[0099] FIG. 54 is a chart showing permeation flux and rejection (%)
of cellulose nanofiber prepared from aqueous solution with 0.05% of
concentration/PAN and PAN 10/PAN400 membranes during 48 hours for
filtration of oil/water emulsion, according to an embodiment of the
present invention;
[0100] FIG. 55 is a chart showing permeation flux and rejection
ratio of cellulose nanofiber membrane during 24 hours of filtration
for sodium alginate solution, according to an embodiment of the
present invention;
[0101] FIG. 56 includes SEM images of PVA (cross-sectional (A) and
top view (B)), PVA/cellulose nanofiber (0.005 wt %)
(cross-sectional (C) and top view (D)), and PVA/cellulose nanofiber
(0.025 wt %) (cross-sectional (E) and top view (F)) composite
membranes, according to an embodiment of the present invention;
[0102] FIG. 57 is a chart showing pure water flux of PVA/CN
composite membranes compared with PAN 10 commercial membrane at
different pressures, according to an embodiment of the present
invention;
[0103] FIG. 58 is a chart showing permeation flux and rejection (%)
of PVA/CN composite and PAN 10 membranes during 24 hours for
filtration of oil/water emulsion, according to an embodiment of the
present invention;
[0104] FIG. 59 includes SEM images of cellulose (0.10 wt % in
EAc)/cellulose nanofiber (0.01 wt %) composite membrane, according
to an embodiment of the present invention;
[0105] FIG. 60 is a chart showing permeation flux and rejection
ratio of cellulose/CN and PAN10 membranes during 24 hours for
filtration of oil/water emulsion, according to an embodiment of the
present invention;
[0106] FIG. 61 shows a cellulose nanofiber membrane cross-linked by
heating process, according to an embodiment of the present
invention;
[0107] FIG. 62 shows a TEM image of cellulose nanofiber membrane
coated on TEM grid and stained by uranyl acetate (2.0%), according
to an embodiment of the present invention;
[0108] FIG. 63 is a chart showing permeation flux and rejection (%)
of cellulose nanofiber membrane during 72 hours for filtration of
oil/water emulsion, according to an embodiment of the present
invention;
[0109] FIG. 64 is a chart showing permeation flux and rejection
ratio of cellulose nanofiber membrane as a function of MWCO,
according to an embodiment of the present invention;
[0110] FIG. 65 is a chart showing permeation flux and rejection
ratio of cellulose nanofiber membrane during 48 hours of filtration
for sodium alginate solution, according to an embodiment of the
present invention;
[0111] FIG. 66 shows a cellulose nanofiber membrane cross-linked by
GA, according to an embodiment of the present invention;
[0112] FIG. 67 shows a cellulose nanofiber membrane cross-linked by
PAA, according to an embodiment of the present invention;
[0113] FIG. 68 is a chart showing permeation flux and rejection
ratio of cellulose nanofiber membrane after PAA cross-linking for
filtration of oil/water emulsion, according to an embodiment of the
present invention;
[0114] FIG. 69 is a chart showing rheological behavior of cellulose
nanofiber solution gelled by an ionic liquid, according to an
embodiment of the present invention;
[0115] FIG. 70 shows the structure of the ionic liquid compounds,
according to an embodiment of the present invention;
[0116] FIG. 71 is a chart showing permeation flux and rejection
ratio of cellulose nanofiber membrane after ionic liquid
cross-linking for filtration of oil/water emulsion, according to an
embodiment of the present invention;
[0117] FIG. 72 is a chart showing permeation flux and rejection
ratio of cellulose nanofiber membrane after PEO-monomer
cross-linking for filtration of oil/water emulsion, according to an
embodiment of the present invention;
[0118] FIG. 73 is a chart showing permeation flux and rejection
ratio of cellulose nanofiber membrane after surface esterification
for filtration of oil/water emulsion, according to an embodiment of
the present invention;
[0119] FIG. 74 is a chart showing permeation flux and rejection
ratio of cellulose nanofiber membrane after polyether-b-polyamide
(PEBAX)-coating for filtration of oil/water emulsion, according to
an embodiment of the present invention;
[0120] FIG. 75 is a chart showing permeation flux and rejection
ratio of chitin nanofiber membrane for filtration of oil/water
emulsion, according to an embodiment of the present invention;
[0121] FIG. 76 is a chart showing permeation flux and rejection
ratio of cellulose nanocrystal membrane for filtration of oil/water
emulsion, according to an embodiment of the present invention;
[0122] FIG. 77 is a chart showing thermal stability of
polysaccharide nanofibers, according to an embodiment of the
present invention;
[0123] FIG. 78 is a diagram illustrating a strategy of modification
of cellulose nanofiber membrane for the adsorption of viruses,
according to an embodiment of the present invention;
[0124] FIG. 79 includes charts showing adsorption capacity of BSA
in cellulose nanofiber membrane as a function of time, according to
an embodiment of the present invention; and
[0125] FIG. 80 shows hydrolysis of PAN E-spun membrane for the
adsorption of viruses, according to an embodiment of the present
invention.
DETAILED DESCRIPTION OF EMBODIMENTS OF THE PRESENT INVENTION
[0126] Embodiments of the present invention are described in detail
with reference to the accompanying drawings. The same or similar
components may be designated by the same or similar reference
numerals although they are illustrated in different drawings.
Detailed descriptions of constructions or processes known in the
art may be omitted to avoid obscuring the subject matter of the
present invention.
[0127] The terms and words used in the following description and
claims are not limited to their dictionary meanings, but are used
to enable a clear and consistent understanding of the invention.
Accordingly, it should be apparent to those skilled in the art that
the following description of embodiments of the present invention
are provided for illustrative purposes only and not for the purpose
of limiting the invention, as defined by the appended claims and
their equivalents.
[0128] It is to be understood that the singular forms "a," "an,"
and "the" include plural referents unless the context clearly
dictates otherwise. Thus, for example, reference to "an identifier"
includes reference to one or more of such identifiers.
[0129] With the combined nanofiber and cellulose advances in
membrane technology, one can benefit from long, safe and
inexpensive nanofibers of different diameters. For example, it is
possible to design a robust, inexpensive, hand-held device, based
on the high-flux and low-pressure nanofibrous membranes, to purify
drinking water in developing countries, as shown in FIG. 16. Such a
system could largely eliminate water infested with microbes and
bacteria that result in dysentery, cholera or typhoid.
[0130] A Thin Film Composite (TFC) membrane has a three-tier
composite structure. The top cellulose coating layer is the key for
separation of solute and permeates. The middle layer serves as a
support, in which the high-flux nano-fibrous membrane with large
porosity and uniform structure is used. This nanofibrous mid-layer
is often fabricated by electrospinning that replaces the
conventional asymmetric porous membrane, normally fabricated by the
phase inversion method. The bottom layer is a non-woven
microfibrous support possessing strong mechanical property.
[0131] Another embodiment of the present invention, as shown in
FIG. 17, includes a very thin, strong and functional nanocomposite
barrier layer, imbedded with interconnected and directed water
channels guided by the surface of nanofibers. This new membrane
configuration offers two critical advantages for the separation of
water and small molecules/ions: (1) increase flux, by a factor of
10 or more, while maintaining the same high rejection ratio; and
(2) reduce fouling by providing a smooth hydrophilic barrier layer,
reducing particulate clogging and adsorption of hydrophobic
materials. Further, the integration of a nanofibrous scaffold with
the nanocomposite barrier layer also creates a mechanically strong
nanocomposite thin film that can withstand frequent back flushing
operations for membrane cleaning, and the presence of nanofibers
permits surface modifications, yielding directed water pathways to
enhance throughput.
[0132] The embodiments of the present invention enable the
fabrication of water purification systems with a performance/cost
ratio exceeding existing commercial systems by factors of 100-1000.
For ultra-filtration, an increase in filter throughput is
demonstrated by a factor of more than 10. The embodiments of the
present invention are based on a unique nanofibrous membrane
concept which forms a revolutionary platform suitable for all
segments of water purification processes, including
micro-filtration (MF), ultra-filtration (UF), nano-filtration (NF)
and reverse osmosis (RO). Better flux means less time and less
energy to filter the same amount of water, which in turn greatly
increases cost efficiency.
[0133] A correlation between pore size and fiber diameter is
demonstrated based on an ideal non-woven structure with a total
porosity about 80%. The maximum pore size of the nanofibrous
membrane (determined by the bubble point measurement) is
approximately three times of mean pore size of the membrane (also
determined by the bubble point measurement or the SEM image
analysis), and the mean pore size of the nanofiber membrane is
approximately three times the fiber diameter, as shown in FIG.
18.
Oxidized Cellulose Micro fibers
[0134] The embodiments of the present invention utilize techniques
involving the oxidation of cellulose microfibers with
TEMPO/NaBr/NaClO in an aqueous solution at ambient temperatures.
These techniques are based on the oxidation reaction of the primary
hydroxyl group of polysaccharides, such as cellulose, by sodium
hypochlorite and a catalytic amount of sodium bromide mediated with
TEMPO radicals. After oxidation, the carboxyl group and the
aldehyde group are formed, instead of the primary hydroxyl group of
cellulose. As a result, the structure of cellulose microfiber
becomes very loose, even forming a `balloon` structure like that in
some solvents of cellulose. The loose cellulose fibers can easily
be defibrillated by using only mild mechanical treatment, such as a
homogenizer, and then cellulose nanofibers dispersed in water are
obtained. The yield of oxidized cellulose microfibers is close to
100% using this process. Thus, such a preparation process can
provide a new platform, permitting fairly easy preparation of
cellulose nanofibers without special instrumentation.
[0135] 10.0 g of Biofloc 92 MV (2.2 g of cellulose) is dispersed in
192 g of water. 0.2 g of sodium bromide and 0.04 g of TEMPO is
dissolved in the suspension solution. The reaction is started by
adding a desired amount (e.g., 20 mmol/g cellulose) of sodium
hypochlorite solution under stirring condition for 24 h. The pH
value is kept at about 10.0 to 10.3 monitored with a pH meter by
adding 0.5 mol/L of sodium hydroxide aqueous solution. The reaction
10 is stopped by adding 10 mL of ethanol followed by stirring for
20 minutes. The rough product was separated by centrifuging (5000
rpm) of the reaction mixture and then decanting the supernatant.
The product was washed with de-ionized (DI) water 5 times and
separated finally by centrifugation. The oxidized cellulose
microfiber slurry was kept in a refrigerator with the dry oxidized
cellulose microfibers being obtained after freeze drying the
product for one day.
[0136] 0.01 g of oxidized cellulose microfibers (0.20 g of oxidized
cellulose slurry prepared with 20 mmol/g cellulose NaClO) are
dispersed in 100 g of water and sonicated for 5 min with a
homogenizer. Almost all the oxidized cellulose microfibers were
defibrillated and the suspension was centrifuged at 5000 rpm. The
supernatant was the cellulose nanofiber suspension in water with a
concentration of .about.0.01 wt %. SEM and TEM images on selected
stages of the preparation process of cellulose nanofibers, are
shown in FIG. 19.
[0137] Different oxidized cellulose microfibers from different
cellulose sources, such as wood and cotton pulps, were prepared by
using the TEMPO/NaBr/NaClO oxidation system, which contributed to
loosen the cellulose microfibers by introducing carboxyl groups
onto the surface. The negatively charged nanofibers with repulsion
among the same charges helps to defibrillate the cellulose
microfibers. Also, the oxidation reaction was carried out in an
alkali environment, typically with a pH value between 10 and 10.3,
which is partially responsible for removal of the pectin,
hemicellulose, and lignin remaining in the cellulose fibers. The
yield and morphology of the oxidized cellulose microfibers are
listed in FIG. 20.
[0138] FIG. 20 shows that the cellulose microfibers have an open
`balloon`-like morphology after the oxidation reaction, and the
opening degree is dependent on the amount of oxidizing reagent
NaClO. With more amounts of hypochlorite, more opening fibers
become available, which means that the cellulose nanofibers become
more easily available by subsequent mechanical treatment. However,
the yield of cellulose microfibers was affected by the degree of
oxidation. Heavy oxidation could actually decrease the yield of the
oxidized cellulose microfibers because part of the cellulose and
other polysaccharides could be dissolved into the base reaction
system and were separated out by later centrifugation. 20 mmol/g
cellulose with NaClO was preferable for the preparation of the
cellulose microfibers for keeping the yield to about 90%.
[0139] After oxidation, cellulose microfibers with carboxyl groups
on the surface were obtained. The results of conductivity titration
performed to measure the content of carboxylate groups of oxidized
cellulose microfibersity titration is shown in FIG. 21.
[0140] The contents of the carboxylate group were increased as the
amount of NaClO increased, while the solubility of cellulose
microfibers in the base aqueous solution also increased as the
content of the carboxylate group increased. As described above, the
carboxylate group of the oxidized cellulose microfibers with 20
mmol/g of NaClO was approximately 0.7 mmol/g cellulose, which means
that approximately 12% of the primary of hydroxyl group of
cellulose was oxidized. Further chemical modification of the
cellulose nanofibers is dependent upon the amount of the
carboxylate group, as discussed in greater detail below.
[0141] Cellulose nanofibers can be prepared from either slurry or
dry oxidized cellulose microfibers, which can be obtained by freeze
drying of the slurry for one day. The slurry can be very easily
dispersed further in water and almost all of cellulose therein can
be chopped into cellulose nanofibers, while the dry oxidized
cellulose microfibers are more difficult to disperse, and only
about 50% of the cellulose can be transferred into cellulose
nanofibers using the current mechanical treatment. For example,
longer time sonication was needed to increase the yield of
cellulose nanofibers.
[0142] After centrifuging, the supernatant part of the solution was
collected which contained the cellulose nanofibers. It should be
noted that the morphology of the cellulose nanofibers after freeze
drying was dependent on the concentration. FIG. 22 shows cellulose
nanofibers prepared by using different concentrations of the
solution (0.7 mmol carboxyl group/g cellulose) after the freeze
drying process. The concentrations are (a) 0.30%, (b) 0.18%, (c)
0.15%, (d) 0.05%, (e) 0.01%, (f) 0.005%, respectively. Diameters of
cellulose nanofibers are about (a) N/A, (b) 1000 nm, (c) 500 nm,
(d) 200 nm, (e) 100 nm, (f) 50 nm, respectively. The diameter of
cellulose nanofibers decreased with decreasing cellulose
concentration of the prepared solution. The diameter of cellulose
nanofibers estimated from SEM images ranged from about 1000 to 50
nm with fiber length of more than 100 .mu.m.
[0143] Fine cellulose nanofibers with 5 to 10 nm of diameter were
observed when the cellulose concentration was about 0.05% or lower,
as shown in FIG. 23. TEM images of cellulose nanofibers at
cellulose concentrations of (a) 0.005%, (b) 0.01%, (c) 0.05%, (d)
0.10%, (e) and (f) 0.15% stained by uranyl acetate (2.0%)
shown.
[0144] The viscosity of the cellulose nanofibers aqueous solution
was dramatically higher (more than 100 Pas) than that of pure water
(0.001 Pas), even when the cellulose concentration was only 0.70%,
as shown in FIG. 24.
[0145] Based on the average length of cellulose nanofibers, an
overlap concentration is estimated by Equation (3):
C*.about.1/L.sup.3N.sub.A (3)
L is the length of an isolated chain, and N.sub.A is the Avogadro
constant. An approximate estimate suggested that C* is
approximately 10.sup.-8 mol/L.
[0146] The overlap concentration of cellulose nanofibers is
significantly lower than those of polymer solutions, implying that
the viscosity of a cellulose nanofiber `solution` should be very
high even at very low concentrations in terms of regular polymer
solutions. Over the normal frequency region, the complex viscosity
of a cellulose nanofiber aqueous `solution` (actually a suspension)
changes very little when their concentrations are lower than 0.20%,
with their rheological behavior being like that of a Newtonian
fluid. When the concentration of the solution is higher than 0.30%,
it is still very low although, however, there is an obvious shear
thinning effect, similar to that of concentrated polymer solutions.
Thus, the rod stiffness has a remarkable effect on the measured
viscosity because there is not much entanglement like that among
more flexible polymer chains for more rigid cellulose nanofibers in
suspension.
[0147] The solution viscosity is pH sensitive. The viscosity of
0.20% cellulose nanofiber solution at different pH values is shown
in FIG. 25(a). The viscosity of cellulose nanofiber solution
changed from 0.005 to 0.01 Pas when the pH varied from 10.0 to 4.0.
However, there was a sharp increase (the peak value was 230 Pas at
pH .about.3.2) when the pH value was below 4.0 and gelation
appeared due to a hydrogen bond formation between carboxyl groups
of the cellulose backbone and hydroxyl groups. The gel became
harder with further decrease in the pH value, until almost all
cellulose nanofibers participated in the gel network formation
whereby the aqueous solution became heterogeneous. The gelation
process was reversible according to the viscosity changes of the
cellulose nanofiber solutions, as shown in FIG. 25(b).
[0148] 0.20% of cellulose nanofiber aqueous solution was used in
determining the effect of ionic strength changes on the solution
viscosity. When the concentration of sodium chloride was lower than
0.002 mol/L, nanofibers with approximately 1000 nm diameters after
freeze drying were observed. However, the nanofibers could be
further aggregated by increasing the ionic strength to above 0.008
mol/L and gelation began to occur, as confirmed from the
rheological result, as shown in FIG. 26. The gelation phenomenon
occurred very fast when the concentration of sodium chloride was
higher than 0.01 mol/L. Ionic liquid EAc
(1-methyl-3-ethylimidazolium acetate, melting point:
<-20.degree. C., while the melting point of NaCl is 801.degree.
C.) can also be used to adjust the ionic strength. The viscosity of
cellulose nanofiber aqueous solutions is shown in FIG. 63. The
gelatin was observed again when the concentration of EAc was above
0.033 mol/L.
[0149] Different carboxyl contents of oxidized cellulose
microfibers affects the yield of the cellulose nanofibers, based on
the same mechanical treatment, as shown in FIG. 27. The viscosity
of the cellulose nanofiber solution changed with different amounts
of oxidized cellulose microfibers. The yields were almost the same
when the carboxyl group content was higher than 10 mmol/g
cellulose.
[0150] The miscibility of cellulose nanofiber aqueous solution in
different organic solvents was tested with UV-Vis spectroscopy.
Four different organic solvents, including DMF, acetone, THF, and
ethanol, which were miscible with water, were employed to determine
the miscibility of the cellulose nanofiber aqueous solutions. The
results are listed in Table 4, in which, "CNAS" means a cellulose
nanofiber aqueous solution, weight ratio of CNAS/organic solvent,
"S" means miscible, "PM" means partially miscible and "I" means
immiscible.
TABLE-US-00004 TABLE 4 Solvents 10:1 1:1 1:10 CNAS/Water M M M
CNAS/DMF M M M CNAS/Acetone M PM I CNAS/THF M PM I CNAS/Ethanol M I
I
[0151] Table 4 shows that the cellulose nanofiber aqueous solution
was completely miscible with DMF as well as with water, and
partially miscible with acetone and THF (weight ratio is 1:1).
However, only a small amount of the cellulose nanofiber solution
(1:10) can be dispersed into acetone, THF or ethanol.
[0152] FIG. 28 is a TEM image of CNAS/DMF showing no gel formation
or precipitates observed when CNAS was dispersed into DMF with any
weight ratio of DMF to water, indicating that DMF may be regarded
as a co-solvent for cellulose nanofibers.
[0153] FIG. 29 shows thermal stability of oxidized cellulose
microfibers. Oxidized cellulose fibers started to decompose at
about 250.degree. C. (.about.5.0% weight loss), when compared with
that of the wood pulp (336.degree. C.). All the oxidized cellulose
microfibers have similar decomposition temperatures except for the
oxidized cellulose microfibers prepared with 1.25 mmol/g cellulose
of NaClO, which showed decomposition at a slightly higher
temperature if 259.degree. C. The lower the degree of oxidation,
the lower the yield of cellulose microfibers became and the higher
the thermal stability.
[0154] FIG. 30 shows thermal stability of cellulose nanofibers
prepared by using the mechanical treatment of the oxidized
cellulose microfibers (0.7 mmol/L carboxylate content). The
decomposition onset of cellulose nanofibers started at about
240.degree. C. for all concentrations of cellulose nanofibers.
However, the cellulose nanofibers decompose completely at
275.degree. C. when the concentration is below 0.20%, which matches
with FIG. 22, demonstrating that cellulose nanofibers were observed
from SEM images after freeze drying at 0.20%. The cellulose
nanofibers are completely decomposed at 400.degree. C. at
concentrations higher than 0.30%, when only pieces of film were
observed from SEM images.
[0155] FIG. 31 illustrates changes of crystallinity of cellulose
microfibers before and after oxidation, as well as cellulose
nanofibers prepared by post-mechanical treatment. WAXD patterns of
cellulose microfibers showed nearly no change before and after
oxidation, with little change in crystallinity. This confirms that
the oxidation reaction occurred at the surface of the crystalline
core or amorphous regions. As the oxidation reaction did not
substantially affect the degree of crystallinity of cellulose
nanofibers, the process will not change the mechanical strength of
modified cellulose nanofibers, even though there could be many
carboxyl or aldehyde groups located on the surface of those
modified cellulose microfibers. After mechanical treatment, the
crystallinity of cellulose nanofibers was 63.6%, being comparable
with initial samples having crystallinity of 64.3 and 65.5%,
respectively. According to the Scherer equation shown in Equation
(4):
D = K .lamda. .beta.cos.theta. , ( 4 ) ##EQU00002##
where K is 0.89, .lamda. is the X-ray wavelength, and .beta. is the
Full Width at Half Maximum (FWHM), which is obtained after the peak
fit. The crystal size (D) of (200) diffraction pattern was
estimated, as shown in FIG. 31. The crystal sizes of cellulose
fibers were 6.5, 5.6 nm for initial microfibers and 6.1 nm for
cellulose nanofibers, which could be matched with the result
measured from TEM images (FIG. 23), with a fiber diameter of about
5 nm.
[0156] Cellulose nanofibers can be chemically modified to further
expand their physical and chemical properties for different
applications. This is particularly important and relatively unique
for cellulose and the advantages are desirable not only for liquid
filtration but also for air filtration. All the chemical
modifications are based on the reactions with carboxyl, hydroxyl,
and aldehyde groups located on cellulose nanofibers backbones, and
are shown as examples, as the diversity for cellulose modifications
is exceptional.
[0157] The primary hydroxyl groups of cellulose are about 6.0
mmol/g cellulose, while the secondary hydroxyl groups are about
12.0 mmol/g cellulose. After oxidation, a part of the primary
hydroxyl groups is oxidized into carboxyl groups and aldehyde
groups, which are affected by the amount of oxidants and the pH
value of the reaction system as mentioned before. The carboxyl
group content was about 0.7-1.0 mmol/g cellulose, as determined by
conductivity titration. Thus, 12.0 to 17.0% (mol %) of primary
hydroxyl groups could be oxidized into carboxyl groups. When
functional groups are introduced, the density of functional groups
located on the surface of the cellulose nanofibers will depend on
the content ratio of hydroxyl, carbonyl, and aldehyde. These
chemical modifications, including oxidation of 6-position hydroxyl
group, provide anti-bacterial properties to the cellulose
nanofiber.
[0158] The modifications of cellulose nanofibers in aqueous or
non-aqueous solvents are shown below. Characterization of cellulose
nanofiber derivatives can be carried out using standard physical
techniques.
[0159] Negatively charged cellulose nanofibers were prepared by
oxidation of the cellulose microfibers, followed by mechanical
treatment. The synthetic preparation of cellulose nanofibers by
TEMPO/NaBr/NaClO oxidation is shown in FIG. 32. A series of the
cellulose nanofibers with a negatively charged surface and a fine
diameter (.about.5 nm) are prepared. The degree of modification is
indicated by the carboxyl group content, which is determined by
conductive titration, as shown in FIG. 1.
[0160] The cationic functionalization of the surface of cellulose
nanofibers through a reaction with epoxy-substituted ammonium in
alkali aqueous solution was also carried out, as shown in FIG. 33.
The size and shape of the crystals remains unchanged, however, the
functional process reversed the surface charge and led to a
reduction of the total surface charge density.
[0161] Cationic cellulose nanofibers were prepared by mixing
cellulose nanofibers suspension 200 g (0.20%, 0.4 g of cellulose)
and 200 mL (2 mol/L) of NaOH followed by adding 2.3 g of
glycidyltrimethylammonium chloride. The mixture was stirred at
40-50.degree. C. for two days. After reaction, the product was
washed three times with ethanol and dried.
[0162] The onset decomposition temperature of the cationic
cellulose nanofibers is higher than that of original cellulose
nanofibers. Two step decomposition, from 250.degree. C. to
340.degree. C. and from 340.degree. C. to 500.degree. C.,
respectively, is observed from FIG. 34, due to the introduction of
the ammonium substituted group. The endothermic peaks were shifted
to higher temperatures from cellulose nanofibers to cationic
cellulose nanofibers.
[0163] The crystallinity of cationic cellulose nanofibers was
investigated with WAXD patterns. FIG. 35 shows the X-ray
diffraction profiles of cellulose nanofibers and cationic cellulose
nanofibers. The profile of cationic cellulose nanofibers is quite
different from that of original cellulose nanofibers. Moreover, the
evaluated crystallinity of cationic cellulose nanofibers increased
from original 63.5% to 71.6%. These features also clearly indicate
that ammonium groups were introduced to the surface of cellulose
nanofibers.
[0164] To alter the hydrophilic nature of cellulose nanofibers,
cellulose nanofibers could be made more hydrophobic by an
esterification reaction using acetic anhydride and a small amount
of perchloric acid (HClO.sub.4) as a catalyst, as shown in FIG. 36.
This reaction is surface modification of cellulose nanofibers by
partially acetylating hydroxyl groups, which will provide more
hydrophobic property to cellulose nanofibers. The acetylation
degree was controlled by the amount of acetic anhydride. The
perchloric acid was used as the catalyst. This reaction has to be
achieved in a nonaqueous solvent, such as toluene. Freeze dried
cellulose nanofibers 0.23 g were immersed in a mixture of 40 mL of
acetic acid, 50 mL of toluene, and 0.2 mL of HClO.sub.4 (60%).
Then, 10 mL of acetic anhydride was added with stirring. The
mixture was allowed to stand at room temperature for 16 hours.
After acylation, the cellulose nanofibers were washed thoroughly
with ethanol and dispersed in DMF, followed by treatment with a
homogenizer for five minutes.
[0165] The thermal stability of acetyl cellulose nanofibers is
compared to cellulose nanofibers, cellulose acetate and cellulose
triacetate in FIG. 37. The decomposition temperature is
270.9.degree. C. It is much higher than that of cellulose
nanofibers, which started at 240.degree. C. However, it is lower
than that of cellulose acetate (295.8) and cellulose triacetate
(305.4). The TGA curve of acetyl cellulose nanofibers is very
similar to that of cellulose acetate/triacetate, but different from
cellulose nanofibers which decomposed very quickly. The DSC results
provide a similar result. The endothermic peaks were shifted to a
higher temperature, indicating that the acetyl groups have been
introduced into the cellulose nanofibers.
[0166] The crystallinity of acetyl cellulose nanofibers was
determined with WAXD patterns. FIG. 38 shows the X-ray diffraction
profiles of cellulose nanofibers, acetyl cellulose nanofibers, and
cellulose acetate/triacetate. The profile of acetyl cellulose
nanofibers differs from that of original cellulose nanofibers but
is similar to that of cellulose acetate/triacetate. Moreover, the
evaluated crystallinity of acetyl cellulose nanofibers decreased
from original 63.5% to 39.7%, but still higher than that of
cellulose acetate/triacetate (24.0 and 26.2%). These features also
indicate that acetyl groups were introduced from the surface to the
core of BC nanofibers as described in some references.
[0167] Furthermore, the dispersibility of modified cellulose
nanofibers was determined with water and DMF, respectively, as
shown in Table 5. The cellulose nanofibers are dispersed well
either in water or in DMF. After acetylation, they could never be
dispersed in water again, but are easily dispersed in DMF with high
concentration. The cationic cellulose nanofibers aggregate together
because there are both positive (ammonium) and negative
(carboxylate) charges in the system. Interaction between ammonium
and carboxylate will occur either in water or in organic solvents.
In Table 5, a " " indicates good dispersion, "x" indicates poor
dispersion or precipitate; "*" indicates a concentration of
suspension of 0.05 wt % and "**" indicates a concentration of 0.01
wt %.
TABLE-US-00005 TABLE 5 H.sub.2O DMF Cellulose nanofibers* Acetyl
cellulose x nanofibers* Cationic cellulose x x nanofibers**
[0168] An alternative method to prepare cationic cellulose
nanofibers, which can be dispersed in water, 10.0 g of cellulose
microfibers (Biofloc 92, cellulose content is 2.5 g) are dispersed
into 600 mL of NaOH (2 mol/L) aqueous solution followed by adding
7.7 g of glycidyltrimethylammonium chloride. The mixture is stirred
at 60.degree. C. for three days. After thoroughly washing the
modified cellulose fibers, homogenizer was used to chop up the
microfiber into nanofibers. The TEM image of cationic cellulose
nanofibers was showed in FIG. 39. The fiber diameter is about
10.about.20 nm with the length of the nanofiber more than 5
.mu.m.
[0169] The rheological behavior of acetyl cellulose nanofibers in
DMF at ambient temperature is shown in FIG. 40. The viscosity of
acetyl cellulose nanofibers was much lower than that of cellulose
nanofibers, even lower than that of cellulose triacetate at the
same concentration. It matches the previous X-ray results
indicating the acetylation occurred from surface to the core of
cellulose nanofibers, which changed their properties.
Cellulose Nanofiber Membrane
[0170] Cellulose nanofiber aqueous solutions with different
concentrations were used to fabricate coating layers on PAN and PES
e-spun membranes, as shown in FIG. 41. A taped PAN/PES E-spun
membrane is soaked in water in step 402. Excess water is drained
from the E-spun membrane in step 404. Cellulose nanofiber
suspension is applied on one side of the membrane in step 406. The
solution is cast evenly on the membrane surface in step 408. The
membrane is dried at room temperature in step 410. All membranes
were stored at room temperature in a dry box and wetted with water
before use.
[0171] Cellulose nanofiber aqueous solutions can be applied in
water filtration process. One embodiment of the present invention
uses a concentrated cellulose nanofiber aqueous solution (with
concentration being higher than 0.05%) as a coating solution to
produce a coating layer directly on electrospun substrates. A
second embodiment of the present invention uses .about.5 nm
diameter cellulose nanofibers (with a variable concentration being
often lower than 0.05%) as an additive to strengthen the top
barrier as a nano-composite and to create liquid (in this case,
water) nano-channels in order to increase the permeation flux of
the membrane.
[0172] There are many advantages to using ionic liquid as the
solvent in fabricating cellulose as the barrier layer in the
separation membrane. First, the cellulose nanofibers could be
dispersed in water, which is regarded as a green solvent. It is
environmentally benign when compared with most other coating
processes. Second, water can be evaporated directly after coating
without further treatment. When an ionic liquid is used, which is
also regarded as a green solvent, one more step has to be
performed, i.e., removal of the ionic liquid by water or ethanol
extraction. Third, with concentration of the cellulose nanofiber
aqueous solution being much lower (about 0.05% to 0.20%, and
partially because of their high viscosities at higher
concentrations), a very thin barrier layer (for example, the range
of barrier layer is from 0.05 to 1.0 .mu.m) can easily be achieved.
Finally, many cellulose nanofibers with .about.5 nm diameter can
serve as water channels to improve the permeation flux of
water.
[0173] The formation of a TFC was used to fabricate this unique
class of high-flux membranes containing an integrated nanocomposite
cellulose nanofiber coating layer. Typically, the TFC membrane
consists of a three-tier composite structure. The top layer is the
key for separation of solute and permeates. The middle layer serves
as a support, in which the high-flux nano-fibrous membrane with
large porosity and uniform structure is used in the present study.
This nanofibrous mid-layer is often fabricated by electrospinning
that replaces the conventional asymmetric porous membrane, normally
fabricated by the phase inversion method. PAN E-spun membrane was
used as the middle layer. The bottom layer is a non-woven
microfibrous support possessing strong mechanical property. The top
cellulose nanofiber coating layer is suitable for both the
conventional TFC format and the high flux nano-fibrous format.
[0174] The PAN E-spun membrane, as shown in FIGS. 41(a)-(c), was
prepared under prescribed conditions. The surface morphology of the
PET non-woven substrate is also included in FIG. 42, with a top
view provided in FIG. 42(a), and views tops and cross-sectional
view of a PAN electrospun (E-spun) membrane provided in FIG. 42(b)
and (c). The average fiber diameter of E-spun membrane estimated
from SEM images was about 159.7.+-.60.7 nm, which was about 180
times smaller than that of the PET non-woven support
(28,900.+-.6,200 nm). From the cross-sectional image of the E-spun
membrane, the fiber diameter was estimated at about 159.0.+-.58.6
nm, in very good agreement with that estimated from the top
view.
[0175] Table 6 lists maximum pore size of the E-spun membrane and
that of the PET support based on bubble point tests with water as
the fluid. The maximum pore size of PET was above 400 .mu.m,
representing about one hundred times higher than that of the PAN
E-spun membrane (.about.3.6 .mu.m). In the embodiments of the
present invention, although the porosity of the PAN E-spun membrane
was about 83.2%, while that of the PET substrate was approximately
62.1%, as measured by the weighing method, the pure water flux of
PAN E-spun membrane was about 20 times lower than that of PET at
low pressures (e.g., .about.2.3 psi). This discrepancy is due to
the uneven pore size and blockage of the E-spun membrane.
TABLE-US-00006 TABLE 6 Pure Ultimate Fiber Pore water Young's
tensile Elongation Thickness diameter size Porosity flux modulus
strength at break (.mu.m) (nm) (.mu.m) (%) (L/m.sup.2h) (MPa) (Mpa)
(%) PET 135.2 .+-. 17.8 28900 .+-. 6200 >400 62.1 92000 .+-. 200
276.2 .+-. 113 27.2 .+-. 5.3 16.2 .+-. 6.0 PAN E- 54.0 .+-. 5.0
159.7 .+-. 60.7 ~3.6 83.2 4000 .+-. 20 125.2 .+-. 2.6 6.3 .+-. 0.1
48.0 .+-. 1.0 spun Cellulose 0.1 .+-. 0.05 -- <0.055 -- 140.0 --
-- -- nanofiber barrier PAN10 151.8 .+-. 7.2 -- <0.013 17.3 25.1
-- -- -- PAN400 141.0 .+-. 4.0 -- <0.055 15.5 84.2 -- -- --
[0176] The mechanical property of the PAN E-spun membrane as well
as that of PET had been determined by tensile experiments, as
listed in Table 6. The Young's modulus of the PET was 2-3 times
higher than that of the PAN E-spun membrane. Meanwhile, the
ultimate tensile strength of the PET was four times higher than
that of the PAN E-spun membrane at comparable elongation to break.
All results definitely indicated that the PET was a stronger
substrate than the PAN E-spun membrane. Thus, the composite
membrane of the present invention, the PET was used as the bottom
substrate, supplying the needed high mechanical strength.
[0177] The PAN E-spun membrane was immersed in DI water (pH
.about.2) to be wetted and to be filled with water in order to
prevent heavy penetration of the cellulose nanofiber solution. At
the interface between acidic water and cellulose nanofiber
suspension, a gel is immediately formed which slows down the
diffusion of the cellulose nanofiber solution from aqueous phase to
support phase. As a result, only one or two layers of the E-spun
nanofibers are incorporated into the barrier layer. The cellulose
nanofiber solution is cast with a coating rack having a thickness
of the barrier layer as controlled by the height of the tapes used.
After coating, the cellulose nanofiber-coated membrane is dried at
room temperature or 100.degree. C. The cellulose nanofiber membrane
is stored for use at room temperature after drying.
[0178] FIG. 43 shows the thermal stability of the cellulose
nanofiber membranes prepared from cellulose nanofiber solutions
with different concentrations. The cellulose nanofiber membranes
were dried by freeze drying. The onset of decomposition of the
membranes are almost same at 240.degree. C., however, the
decomposition of the cellulose membrane prepared from lower
concentration (e.g., 0.01 or 0.05 wt %) is slower than that of
higher concentration of cellulose. This trend is opposite that of
FIG. 30, in which the higher concentration, the slower
decomposition. The completely decomposed temperature for all
membranes is above 310.degree. C., which is higher than that of the
original cellulose nanofibers, which is below 280.degree. C. This
difference is attributed to the gelatin process of cellulose
nanofibers at acidic conditions.
[0179] Cellulose nanofiber membranes on PAN/PES substrate, prepared
by using different concentrations of cellulose nanofiber solutions
were obtained by following the procedure outlined in FIG. 41. FIG.
44 shows their SEM images, namely, (a) PAN e-spun membrane; (b)
Cellulose nanofiber/PAN coated with the concentration 0.20%; (c)
Cellulose nanofiber/PAN coated with the concentration 0.30% (insert
is the different part on the membrane); (d) Cellulose nanofiber/PAN
coated with the concentration 0.50% (insert is the cross-section of
the membrane); (e) PES e-spun membrane; (f) Cellulose nanofiber/PES
coated with the concentration 0.20%; (g) Cellulose nanofiber/PES
coated with the concentration 0.30% (insert is the different part
on the membrane); (h) Cellulose nanofiber/PES coated with the
concentration 0.50% (insert is the cross-section of the
membrane).
[0180] As shown in FIG. 44, after coating with cellulose nanofiber
aqueous solution (0.20%), the surface fibers were still present due
to the heavy penetration of the coating solution. As described
above, the viscosity of cellulose nanofiber aqueous solution with
0.20% concentration was 0.003 Pas, which was much lower than that
of the cellulose coating solution in an ionic liquid (0.10%, 0.18
Pas). Buy increasing the concentration of cellulose nanofiber
aqueous solution to 0.30%, the viscosity is increased to
.about.0.08 Pas, thereby improving the coating layer quality due to
a decrease in the penetration of the substrate. However, at the
same time, the coating solution was distributed less evenly so that
some part could not be covered very well by cellulose nanofibers
(see FIG. 44 (c)). Further increasing the concentration to 0.50%, a
flat and evenly coated layer was observed from FIG. 44 (d), and the
viscosity of the coating solution was increased to about 2.63 Pas.
This cellulose nanofiber membrane was selected to test the water
filtration process. However, the thickness of the coating layer was
about 1.5 .mu.m. It was too thick when compared with the cellulose
membrane prepared by using an ionic liquid, which has a top layer
thickness of only 0.3 to 0.5 .mu.m. Cellulose nanofiber/PES
membrane showed similar results when compared with that of
cellulose nanofiber/PAN, with the thickness of the top layer being
about 2.5 .mu.m when 0.50% of cellulose nanofiber aqueous solution
was used. Top layer thickness optimization is dependent upon
solution viscosity, the solvent used, and the processing procedure,
which can be achieved in a continuous large-scale processing
procedure by appropriate adjustment of the variables.
[0181] Pure water fluxes of cellulose nanofiber membranes are shown
in FIG. 45. When the coating layer was too thick, the pure water
flux of cellulose nanofiber membrane/PAN/PES was lower when
compared with a commercial membrane PAN10 (Sepro). Thus, cellulose
nanofiber membranes with thinner layers (.about.0.5 .mu.m) are
required without heavy penetration into the substrate,
specifically, with concentration of cellulose nanofiber aqueous
solution lower than 0.50%.
[0182] As described above, the cellulose nanofiber aqueous solution
is sensitive to pH value and ionic strength, and decreasing pH or
increasing ionic strength of the solution will promote the
formation of gelatin. The e-spun PAN/PES membrane is immersed in
water with the pH value less than 2.0, or in water with the
concentration of sodium chloride more than 0.04 mol/L before
coating the cellulose nanofiber aqueous solution. At the
interfacial phase between E-spun fibers and coating solution,
gelatin was formed which avoids penetration even at very low
concentrations of cellulose nanofiber solution. A cellulose
nanofiber aqueous solution with a concentration=0.20% was used to
prepare the cellulose nanofiber/PAN membrane. FIG. 46 shows
corresponding SEM images. The surface of cellulose nanofiber
membrane is very flat and the thickness of the top layer was about
0.5 .mu.m. However, in this case, 0.01 mol/L of HCl has to be used
which may challenge steel instruments used in large scale
production.
[0183] Another cellulose nanofiber membrane, prepared with 0.2
mol/L of sodium chloride aqueous solution, followed the same
procedure. The corresponding SEM images are shown in FIG. 47.
Sodium chloride crystals are formed after drying the membrane,
which could produce defects on the surface of the membrane. After
pouring the coating solution on the surface of the E-spun membrane
and waiting one minute, the coated membrane may be inserted into
water to remove salt for one-half hour. The membrane is then dried
at room temperature. FIG. 48 shows the corresponding SEM images of
the membrane with cellulose top layer coating.
[0184] The pure water flux of the cellulose nanofiber membrane
increased with increasing pressure. However, the flux depends
dramatically on the thickness of the top layer. The thinner the top
layer, the higher the flux, according to D'Arcy's law, as shown in
Equation (5):
J = KP .eta. L , ( 5 ) ##EQU00003##
in which K is hydraulic permeability of the membrane, .eta. is
viscosity of the liquid, L is thickness of the membrane and P is
pressure.
[0185] This result suggests that J is inversely proportional to L
and thinner coating is preferred in order to obtain higher
permeation flux without decreasing the rejection ratio. However,
the mechanical stability of the cellulose membrane is also taken
into account, i.e., the thinner coating layer will eventually not
be able to withstand higher pressures, more easily producing
defects on the membrane during the coating process.
[0186] In the present invention, a 0.1-0.2 .mu.m thickness for the
barrier layer is preferable for the cellulose membrane. Compared to
that of PAN10 with a water permeation flux of 3.44 L/(m.sup.2hpsi),
the pure water permeation flux of the cellulose nanofiber membrane
at a barrier layer thickness of approximately 0.1 .mu.m was 61.40
L/(m.sup.2hpsi), about 18 times higher based on the same applied
pressure. Even for PAN400, the permeation flux was approximately
36.90 L/(m.sup.2hpsi), about 1.7 times lower than that of cellulose
nanofiber membrane.
[0187] Following the similar procedure as shown in FIG. 41, a
series of cellulose nanofiber membranes were prepared with PAN
E-spun membranes as the support and with different concentrations
of cellulose nanofiber aqueous solution to prepare the barrier
layer. 0.10 wt % and 0.05 wt % of cellulose nanofiber solutions
were used to prepare cellulose nanofibers membrane in order to
obtain the membrane with a very thin barrier layer. The SEM images
of the cellulose nanofiber membranes were obtained and the surface
morphology as well as the cross-sectional part of the membranes was
observed. FIG. 49 shows at A and B cross-sectional and top views
for a membrane from 0.10 wt %. At C and D of FIG. 49,
cross-sectional and top views are shown for the membrane from 0.05
wt % of solution.
[0188] The thickness of the barrier can be estimated from the
cross-sectional image. For the membrane prepared from cellulose
nanofiber solution with 0.10 wt % of concentration, it is about 0.2
.mu.m thick. The thickness of the membrane prepared from 0.05 wt %
of cellulose nanofiber solution was about 0.1 .mu.m, which is
fairly thin compared to the barrier of other ultrafiltration
membranes and implies a very high permeation flux even at lower
pressure.
[0189] The barrier layer thickness in the resulting nanofibrous
membrane was about 0.1 .mu.m, and its surface was smooth and flat.
From the cross-sectional image, the cellulose nanofiber barrier
layer was shown to be relatively uniform with some segments of the
PAN E-spun nanofibers being imbedded in the barrier layer. This
integrated nanocomposite format reinforces the mechanical strength
of the top coating layer, if the nanofibers are stronger than the
more amorphous top layer. From the top view of the membrane, some
streaks which could come from the contour of the nanofibrous
scaffold were evident. The surface porosity of the nanofibrous
scaffold was about the same as the bulk porosity of the scaffold
(about 83.2%), being several times higher than that of current
commercial asymmetric membranes (about 15.5.about.17.3% estimated
from SEM images, FIG. 50 (A) and (B)) being fabricated by the phase
inversion method.
[0190] The MWCO of the cellulose nanofiber membrane based on the
PAN E-spun membrane support is shown in FIG. 51. The MWCO of the
cellulose nanofiber membrane with 0.1-.mu.m thickness of the top
layer was .about.2,000 KDa while the rejection ratio was higher
than 90%, according to the TOC result. The Stokes-Einstein radius
(r.sub.s in .ANG.) of polydispersed dextran can be calculated from
Equation (6):
r.sub.s=0.33.times.(MW).sup.0.463 (6)
with MW being the molecular weight of dextran in Da. According to
the Equation (6), the radius of the dextran with molecular weight
of 2,000 KDa was 27.3 nm. The MWCO of the cellulose nanofiber
membrane was .about.2,000 KDa, implying that more than 90% of the
pores of the cellulose nanofiber membrane were smaller than the
diameter of the dextran with 2000 KDa which is about 54.6 nm, thus,
the pore size (d.sub.p in .ANG.) of cellulose nanofiber membrane
could be estimated according to Equation (7):
d.sub.p=2r.sub.s (7)
According to the Equation (7), the pore diameter of the cellulose
nanofiber membrane was less than 50 nm (for .about.90% of
pores).
[0191] The MWCO of PAN10 was about 70 KDa, while that of PAN400 was
about 2,000 Kda, comparable with that of the cellulose nanofiber
membrane.
[0192] Oil/water emulsion is used a simulated model for
purification of bilge water in naval and other ships. Bilge water
is a major pollutant, especially of the ocean. The oil particle
size of oil/water emulsions could be estimated from 0.2 to 5.0
.mu.m, being much higher than the MWCO of the cellulose nanofiber
membrane, indicating that the fabricated cellulose nanofiber
membrane could be used to separate impurities beyond the oil/water
emulsion, although for oil molecules, smaller pores would be
needed. The oil concentration of the permeate after filtration with
the cellulose nanofiber membrane was less than 6.75 ppm (as the
rejection ratio is above 99.5%), which satisfies environmental
wastewater discharge standards, typically of <10 ppm, indicating
that the cellulose nanofiber membrane is a good candidate for the
treatment of oil/water wastewater.
[0193] The filtration efficiency was calculated in terms of the
rejection percentage (R %) as follows in Equation (8):
R % = C f - C p C f .times. 100 % , ( 8 ) ##EQU00004##
where C.sub.f and C.sub.p are concentrations of the feed solution
and the permeation solution, respectively. The concentrations were
determined by UV at a wavelength of 240 nm.
[0194] Cellulose nanofiber membranes were employed to filter the
oil/water emulsion at different pressures. FIG. 52 shows the
permeation flux and the rejection %. The permeation fluxes of
cellulose nanofiber membranes with different thickness of barrier
layers increased with increasing pressure, while keeping the
rejection ratio higher than 99.6%.
[0195] The permeation flux of thinner cellulose nanofibers
membranes increases slower than that of the thicker one, implying
that the thinner cellulose nanofiber membrane can most likely be
used very effectively at low pressures. The permeation flux of
thinner cellulose nanofiber membrane was about 400 L/m.sup.2h at 30
psi. However, for the thicker cellulose nanofiber membrane, higher
pressure will be better if high permeation flux is required.
[0196] FIG. 53 shows the thicker cellulose membrane used for 1 day
at 30 and 90 psi. The permeation flux of cellulose nanofiber
membrane decreased after a 1-day filtration of oil/water emulsion.
Only about 20% decrease in the permeation flux of cellulose
nanofiber membrane was observed at 30 psi. The permeation flux
decreased by about 48% at 90 psi, implying less fouling at lower
operating pressure. However, the permeation flux was still
6.about.7 times higher than that of PAN 10 (about 50% decreased in
permeation flux at 30 psi for one day). The rejection ratio was
kept higher than 99.6% during the performance.
[0197] The performance of cellulose nanofibrous membrane was
compared with that of two commercial PAN UF membranes: PAN10 and
PAN400. Although the membrane PAN10 has a lower MWCO (.about.70
KDa), it shows a similar rejection ratio for oil/water emulsion to
that of the cellulose nanofiber membrane. FIG. 54 shows that the
fluxes of both membranes decreased slowly during the filtration
process because of the combined effects of the scaffold compaction
as well as the fouling of oil on the surface of the membranes,
while the corresponding rejection ratio increased slightly. After
48 hours of operation, the flux became more stable. The permeation
flux of the cellulose nanofiber membrane was decreased by
.about.31%, while that of PAN10 was seriously decreased by
.about.66% of the starting value. The permeation flux of cellulose
nanofiber membrane was consistently approximately eleven times
higher than that of PAN10, while retaining the similar rejection
ratio (above 99.6%) in both systems.
[0198] As for PAN400, the MWCO was about 2,000 KDa, similar to that
of the cellulose nanofiber membrane. However, it had a much lower
rejection ratio of only about 90.0% for the oil/water filtration at
the start of the filtration process and 98.2% after operation for
24 hours, being much lower than that of the cellulose membrane
(approximately 99.7%), while the permeation flux of the cellulose
nanofiber membrane was still two and one-half times higher than of
PAN400.
[0199] Membrane fouling remains a major problem for ultrafiltration
membranes in the application of oil/water separation, with usual
concerns being surface morphology and fluid affinity. Two kinds of
fouling have to be considered during the ultrafiltration process:
reversible and irreversible. Reversible oily fouling can be reduced
by making the surface morphology smoother or enhancing surface
hydrophilicity, while irreversible fouling would depend on surface
pore size and its distribution which could be manipulated by
considering the materials property and the coating process. The
surface hydrophilicity can be estimated on the basis of contact
angle measurements (CAM200 Optical Contact Angle Meter, KSV
Instruments, LTD. In this test, Milli-Q water was used as the probe
liquid.). For the cellulose nanofiber membrane, the contact angle
of water was about 18.7.degree., while that of PAN E-spun membrane
was about 57.6.degree., as well as that of PAN UF membrane at
approximately 50.degree., implying that the cellulose nanofiber
membrane should have lower fouling than that of commercial
PAN10/400 membranes. From FIG. 49, the PAN E-spun nanofibers were
partially imbedded in the cellulose barrier layer. Thus, the
surface morphology of the cellulose membrane would be affected by
that of the PAN E-spun mid-layer. Furthermore, the thinner fiber
diameter would produce a smoother top layer surface. Thus, the
effects of E-spun mid-layer support have a dual effect on the
performance of the top barrier layer.
[0200] Another the fouling test was carried out with sodium
alginate (500 ppm, 80-120 KDa), a microbial polysaccharide, which
usually represented extracellular polymeric substances, was added
in water. The permeation flux and the rejection ratio of the
cellulose nanofiber membrane were carried out for a period of 24
hours. The results are shown in FIG. 55. The permeation flux
decreased with time to approximately 55% of the starting flux,
implying a higher fouling rate than that of the oil/water emulsion
because of the adsorption of sodium alginate on the more
hydrophilic surface of the cellulose nanofiber membrane. However,
the rejection ratio was higher than 97.7% according to the TOC
results.
Cellulose Nanofiber Composite
[0201] In another embodiment of the present invention, the
cellulose nanofibers are introduced into the barrier layer by
simply mixing the cellulose nanofiber aqueous solution and PVA
solution at a different ratio in order to obtain a coating solution
with a certain amount of cellulose nanofiber. The preparation of
PVA (2 wt % and 4 wt %) coating solution is performed by adding 0.6
g (or 1.2 g of PVA for 4 wt %) of PVA in 29.4 g of water following
by heating the solution for 1 day at 90.degree. C. Cellulose
nanofiber aqueous solutions having concentrations of 0.01%, 0.05%,
0.10%, 0.20%, and 0.40% are added into the PVA aqueous solution
(4.0%) at equal weight ratio. The mixture is stirred at room
temperature over night. A clear solution is obtained containing PVA
(2.0%) and cellulose nanofibers (from 0.005 to 0.20%). However, the
PVA solutions with 0.10 and 0.20% of cellulose nanofibers were
heterogeneous under the mixing conditions (a phase separation was
observed).
[0202] The PVA/CN composite membrane is prepared according to the
following procedure. The procedure for the coating of the
cross-linked PVA top layer is as follows. (1) Soak the PAN E-spun
membrane in H.sub.3BO.sub.3 aqueous (0.8 mol/L) solution and drain
after putting on PET support. (2) Tape the edges of membrane to
control the thickness of the top coating layer of the membrane. (3)
2 wt % PVA aqueous solution (cross-linked by GA,
r=[--OH]/[GA]=0.25, 19.5 minutes reaction with GA.) is cast-coated
on the surface of PAN E-spun membrane. (4) The membrane is
incubated in an oven (humidity: 100%) at room temperature for
twelve hours before test.
[0203] SEM images of PVA and PVA/cellulose nanofiber composite
membranes are shown in FIG. 56. The thickness of the barrier of all
membranes was about 0.5-0.8 .mu.m. The surface morphology of the
membranes was very flat, which is a very important factor to affect
on the low fouling property of PVA membranes.
[0204] 0.01 and 0.05 wt % of cellulose nanofiber aqueous solutions
were employed to prepare PVA/CN composite membranes. After
dilution, the cellulose nanofiber concentration of the coating
solutions were 0.005 wt % and 0.025 wt %, respectively.
[0205] FIG. 57 shows pure water flux of PVA/CN composite membranes
compared with PAN 10 commercial membrane at different pressures,
while the temperature was about 25.degree. C. The thickness of the
barrier layers of all PVA membranes were about 0.5 .mu.m. It is
demonstrated that the pure water flux increases with increased
content of cellulose nanofibers in the PVA composite membrane,
which proved the creation of water channels in the PVA matrix. The
pure water flux of PVA/CN (1.25% to PVA) composite membrane is
shown as being two times higher than that of pure PVA without
cellulose nanofibers.
[0206] FIG. 58 shows the permeation flux and rejection % of PVA/CN
composite membrane during the filtration of oil/water emulsion. The
permeation flux PVA/CN composite membrane increased by
approximately 70% compared with that of pure PVA when cellulose
nanofibers (0.25 wt % to PVA) had been introduced into the PVA
matrix. The permeation flux was ten times higher than that of PAN
10 (Sepro).
[0207] In order to introduce higher amounts of cellulose nanofibers
into the barrier layer without phase separation or gelation during
the coating, the E-spun membrane is first coated with cellulose
nanofibers, then the holes are filled with a coating material such
as cellulose. An SEM image of the cellulose/CN composite membrane
is shown in FIG. 59.
[0208] FIG. 60 shows the results of oil/water emulsion for the
cellulose/CN composite membrane. The permeation flux of
cellulose/CN composite membrane was seven times higher than that of
PAN 10 while keeping similar rejection (%) above 99.5%.
[0209] Cellulose and cellulose nanofibers prepared from oxidized
cellulose were employed to prepare ultrafiltration membranes that
serve in filtration of oil/water emulsion. Chemically inert
cellulose membrane is well known, but can be consumed by bacteria
in an aerobic environment. Oxidized cellulose with 18-25% of
oxidation degree was used as hemostat in medical area. Thus, the
cellulose nanofiber membrane was anti-bacterial.
Membrane Adaptation
[0210] The chemical resistance study of cellulose membranes for
bilge water filtration was carried out to identify any potential
material compatibility problems associated with the know
constituents present in bilge water. After testing at 80.degree. C.
for 7 days, the cellulose nanofiber membrane still keeps higher
flux and higher rejection as before. The chemical resistance of
cellulose nanofiber membrane is higher than that of cellulose
membrane.
[0211] The chlorine resistance of cellulose and cellulose nanofiber
membranes was investigated. The percent rejection of cellulose
nanofiber membrane remained higher than 99.6% at pH 10, meaning
that the cellulose nanofiber membrane has higher hypochlorite
resistance and could be used in hypochlorite-existing solution.
[0212] The anti-bacterial property of cellulose nanofiber membrane
was studied with E-coli (the concentration is 5.5.times.10.sup.5
cfu/mL) as the model. The experiment is carried out at 37.degree.
C. for 3 days. The cellulose nanofiber has good tolerance to
bacteria because it is produced from oxidized cellulose which
usually is used as anti-bacterial materials (hemostat). Besides,
cellulose nanofiber membrane has a good stability in wide pH range
from 4 to 10, as well as cellulose membrane.
[0213] The stability of cellulose and cellulose nanofiber membranes
is summarized in Table 7, in which " " indicates good resistance
and "x" indicates poor resistance.
TABLE-US-00007 TABLE 7 Cellulose Cellulose nanofiber Before After
Before After Chemical Reducing Flux (L/m.sup.2h) 83.1 54.9 Reducing
Flux 60.3 69 resistance (L/m.sup.2h) Rejection % 99.9 96.0
Rejection % 99.8 99.7 Oxidizing Flux (L/m.sup.2h) 75.2 77.1
Oxidizing Flux 67.5 86 (L/m.sup.2h) Rejection % 99.9 98.0 Rejection
% 99.9 99.4 Bacteria x Flux (L/m.sup.2h) 112.7 95.3 Flux 100.9
124.6 resistance (L/m.sup.2h) Rejection % 99.9 99.1 Rejection %
99.8 99.7 Chlorine pH = 4 Flux (L/m.sup.2h) 94.9 80.1 x pH = 4 Flux
124.6 92 resistance (L/m.sup.2h) Rejection % 99.9 99.9 Rejection %
99.7 98.2 x pH = 10 Flux 94.9 111.3 pH = 10 Flux 124.6 118.7
(L/m.sup.2h) (L/m.sup.2h) Rejection % 99.9 98.3 Rejection % 99.7
99.6 pH pH = 4 Flux 71.2 65.3 pH = 4 Flux 142.4 169.1 sensitive
(L/m.sup.2h) (L/m.sup.2h) Rejection % 99.9 99.9 Rejection % 99.9
99.9 pH = 10 Flux 71.2 89.0 pH = 10 Flux 142.4 151.3 (L/m.sup.2h)
(L/m.sup.2h) Rejection % 99.9 99.9 Rejection % 99.9 99.9
[0214] In industry, an ultrafiltration membrane has to be used for
long time, e.g., half of a year, before being discarded. Moreover,
two weeks can be required for cleaning, to save cost and energy.
That means that the life of a membrane must long enough for scale
up production and practical application. Further decreasing fouling
in order to increase the efficiency is also an urgent task for
water purification. Based on those requirements, the cellulose
membrane is modified by further cross-linking reactions.
[0215] As described above, the cellulose nanofiber has three
different functional groups, hydroxyl, carboxylic, and aldehyde
groups that provide a platform to modify the cellulose nanofiber
membrane. Considering the continuous production process in the
future, as well as the property of cellulose nanofiber aqueous
solution, there are different strategies to do the
cross-linking.
[0216] There are two possible places to add the cross-linking
reagent. First, the cross-linking agent may be added to the
cellulose nanofiber coating solution. Second, the cross-linking
process may be added in the aqueous bath, which serves as the
protection of heavy penetration to E-spun membrane. Some
cross-linking reagents can be dissolved into the water bath before
coating without negative effect on the coating process.
[0217] The cross-linking preferably occurs during the drying of the
membrane under heating conditions, or through post-cross-linking
which requires one more step to do the cross-linking reaction. A
first method is a heating cross-linking reaction based on the
aldehyde groups and carboxylic groups located on the surface of
cellulose nanofibers produced by TEMPO/NaBr/NaClO oxidizing system.
Such a cross-linking mechanism is shown in FIG. 61.
[0218] The content of aldehyde is determined by hydroxylamine
hydrochloride titration experiment. The aldehyde group content is
about 0.25 mmol/g cellulose in cellulose nanofiber. The carboxylic
group content is determined as .about.0.70 mmol/g cellulose. After
coating, the membrane is heated at 100 for 20 min in an oven, the
thickness of the barrier is about 0.2 .mu.m. The TEM image of the
cellulose nanofiber film prepared from cellulose nanofiber aqueous
solution (0.05 wt %) coated on TEM grid and stained by uranyl
acetate (2.0%) was obtained, as shown in FIG. 62. The surface
morphology of cellulose nanofiber membrane was completely composed
with cellulose nanofibers and very dense after formation of the
membrane, which implies that the cellulose nanofiber membrane could
be applied for ultrafiltration process. To estimate the new
filtration performance, the membrane is tested with oil/water
emulsion.
[0219] The filtration performance pressure is 30 psi and
temperature is .about.39.degree. C. The % rejection remains higher
than 99.8% with little higher fouling after filtration for 72
hours, as shown in FIG. 63. As compared with previous results, the
membrane structure is more stable during the filtration,
demonstrating successful cross-linking of the cellulose nanofiber
membrane through the heating process.
[0220] The MWCO of the membrane remains similar to that of a
membrane without a heating treatment, i.e., about 2,000 KDa, as
shown in FIG. 64.
[0221] The fouling test was also carried out with sodium alginate
(500 ppm, 80-120 KDa) in water. The permeation flux and the
rejection ratio of the cellulose nanofiber membrane were obtained
after a period of 48 hours, and the results are shown in FIG. 65.
The permeation flux decreased with time to approximately 30% of the
starting flux, implying a lower fouling rate than that of the
cellulose nanofiber membrane without heating. However, the
rejection ratio was only about 96.3.degree. A) according to the TOC
results.
[0222] To further cross-link cellulose nanofibers, GA with
different concentration was employed in the coating process by
immersing PAN e-spun membrane into GA acidic aqueous solution. The
cross-linking reaction is shown in FIG. 66. A concentration of GA
lower than 0.0015 mol/L is more helpful than that of higher
concentration of GA in this coating recipe. Furthermore, glyoxal is
another good cross-linker for cellulose.
[0223] Considering that aldehyde reagents are slightly toxic when
only a small amount of GA or glyoxal is used in the coating
process, another good cross-linking reagent regarded as non-toxic
and also very cheap is PolyAcrylic Acid (PAA) with different
molecular weight. FIG. 67 shows a cellulose nanofiber membrane
cross-linked by PAA.
[0224] PAA (molecular weight is 450 KDa) could be added into the
cellulose nanofiber aqueous solution because the pKa of PAA is
4.30, which does not encourage cellulose nanofiber to form a gel.
The concentration of PAA seriously affects the anti-fouling
property of the cellulose nanofiber membrane, as shown in FIG. 68.
A catalyst, Sodium HypoPhosphite (SHP), may be employed to promote
the cross-linking reaction with PAA. Following the same procedure
as above, except using water bath (pH=2.02) containing
NaH.sub.2PO.sub.2 (0.01 mol/L) instead of the original.
[0225] A similar cross-linker, PolyVinylamine Hydrochloride, may be
used to cross-link the cellulose nanofiber membrane. PVAH can not
be added into cellulose nanofiber solution directly because is may
form a gel before the coating process due to its strong acidic
property. However, PVAH (0.0015 mol/L) can be added into the water
bath with pH=1.90. The membrane fouling is a little heavy, however,
the rejection remains very high .about.99.8%.
[0226] Epichlorogydrin (ECH) is a common cross-linking reagent in
the food industry for polysaccharides. It based on the reaction
between epoxy group, chlorine, and hydroxyl groups under basic
conditions. This cross-linking reaction can not be carried out
during the coating process. An additional cross-linking step has to
be done after preparation of the cellulose nanofiber membrane.
[0227] As described above, cellulose nanofibers can gelatin with an
increase in the ionic strength of the solution. The gel forms
completely and rapidly when the concentration of sodium chloride is
higher than 0.033 mol/L. Ionic liquid, such as
1-ethyl-3-methylimidazolium acetate, is also a salt but liquid at
room temperature. Thus, it could be used to form a gel of cellulose
nanofiber instead of sodium chloride. The rheological behavior of
cellulose nanofiber aqueous solution is shown in FIG. 69. The
gelatin occurred when the concentration of EAc is above 0.033
mol/L, the viscosity of the gel solution is about 16.3 Pas.
[0228] A structure of new ionic liquid cross-linkers (C2IL) is
shown in FIG. 70. The structure of the compounds was confirmed by
.sup.1H NMR. For the samples when n=0 and 8, The .sup.1H NMR data
(D.sub.2O as the solvent) are shown below: (n=0, .delta., ppm):
9.059 (N--CH--N, s, 2H), 7.779 (N--CH--CH, s, 2H), 7.489
(N--CH--CH, s, 2H), 7.070 (N--CH.dbd., m, 2H), 5.772 and 5.411
(N--CH.dbd.CH.sub.2, s, 4H), 4.769 (N--CH.sub.2, t, 4H). (n=8,
.delta., ppm): 8.940 (N--CH--N, s, 2H), 7.676 (N--CH--CH, s, 2H),
7.486 (N--CH--CH, s, 2H), 7.032 (N--CH.dbd., m, 2H), 5.730 and
5.345 (N--CH.dbd.CH.sub.2, s, 4H), 4.135 (N--CH.sub.2, t, 4H),
1.791 (N--CH.sub.2--CH.sub.2, m, 4H), 1.203
(N--CH.sub.2--CH.sub.2--(CH.sub.2).sub.6, m, 12H).
[0229] The new ionic cross-linker (n=0) was employed to incorporate
into the cellulose nanofiber matrix during coating process, and
polymerized to form a network with K.sub.2S.sub.2O.sub.8 as the
thermal initiator. Specifically, K.sub.2S.sub.2O.sub.8 was
dissolved in water bath with the concentration of 0.017 mol/L (0.2
wt %), the C2IL was also dissolved into the water bath with the
concentration of 6.2 g/L. After coating the membrane was dried at
100.degree. C. for 30 min. The thickness of barrier layer is
.about.0.2 .mu.m. The permeation flux decreased after one day of
filtration, while kept higher rejection .about.99.8%, as shown in
FIG. 71. The ionic surface of the membrane might lead to a little
heavy fouling, however, it is very benefit to adsorb viruses during
the filtration.
[0230] PolyEthylene Oxide (PEO) usually was considered as
anti-fouling materials, however, incorporated PEO into the membrane
by simply blending is not a good method because it may be washed
out during the filtration. In-situ cross-linking the PEO by thermal
or photo initiating the monomer containing two/three vinyl groups
could be helpful to immobilize PEO in the membrane.
[0231] A di-functional monomer, polyethylene glycol400 diacrylate
(SR344), was incorporated into the cellulose nanofiber coating
solution (0.05 wt %) with the concentration of 0.025 wt %. The
thermal initiator, K.sub.2S.sub.2O.sub.8 (0.01 wt %), was dissolved
in the water bath with pH=1.92. After coating, the membrane was
dried at 100.degree. C. for 25 min. The thickness of the barrier is
.about.0.2 .mu.m.
[0232] The permeation flux was about 260 L/m.sup.2h after two days
of filtration and going to a constant value while keeping the
higher rejection than 99.7%, as shown in FIG. 72. This indicated
that the introduction of PEO segment definitely decreases the
fouling during the filtration of oil/water emulsion, and the
membrane could be used for long time with high rejection.
[0233] The surface modification of cellulose nanofiber membrane had
been also carried out based on the chemical modification of
cellulose nanofiber. This modification has to be achieved after
preparation of the membrane because it occurred in an organic
solvent, such as toluene. The cellulose nanofiber membrane was
prepared first following the same procedure: cellulose nanofiber
coating solution (0.05 wt %) was coated on the PAN E-spun membrane
immersed into water bath at pH=1.70. The membrane was dried at
100.degree. C. for 20 min. The thickness of the barrier is about
0.3 .mu.m. The dry membrane was immersed in a mixture (HClO.sub.4:
0.4 mL; Acetic anhydride: 20 mL; acetic acid: 80 mL; toluene: 100
mL) for thirty minutes at room temperature. After wash it with
ethanol, the membrane was dried at room temperature before
using.
[0234] The permeation flux is relatively low, as shown in FIG. 73,
because the hydrophobic property of the membrane after
esterification which implies the different application for this
membrane instead of the filtration of oil/water emulsions. However,
the acetyl groups were introduced into the membrane, which will
enable anti-bacterial property to the cellulose membrane since
cellulose acetate/triacetate has higher bacterial tolerance.
[0235] Another strategy to decrease fouling by oil is the
application of polyether-b-polyamide (PEBAX). This material has a
super anti-fouling property due to the composition of the block
copolymer. Thus, PEBAX 1074 is introduced on the surface of
cellulose nanofiber membrane and check the anti-fouling
performance, results of which are shown in FIG. 74. The coating
conditions are: cellulose nanofiber solution (0.05 wt %) was coated
on PAN E-spun membrane immersed in water bath first at pH=1.79. The
thickness of the barrier is .about.0.2 .mu.m and the membrane was
dried at 100.degree. C. for 10 min. The PEBAX (0.01 wt % in
butanol) was coated on the surface (the thickness of the PEBAX
layer is .about.15 nm), and the membrane was dried at 100.degree.
C. for 10 min again.
[0236] Additional polysaccharides can also be employed to prepare
nano-scale materials for production of ultrafiltration membranes.
Chitin is a universal material and its production is only less than
cellulose on the earth. Chitin is a good candidate compared to
cellulose when in the form of nanofibers or dissolved in ionic
liquids for use in ultrafiltration membranes with anti-bacterial
and low fouling properties. Chitin nanofibers are prepared from
Chitin powder (Aldrich, from crab shell, 90%) following a similar
procedure as the preparation of cellulose nanofibers. Briefly, the
chitin powder is oxidized with TEMPO/NaBr/NaClO system followed by
homogenizer-treatment.
[0237] 10.0 g of Chitin powder is dispersed in 192 g of water. 0.2
g of sodium bromide and 0.04 g of TEMPO is dissolved in the
suspension solution. The reaction is started by adding 20 g of
sodium hypochlorite solution (10-13%) under stirring condition for
24 h. The pH value is kept at about 10.0 to 10.3 monitored with a
pH meter by adding 0.5 mol/L of sodium hydroxide aqueous solution.
The reaction is stopped by adding 10 mL of ethanol followed by
stirring for 20 minutes. The rough product is separated by
centrifuging (5000 rpm) of the reaction mixture and then decanting
the supernatant. The product was washed with De-Ionized (DI) water
5 times and separated finally by centrifugation. The yield is
approximately 80%.
[0238] 1.50 g of oxidized chitin slurry are dispersed in 70 g of
water and sonicated for 5 min with a homogenizer. Almost all the
oxidized chitin was defibrillated and the suspension was
centrifuged at 5000 rpm. The supernatant was the chitin nanofiber
suspension in water with a concentration of 0.10 wt %. Chitin
nanofiber/PAN 0.1 wt % aqueous solution is used as the coating
solution on the support PAN E-spun membrane immersed in pH=1.86
aqueous solution. The membrane was dried at room temperature after
coating and the thickness of coating layer was about 0.5 .mu.m.
FIG. 75 shows Permeation flux and % rejection of chitin nanofiber
membrane for filtration of oil/water emulsion. The filtration
performance pressure is 30 psi and temperature is 35.degree. C.
[0239] The permeation flux remains high after 1 day's test, and %
rejection is approximately 99.9%. The permeation flux of the chitin
nanofiber membrane decreases very slowly, which implies that the
fouling is lower than that of cellulose nanofiber membrane.
[0240] Chitosan nanofibers could also be prepared from chitin
nanofibers by hydrolysis of chitin nanofibers with sodium hydroxide
aqueous solution (0.1 mol/L) at ambient temperature for 24 h. After
the reaction, the chitosan nanofibers were separated by filtering
the suspension with microfiltration membrane (0.1 .mu.m) and
washing thoroughly with water.
[0241] Cellulose nanocrystals are also universally used materials,
and specifically used as additives in optical devices. The
permeation flux of the cellulose nanocrystal membrane decreases
slowly, which implies that the fouling is lower than that of
cellulose nanofiber membrane, as shown in FIG. 76. Cellulose
nanocrystal is a good candidate for an ultrafiltration process with
high flux and high percentage rejection for oil/water emulsion, and
with high crystallinity for potential special applications.
[0242] Cellulose nanocrystal is prepared from microcrystal
cellulose, which is a commercially available material (Aldrich),
with same method as the preparation of cellulose nanofibers. The
crystallinity of cellulose nanocrystals at greater than 85%
provides for more mechanical and other special properties. For
example, the high crystallinity of cellulose nanocrystals might be
a better material for anti-bacterial application. The aqueous
solution of cellulose nanocrystal (0.1 wt %) is completely
transparent which is differs from that of cellulose nanofibers.
[0243] Following the same coating procedure, cellulose nanocrystal
membrane was prepared and tested in oil/water filtration process.
The cellulose nanocrystal (0.1 wt %) aqueous solution is the
coating solution of the PAN E-spun membrane immersed in the aqueous
solution at pH=1.86. The membrane was dried at room temperature and
the thickness of barrier is about 0.2 .mu.m. FIG. 76 shows the
permeation flux and % rejection of cellulose nanocrystal membrane
for filtration of oil/water emulsion. The filtration performance
pressure is 30 psi and temperature is 43.degree. C.
[0244] The permeation flux remains relatively high after 2 days of
filtration, and % rejection is about 99.7%. The permeation flux of
the cellulose nanocrystal membrane decreases slowly, which implies
that the fouling is lower than that of cellulose nanofiber
membrane.
[0245] Similarly, as a comparable example, microcrystal cellulose
may be dissolved into the ionic liquid (EAc) for preparation of a
cellulose microcrystal membrane. Microcrystal cellulose may be a
good candidate for ultrafiltration process with high flux and high
% rejection for oil/water emulsion.
[0246] The diameter and length of nanofibers are estimated from TEM
images and listed in Table 8, showing diameter and length of
polysaccharide nanofibers. All polysaccharide nanofibers have
ultra-fine fiber diameter compared to that of E-spun nanofibers
which usually higher than 100 nm in diameter. The fiber diameter of
wood cellulose nanofibers is smallest and its length is higher than
that of other polysaccharide nanofibers.
TABLE-US-00008 TABLE 8 Nanofibers Diameter (nm) Length (.mu.m) Wood
cellulose 5~8 1~3 Chitin 15~20 0.3~0.5 Microcrystal cellulose 5~10
0.3~0.5 Cotton cellulose 20~30 0.2~0.5
[0247] FIG. 77 shows the thermal stability of polysaccharides
nanofibers including wood and cotton nanofibers, cellulose
nanocrystals from microcrystal cellulose, and chitin
nanofibers.
[0248] Pathogenic enteroviruses such as poliovirus and hepatitis A
virus, usually discharged from sewage system, are surviving in
water environment involving rivers, lakes, and ponds, etc., which
are often the reservoirs of drinking water. Modified cellulose
materials including sulfated cellulose, nitrocellulose, and
phosphate cellulose usually have been made into affinity membranes
to adsorb viruses from water in order to clean the drinking water.
The mechanism of the adsorption is mainly based on two important
interactions: electrostatic interaction and hydrophobic
interaction. Among them, the electrostatic interaction is often
considered as the foundation of adsorption. The reason is that the
basic composition of viruses is proteins, which have net charges in
water environment when the pH value is lower or higher than the
isoelectric point (pI). Below pI, the protein has positive charges
which much easily to be adsorbed onto the surface of negative
charged membrane (e.g., sulfated cellulose), while above pI, the
protein will be captured with positive charged membrane (e.g.,
ammonium modified cellulose). At the isoelectric point, the net
charge of the protein was zero which will has minimum adsorption.
There is special interaction between sugar and protein called
pseudo-affinity binding is also an important factor which affects
on the adsorption of viruses.
[0249] Commercial sulfated cellulose including beads or membrane
had been used as the adsorbed materials many years ago. The higher
efficiency and flexible performance of sulfated cellulose membrane
is the advantages over that sulfated cellulose beads and
ion-exchange chromatography. Nitrocellulose is a common material
which could be used to adsorb proteins due to the negative charged
surface at neutral environment. An alternative strategy is tethered
special ligands (e.g., Cibacron blue dye) onto the surface of the
membrane for adsorption of bovine serum albamin (BSA).
[0250] A modification strategy of cellulose nanofibers for
adsorption of viruses is shown in FIG. 78. The sulfated cellulose
nanofiber membrane was prepared following the procedure described
in WO 2008/125361, the contents of which are incorporated herein by
reference, with slight modification. Chlorosulfonic acid (24 mL)
was dropwise added to pyridine (400 mL) in an ice bath. Then, the
solution was heated to 60.degree. C. and 200 mL of additional
pyridine was added. After completely dissolving the precipitated
components, the solution was cooled to 37.degree. C. and 10 pieces
of cellulose nanofiber membranes were added. Those Membranes were
incubated for 16 hours at 37.degree. C. Subsequently, membranes
were washed with phosphate buffered saline (PBS) and pure water
thoroughly to remove the residue pyridine and dried at room
temperature. The sulfate content of the sulfated cellulose
membranes was determined to .about.4.9 wt % by elemental
analysis.
[0251] The molecular weight of BSA is 69.3 KDa, and pI in water is
4.7 at 25.degree. C. The optical absorbance of 1 mg/mL is at 279 nm
which could be determined by UV. The dimension of BSA is
4.times.4.times.14 nm.sup.3. The adsorption capacity of BSA in the
cellulose nanofiber membrane was measured batch wise. 0.328 g of
cellulose nanofiber membrane was immersed in 10 mL BSA solution
(1.0 mg/mL) in PBS (pH=7.2) on a shaking bed for 12 h. The amount
of the BSA adsorbed on the membrane was calculated from the
concentration change of the BSA solution before and after the
adsorption determined by optical absorption at 280 nm. The BSA
adsorptive capacity as the function of time was determined, as
shown in FIG. 79. After twelve hours, the concentration of BSA
reaches to equilibrium and the adsorption of cellulose nanofiber
membrane to BSA was saturated and the capacity of the adsorption
reaches the maximum level at this conditions. The adsorption
capacity in 1 mg/mL of BSA solution (pH=7.2) is approximately 16.8
mg/g membrane, excluding the PET substrate.
[0252] The recycling and reusing of cellulose nanofiber membrane
was investigated by desorption of BSA adsorbed membrane in the
elution buffer (PBS+2M NaCl, pH 11) for six hours under stirring.
The membrane was then rinsed with DI water thoroughly. The BSA
adsorption test described above was repeated to measure the amount
of the BSA newly captured by the recycled membrane. The cycle was
repeated twice for the same membrane to study the reusability of
the membrane, as shown in Table 9 of BSA adsorption capacities of
the recycled cellulose nanofiber membranes.
TABLE-US-00009 TABLE 9 Recycle times Adsorption capacity (mg/mL) 0
16.8 1 14.9 2 15.4
[0253] PAN E-spun membrane was another good candidate for the
adsorption of proteins because of the big surface area to volume as
mentioned before. In order to introduce negative charges on the
surface of PAN nanofibers, the membrane was treated by sodium
hydroxide aqueous solution. The hydrolytic reaction occurred at the
interface of the solid and the solution, thus those nitrile groups
contacting the solution can be converted into carboxylic group, as
shown in FIG. 80.
[0254] The PAN E-spun membrane was modified by immersion in 1 mol/L
NaOH solution for five days. After modification, the membrane was
thoroughly washed with water to remove the excess NaOH. The
hydrolysis converted some of the surface nitrile to carboxylic
groups. The number of carboxylic group increases with the time, the
temperature, and the concentration of the NaOH solution
[0255] The product is polyacrylamide and polyacrylic acid. Both are
hydrophilic, and PAA is ionizable in water. Therefore, hydrolyzed
PAN E-spun nanofibers can serve as virus-adsorbers.
[0256] While the invention has been shown and described with
reference to certain embodiments thereof, it will be understood by
those skilled in the art that various changes in form and detail
may be made therein without departing from the spirit and scope of
the invention as defined by the appended claims and their
equivalents.
* * * * *