Chi LEUNG, et al.
Cellulose Nano-Crystals (
Wood pulp extract stronger than carbon fiber or
By Brian Dodson
The Forest Products Laboratory of the US Forest Service has opened
a US$1.7 million pilot plant for the production of cellulose
nanocrystals (CNC) from wood by-products materials such as wood
chips and sawdust. Prepared properly, CNCs are stronger and
stiffer than Kevlar or carbon fibers, so that putting CNC into
composite materials results in high strength, low weight products.
In addition, the cost of CNCs is less than ten percent of the cost
of Kevlar fiber or carbon fiber. These qualities have attracted
the interest of the military for use in lightweight armor and
ballistic glass (CNCs are transparent), as well as companies in
the automotive, aerospace, electronics, consumer products, and
* Three-dimensional ball and stick model of the cellulose polymer
* Micrographs of randomly oriented CNCs
* The upper figure shows the structure of the cellulose polymer;
the middle figure shows a n...
* Cross-sectional structure of various types of cellulose
nanocrystals showing various cryst...
of cellulose fibers from wood pulp
Cellulose is the most abundant biological polymer on the planet
and it is found in the cell walls of plant and bacterial cells.
Composed of long chains of glucose molecules, cellulose fibers are
arranged in an intricate web that provides both structure and
support for plant cells. The primary commercial source for
cellulose is wood, which is essentially a network of cellulose
fibers held together by a matrix of lignin, another natural
polymer which is easily degraded and removed.
Wood pulp is produced in a variety of processes, all of which
break down and wash away the lignin, leaving behind a suspension
of cellulose fibers in water. A typical cellulose wood fiber is
only tens of microns wide and about a millimeter long.
The cellulose in wood pulp, when dry, has the consistency of fluff
or lint - a layer of wood pulp cellulose has mechanical properties
reminiscent of a wet paper towel. Not what you might expect to be
the source of one of the strongest materials known to Man. After
all, paper is made from the cellulose in wood pulp, and doesn't
show extraordinary strength or stiffness.
Further processing breaks the cellulose fibers down into
nanofibrils, which are about a thousand times smaller than the
fibers. In the nanofibrils, cellulose takes the form of
three-dimensional stacks of unbranched, long strands of glucose
molecules, which are held together by hydrogen bonding. While not
being "real" chemical bonds, hydrogen bonds between cellulose
molecules are rather strong, adding to the strength and stiffness
of cellulose nanocrystals.
The upper figure shows the structure of the cellulose polymer; the
middle figure shows a nanofibril containing both crystalline and
amorphous cellulose; the lower figure shows the cellulose
nanocrystals after the amorphous cellulose is removed by acid
Within these nanofibrils are regions which are very well ordered,
in which cellulose chains are closely packed in parallel with one
another. Typically, several of these crystalline regions appear
along a single nanofibril, and are separated by amorphous regions
which do not exhibit a large degree of order. Individual cellulose
nanocrystals are then produced by dissolving the amorphous regions
using a strong acid.
At present the yield for separating CNCs from wood pulp is about
30 percent. There are prospects for minor improvements, but the
limiting factor is the ratio of crystalline to amorphous cellulose
in the source material. A near-term goal for the cost of CNCs is
$10 per kilogram, but large-scale production should reduce that
figure to one or two dollars a kilo.
structure of various types of cellulose nanocrystals showing
various crystalline arrangements of the individual cellulose
polymer molecules (the rectangular boxes)
CNCs separated from wood pulp are typically a fraction of a micron
long and have a square cross-section a few nanometers on a side.
Their bulk density is low at 1.6 g/cc, but they exhibit incredible
strength. An elastic modulus of nearly 150 GPa, and a tensile
strength of nearly 10 GPa. Here's how its strength to compares to
some better-known materials:
The only reinforcing material that is stronger than cellulose
nanocrystals is a carbon nanotube, which costs about 100 times as
much. Stainless steel is included solely as a comparison to
conventional materials. The relatively very low strength and
modulus of oak points out how much the structure of a composite
material can degrade the mechanical properties of reinforcing
As with most things, cellulose nanocrystals are not a perfect
material. Their greatest nemesis is water. Cellulose is not
soluble in water, nor does it depolymerize. The ether bonds
between the glucose units of the cellulose molecule are not easily
broken apart, requiring strong acids to enable cleavage reactions.
The hydrogen bonds between the cellulose molecules are also too
strong in aggregate to be broken by encroaching water molecules.
Indeed, crystalline cellulose requires treatment by water at 320°
C and 250 atmospheres of pressure before enough water intercalates
between the cellulose molecules to cause them to become amorphous
in structure. The cellulose is still not soluble, just disordered
from their near-perfect stacking in the crystalline structure.
But cellulose contains hydroxyl (OH) groups which protrude
laterally along the cellulose molecule. These can form hydrogen
bonds with water molecules, resulting in cellulose being
hydrophilic (a drop of water will tend to spread across the
cellulose surface). Given enough water, cellulose will become
engorged with water, swelling to nearly double its dry volume.
Swelling introduces a large number of nano-defects in the
cellulose structure. Although there is little swelling of a single
CNC, water can penetrate into amorphous cellulose with ease,
pushing apart the individual cellulose molecules in those regions.
In addition, the bonds and interfaces between neighboring CNC will
be disrupted, thereby significantly reducing the strength of any
material reinforced with CNCs. To make matters worse, water can
move easily over the surface/interfaces of the CNCs, thereby
allowing water to penetrate far into a composite containing CNCs.
There are several approaches to make CNC composite materials
viable choices for real world applications. The simplest, but most
limited, is to choose applications in which the composite will not
be exposed to water. Another is to alter the surface chemistry of
the cellulose so that it becomes hydrophobic, or water-repelling.
This is easy enough to do, but will likely substantially degrade
the mechanical properties of the altered CNCs. A third approach is
to choose a matrix material which is hydrophobic, and preferably
that forms a hydrophobic interface with CNCs. While not
particularly difficult from a purely chemical viewpoint, there is
the practical difficulty that interfaces between hydrophobic and
hydrophilic materials are usually severely lacking in strength.
Perhaps the most practical approach will simply be to paint or
otherwise coat CNC composite materials in some material that keeps
water away. For such a prize - inexpensive strong and rigid
materials - we can be sure that innovations will follow to make
the theoretical practical.
Cellulose nanocrystals from renewable biomass
Inventor: LEUNG CHI WOON , LUONG JOHN H T
Applicant: CANADA NAT RES COUNCIL
IPC / EC: C08B15/04 // C08B15/08
Field of the Invention
The present invention relates to a process for producing cellulose
nanocrystals from renewable biomass, and to cellulose nanocrystals
with carboxylic groups produced by the process.
Background of the Invention
Described in 1838 by French scientist Anselme Payen, cellulose has
the molecular formula ([Omicron]6[Eta]10[Omicron]5)[eta]. It is
the most abundant organic polymer, being used in an amount of
about 1.5 * 10<12> tons per year. It has been used as a
renewable, biodegradable and environmentally benign chemical raw
material for 150 years. Cellulose is semicrystalline, having both
crystalline and amorphous regions. It is densely packed with
strong inter- and intramolecular hydrogen bonds conferring
excellent mechanical properties.
Cellulose nanocrystals (CNCs) have emerged as a new class of
nanomaterials for polymer reinforcement and nanocomposite
formulation owing to their exceptionally high mechanical strength
(modulus of 145 GPa; Marks, 1967), tensile strength of 7.5 GPa
(Sturcova, 2005), chemical tunability, and anticipated low cost.
CNCs have also been fostered for diversified applications
including enzyme immobilization (Mahmoud et al., 2009), drug
delivery, and biomedical applications (Dong and Roman, 2007).
In order to produce CNCs, fiber sources from various vegetative
wastes with high initial cellulose contents are being considered
as potential starting materials due to their low costs. The
amorphous regions of the cellulose fibers must be chemically
removed to yield highly crystalline CNCs. Popular acid hydrolysis
using a single concentrated acid or an acid mixture, often with
the aid of an oxidant, is capable of dissolving the amorphous
regions (Revol et al., 1992), leaving behind CNCs with crystalline
rod-like fibers. Such procedures, however, are expensive,
requiring considerably high initial capital investment and having
high operating costs due to the corrosiveness, safety issues and
hazardous waste treatment/disposal requirements of such acids and
their by-products. Additional pre- and/or post-treatment steps
with alkaline or bleaching reagents are required to remove
non-cellulosic fiber contents (e.g. lignin, pectin,
Bai et al. (2009) describe a method for the production of CNCs
with narrow distribution from microcrystalline cellulose (MCC). A
conventional sulfuric acid procedure was used to produce CNCs
(Dong et al., 1998). This process is known to produce a wide range
of size distribution. In order to obtain a narrow size
distribution of CNCs, differential centrifugation with at least
six cycles was required. Even so, the CNCs still exhibited at
least four different aspect ratios. US 2008/0108772 (Oksman et
al., 2008) describes a process for producing cellulose nano
whiskers by treating MCC with HCI, as well as a new extrusion
method to produce a reinforced organic polymeric material. The
production of cellulose nano whiskers using HCI hydrolysis
required pure cellulosic materials (e.g. MCC) and the resulting
cellulose nano whiskers had a large size distribution. Fractions
of cellulose crystals with larger size were isolated by
centrifugation at low speed and discarded. The cellulose nano
whiskers produced had a large size distribution of 100 nm to 1000
nm in length and 5 nm to 15 nm in width.
Persulfates, for example ammonium persulfate, are well known
strong oxidants. In the prior art, for example as described in US
5,004,523 (Springer and Minor, 1991 ), ammonium persulfate has
been used for the isolation of lignin from lignocellulosic
materials. Usually a mixture of ammonium persulfate together with
either strong acid (50% HCI or H2S04) or strong base (KOH or NaOH)
is required. The product isolated from this process is bleached
cellulose, not CNCs.
There remains a need for a simple, cost-effective process for
producing CNCs, especially from vegetative biomass.
Summary of the Invention
It has now been surprisingly found that an inorganic persulfate is
able to produce clean cellulose nanocrystals (CNCs) from
vegetative biomass in one step by dissolving lignin,
hemicellulose, pectin, and other plant contents. In one aspect of
the present invention, there is provided a process for producing
cellulose nanocrystals comprising: providing a cellulosic
material; contacting the cellulosic material with an inorganic
persulfate at an elevated temperature to produce cellulose
nanocrystals; and, recovering the cellulose nanocrystals.
The inorganic persulfate preferably comprises ammonium persulfate
((NH4)2S208), sodium persulfate (Na2S208), potassium persulfate
(K2S2Os) or a mixture thereof. More preferably, the inorganic
persulfate comprises ammonium persulfate, (NH4)2S208.
Any suitable source of cellulosic materials may be used, for
example, vegetative or non-vegetative biomasses. Non-vegetative
biomasses include cellulosic materials that have undergone
considerable pre-treatments, for example, cellulose from papers
and microcrystalline cellulose (MCC). It is an advantage of the
present process that CNCs may be produced in one step from
vegetative biomass. Thus, the cellulosic material preferably
comprises vegetative biomass, more preferably raw vegetative
biomass. Any suitable vegetative biomass may be used, for example,
one or more of hemp material (e.g. raw hemp, pectate-lyase treated
hemp), flax material (e.g. raw flax, pectate-lyase treated flax),
triticale material, wood sources (e.g. wood pulp, cardboard) and
agricultural residues. More preferably, the cellulosic material
comprises one or more of hemp or flax material.
The process is conducted at an elevated temperature. Preferably,
the elevated temperature is in a range of from about 45[deg.]C to
about 80[deg.]C, for example about 60[deg.]C. The persulfate is
preferably provided in an aqueous solution. The aqueous solution
preferably has a concentration of persulfate in a range of from
about 0.5 M to about 2.0 M, more preferably in a range of from
about 0.5 M to about 1.0 M, for example about 1.0 M. Preferably,
the persulfate is stirred with the cellulosic material.
Preferably, contacting the cellulosic material with persulfate is
performed for a period of time in a range of from about 5 hours to
about 24 hours, for example about 16 hours.
In another aspect of the present invention, there is provided a
cellulose-based material comprising nanocrystals of cellulose, the
nanocrystals having surface carboxylic acid groups.
In another aspect of the present invention, there is provided a
cellulose-based material comprising nanocrystals of cellulose, the
nanocrystals having an average diameter of less than about 7 nm
with substantially all of the nanocrystals having diameters within
about 0.5 nm of the average diameter and an aspect ratio (L/D) of
10 or greater. The average diameter is preferably less than about
5 nm. In one embodiment, the average diameter is in a range of
from about 3 nm to about 7 nm, preferably in a range of from about
3 nm to about 4.9 nm. The aspect ratio is preferably in a range of
from about 12 to about 60. Preferably, substantially all of the
nanocrystals have diameters within about 0.3 nm of the average
diameter. CNCs of the present invention preferably have a
crystallinity index (CRI) that is 5% or more greater than the CRI
of the cellulosic material from which the CNCs are made.
Advantageously, the CRI may be 7% or more greater than, or even
10% or more greater than, the CRI of the cellulosic material from
which the CNCs are made. The CRI may be, for example, up to 20%
greater, or up to 17% greater, than the CRI of the cellulosic
material from which the CNCs are made. The values of 5% or more,
7% or more or 10% or more may be lower limits of a range in which
the upper limit is 20% or 17%. Preferably, the surface carboxylic
acid groups are formed by selective oxidation of C6 primary
hydroxyl groups of the CNCs. Preferably, the degree of oxidation
is in a range of from about 0.01 to about 0.20, more preferably in
a range of from about 0.08 to about 0.19, or in a range of from
about 0.05 to about 0.10, for example about 0.08.
The present process is advantageously a one-step process for
producing CNCs from vegetative biomass that has significant
benefits in terms of scalability, safety, sustainability and
low-cost. The present approach may be considered as a chemical
nanoscissor and is effective in processing raw cellulosics,
especially flax and hemp, which contain hemicellulose, lignin and
pectin; in contrast to prior art acid hydrolysis processes which
require pre-purification (e.g. steam exploding) and
post-purification (e.g. bleaching) steps to produce good quality
CNCs. For example, the present process is useful for processing
high-lignin cellulosic materials (e.g. 20% or more lignins), which
obviates the need for steam exploding the cellulosic material
while still resulting in good quality CNCs. The stability and
economic viability of persulfate renders it a suitable replacement
for acids as strong oxidants.
The present facile process provides a commercially viable method
of obtaining CNCs with enhanced uniformity and crystallinity and
with smaller diameter and larger aspect ratio than conventional
processes. The present process directly results in uniform CNCs
with a narrow size distribution, as opposed to conventional
processes as disclosed in Dong et al., 1998, Bai et al., 2009 or
Oksman et al., 2008 for example, which require tedious
post-separation techniques to improve uniformity and which still
do not result in uniformity as good as in the present invention.
Thus, the present process produces high yields of CNCs with narrow
size distribution without requiring tedious separation steps such
as differential centrifugation. Further, CNCs produced in the
present process are surface carboxylated with 8% or higher
functionality, which makes them more reactive and improves their
flexibility and processability in composites. In contrast, acid
hydrolysis produces CNCs having only hydroxyl groups (when HCI is
used) or having only up to 2% sulfonyl groups (when H2S04 is
CNCs have a wide range of applications, including use as
nanofillers in the polymer industry and use as high strength
nanopaper in personal body armor.
Further features of the invention will be described or will become
apparent in the course of the following detailed description.
Brief Description of the
In order that the invention may be more clearly understood,
embodiments thereof will now be described in detail by way of
example, with reference to the accompanying drawings, in which:
Fig. 1A depicts height and phase
mode AFM micrographs of CNCs from flax produced using ammonium
persulfate in accordance with the present invention (5
Fig. 1 B depicts height and phase
mode AFM micrographs of CNCs from hemp produced using ammonium
persulfate in accordance with the present invention (5
Fig. 2 depicts PXRD spectra of
CNCs produced from hemp before and after treatment with 1 M
ammonium persulfate in accordance with the present invention,
with the inset showing deconvoluted cellulose peaks;
Fig. 3 depicts the XPS spectrum
of CNCs from flax prepared by ammonium persulfate treatment in
accordance with the present invention; and,
Fig. 4 depicts IR absorption
bands and their assignments for various cellulosic materials and
corresponding CNCs prepared in accordance with the present
Description of Preferred
Ammonium persulfate, potassium persulfate, and sodium persulfate
were obtained from Aldrich (St. Louis, MO, USA). Avicel PH102
microcrystalline cellulose was obtained from FMC Corp
(Philadelphia, PA, USA). Whatman CFI cellulose powder was
purchased from Whatman Inc. (Piscataway, NJ, USA). Flax (Linum
usitatissimum) and flax shives from Saskatchewan, Canada and hemp
(Cannabis sativa) from Quebec, Canada. Triticale straw extract was
obtained from Dr. G. (Joe) Mazza, Agriculture and Agri-Food
Canada, Summerland, BC, Canada. This freeze dried extract
comprises 54.5% cellulose, 12% hemicelluloses and 20% lignin.
Bacterial cellulose was obtained from Dr. W.K. Wan, University of
Western Ontario, London, ON, Canada. This bacterial cellulose is
produced by the gram-negative bacteria Acetobacter xylinum
BPR2001. The bacterial cellulose is produced in the form of fibers
of diameter less than 50 nm and a degree of polymerization of
between 2000 and 6000. Detailed information about the production,
purification, and characteristics of the bacterial cellulose are
known in the art.
Example 1: Preparation of
cellulose nanocrystals (CNCs) by persulfate
The process described in this example is an environmentally
friendly, one-step procedure for the preparation of CNCs from
different cellulosics. Ammonium persulfate has a very high
solubility in cold water (85 g/100 ml_), while the sodium (55.6
g/100 ml.) and potassium (5.3 g/100 ml.) counterparts (Weast,
1983) have lower solubility. CNCs were prepared by simply heating
cellulosic materials at 60[deg.]C in 1 M persulfate for 16 h with
vigorous stirring, as described in more detail below using
ammonium persulfate as an example. Lignocellulosic fibers such as
flax and hemp were cut into short fragments (about 2-3 mm) prior
to the persulfate treatment. Prolonged reaction time and
persulfate concentrations above 1 M led to excessive hydrolysis,
thereby reducing the yield of CNCs.
Thus, in one example, starting biomass material (0.1 g) was added
to 10 mL of 1 M ammonium persulfate solution (conductivity about
230 mS-cm<"1>). The suspension was heated to 60[deg.]C for
16 h (only 3 h for bacterial cellulose) to give a white suspension
of CNCs. The suspension was centrifuged (18,000 rpm, RCF = 25,400)
for 10 min. The solution was decanted, and about 50 mL of water
was added to the CNC pellet, followed by 5 min of vigorous mixing
and repeated centrifugation. The centrifugation/washing cycles
were repeated 4 times until the conductivity of the solution was
about 5 pS-cm<"1> (pH about 6), close to the conductivity of
deionized water. The product was lyophilized to yield a white
solid. Example 2: Atomic force microscopy (AFM) and transmission
electron microscopy (TEM)
CNCs prepared using ammonium persulfate were sonicated and atomic
force microscopy (AFM) micrographs of such resulting CNCs were
obtained using a Nanoscope(TM) IV (Digital Instruments, Veeco,
Santa Barbara, CA) with a silicon tip operated in tapping mode.
Particle analysis of the AFM micrographs was performed using
TEM micrographs were obtained by a Hitachi transmission electron
microscope (TEM) at 60 kV (model H-7500, Tokyo, Japan). TEMs were
obtained as follows. A small amount of CNCs was suspended in
methanol and sonicated to disperse the material. A 20 [mu][Iota]_
drop of well dispersed suspension was then dried on a Formvar-
carbon coated grid and analyzed. Low Voltage Transmission Electron
Microscopy (LVTEM) micrographs were obtained by a Delong LVEM
(Soquelec Ltd., Montreal, QC, Canada) low-voltage TEM at 5 kV,
with lower accelerating voltages generating higher contrast
between the carbon mesh and CNCs.
AFM (Fig. 1A and Fig. 1 B) and TEM micrographs confirmed the
rod-shape geometry of the CNCs. Such rod-shapes were highly
uniform compared to those obtained by prior art acid treatment.
For flax (Fig. 1A), the diameter of the CNCs was 3.8 +- 0.1 nm
while the length was 144 +- 5 nm. For comparison, prior art acid
hydrolysis of flax gave CNCs with a diameter of 21 +- 7 nm and a
length of 327 +- 108 nm (Cao et al., 2007). For hemp (Fig. 1 B),
the diameter of CNCs was 5.8 +- 0.1 nm, while the length was 148
+- 3 nm. For comparison, prior art acid hydrolysis of hemp gave
CNCs with a diameter of 30 +- 0 nm and a length that was
micrometers in size (Cao et al., 2008).
Nanocrystal size, shape, and size distribution were dependent to a
certain extent upon the starting cellulosic material. As an
example, the cross-section dimension of CNCs prepared from hemp
and flax centered on about 2-6 nm, reflecting elementary fibrils
(about 3-7 nm in diameter) initially present in the starting
materials. Such CNCs are much more uniform and significantly
smaller than CNCs obtained by prior art acid hydrolysis (diameters
ranging from 10-20 nm; Mahmoud et al., 2009). Tables 1 and 2
provide yield, crystallinity index (CRI) and dimensions of CNCs
prepared from various sources using ammonium persulfate. CRIs are
estimated using the integral method and from peak heights (in
parentheses). Length and diameter are reported at 95% confidence
interval. Table 1
Other CNC samples prepared from different cellulosics also showed
a similar mean particle length and length polydispersity. In
comparison, CNCs produced from flax and hemp using prior art acid
hydrolysis procedures have higher average diameters of 16-28 nm
and 20-40 nm, respectively. Further, the average length of CNCs
prepared in accordance with the present invention was about 90- 50
nm such that the average aspect ratio of the CNCs was determined
to be 30 compared to 10 for CNCs obtained by acid hydrolysis.
These are very important findings since the uniformity, small
size, and high aspect ratio of cellulose nanocrystals are critical
for their intended applications as nanofillers. Considering the
dimension of one unit cell (7.8 A, 8.2 A, and 10.4 A), or one
glucose unit of the cellulose chain is equal to about 0.5 nm, the
degree of polymerization of CNCs was estimated to be 20 to 500
using a known method in the art (Nishiyama et al., 2002).
Example 3: Scanning electron
SEM analysis was performed on a Hitachi S 2600N scanning electron
microscope at 2.8 kV. SEM revealed morphological changes on the
surface of the fibers upon ammonium persulfate treatment,
indicating the destruction of the amorphous regions. In contrast,
the fiber remained intact when subjected to heating without
ammonium persulfate. Ammonium persulfate was able to in situ
produce clean CNCs by dissolving lignin, hemicellulose, pectin,
and other plant contents. Free radicals are formed when the
solution containing persulfate is heated (S208<2~> +
heat-> 2S04 <">) (Hsu et al., 2002). Therefore,
persulfates are often used as initiators for emulsion
polymerization reactions in the preparation of polymers and
synthetic rubber. In addition, under the acidic condition used in
this study (pH 1.0), hydrogen peroxide was formed (S208<2">
+ 2H20 -> HS04<"> + H202) (Edgar and Gray, 2003;
Stiernstedt et al., 2006). Collectively, such free radicals and
H202 should be capable of penetrating the amorphous regions to
break down the [beta]-1 ,4 linkage of the cellulose chain to form
CNCs. Both free radicals 2S0 <~> and H202 also opened the
aromatic rings of lignin to decolorize this material. Prior art
acid hydrolysis procedures require alkaline or bleaching agents to
remove the other fiber contents and this treatment often affects
the crystallinity and structure of cellulose (conversion of
cellulose I to cellulose II; Krassig, 1996).
Example 4: X-ray diffraction
(XRD) analysis Wide angle X-ray scattering analyses were
obtained at room temperature on a
Panalytical X'pert PrO diffractometer equipped with a copper
(CuKa, A = 1.54184 A) rotating anode source, along with
instrumental settings of 45 kV and 40 mA. Samples were carefully
deposited on glass slides and inserted in the chamber. The
collected data were analyzed using WinPLOTR
(hllp:llwww.llb.cea.fr/fullweb/winplotrlwinplotr.htm), a graphic
tool for powder diffraction to provide peak position (2[Theta]),
FWHM (full width half maximum), peak deconvolution, and
integration intensity for calculation of the crystallinity index
(CRI). The dhki-spacing is calculated as //2sin0 with A = 1.54184
A. The crystal size is estimated as KA/FWHM.cosd with the form
factor or Scherrer constant ( ) taken as 1 (Scherrer, 1918). CNCs
prepared from various biomass sources were characterized by X-ray
diffraction (XRD). Results are provided in Fig. 2 and in Tables 3
and 4. Fig. 2 shows PXRD spectra of CNCs produced from hemp before
and after treatment with 1 M ammonium persulfate. The inset shows
the deconvoluted cellulose peaks. Table 3 provides a comparison of
crystallite size, dhki-spacing and crystallinity index (CRI)
before and after treatment of biomass samples. Table 4 provides a
comparison of the peak position (2[Theta]) values for the most
intense peaks before and after treatment of biomass.
The CNC diffractograms exhibited the most intense peak (002) with
a shoulder (021 ) and two lower peaks ( 01 and 10-1 ). In some
samples, a very small peak (040) at 35[deg.] was observed. In all
cases, the (002) peak position remained virtually unchanged during
the course of treatment. Such features, including the d-spacing
and average crystallite size as determined by the Debye-Scherrer
formula (Debye, 1915) resembled the diffraction pattern of
cellulose I and confirmed the integrity of the material during the
course of treatment with ammonium persulfate. The crystallinity
index (CRI) of CNCs was then estimated using an integral method
based on the ratio of the areas of crystallines to total scattered
intensity (Jayme and Knolle, 1964) with the results summarized in
Table 1 . CRI estimated using the maximum intensity (peak height)
from (002) plane and the intensity of the background scatter
measured at 2[Theta] of about 18[deg.] often results in
overestimated crystallinity (Segal et al., 1959; Thygesen et al. ,
2005). CNCs prepared in this work can be used to cast smooth thin
films, which are suitable for surface force and friction
measurements (Stienstedt et al., 2006).
In general, CRIs of CNCs were noticeably higher than that of their
parental counterparts except for CNCs prepared from MCC and
Whatman CFI. FWHM (full width half maximum) of the 002 peak was
also smaller than that of the starting material, indicative of
less dissolution of CNCs. For flax shives and hemp, the initial
CRI was about 50-70%, in agreement with the literature data
(Bhatnagar and Sain, 2005). With very high initial crystallinity,
the treatment of MCC and Whatman CFI with ammonium persulfate just
resulted in shorter fibers whereas the CRI remained unchanged.
Example 5: Preparation of CNCs by
sodium persulfate and potassium persulfate
Sodium and potassium persulfate were also capable of producing
CNCs. Similarly to ammonium persulfate treatment, CRIs of the CNCs
produced with sodium and potassium persulfate increased by about
8-16% from their starting materials. The dimensions of the CNCs
obtained from sodium and potassium persulfate are summarized in
Table 5. CRIs are estimated using the integral method and from
peak heights (in parentheses). Length and diameter are reported at
95% confidence interval.
Example 6: X-ray photoelectron
XPS analysis was performed using an AXIS(TM) ULTRA spectrometer
(Kratos Analytical Ltd., Manchester, UK) equipped with a
monochromatic Al Ka source at a power of 225 W. The elemental
composition of the analyzed surface areas was obtained from survey
spectra collected at pass energy of 160 eV. High-resolution C (1
s) and O (1 s) spectra were collected at 20 eV. The pressure in
the analytical chamber was lower than 10<"6> Pa and the
apparatus was calibrated against the following lines: Au (4f), Ag
(3d) and Cu (2p). Since the sample was charging, an electron flood
gun was used during the XPS experiments. Atomic concentrations of
each element were calculated using CasaXPS (Casa Software Ltd.) by
determining the relevant integral peak areas, and applying the
sensitivity factors supplied by the instrument manufacturer; a
Shirley background was used. To compare the high-resolution C (1
s) and O (1 s) peak positions, the spectra were shifted to ensure
that the leading edges of the fitted aliphatic CH* component were
coincident and the detail spectra were fitted with several peaks
using a mixed Gaussian-Lorentzian function. The XPS spectrum of
CNCs from flax prepared by ammonium persulfate treatment is shown
in Fig. 3. The sample shows the main segments for C (1 s) and O (
s) due to CNCs and only insignificant traces of N (1 s), S (2p)
and Si (2p).
Example 7: Infrared spectroscopy
Fourier transform infrared (FTIR) spectra were collected from 4000
to 400 cm<"1> for 64 scans at a resolution of 4 cm<"1>
using a Bruker Tensor 27 FTIR spectrophotometer. Samples were run
as KBr pellets. Near infrared (NIR) analysis for cellulose content
was measured with an Antaris FT-NIR analyzer equipped with an
integrating sphere. An internal gold flag was used as the
instrument background, the spectral resolution was set to 8
cm<"1> and 300 scans were processed mathematically to
generate one spectrum. The spectra for each sample were processed
with the 2nd derivative transformation to remove the spectral
baseline drift due to the color and scattered light from the
particles. The PLS (Partial Least Square) algorithm was used to
develop the correlation models for the cellulose (%), lignin (%),
and hemicelluioses (%) concentration measurements (i.e.
calibration curves) (Guhados et al., 2005).
CNCs prepared from various biomass sources were characterized by
FTIR spectroscopy. Fig. 4 lists the FTIR absorption bands and
their assignments for the various cellulosic materials and their
corresponding CNCs. The increase of CRI of various cellulose
materials after their treatment with persulfate also correlated
well with FTIR data. FTIR spectra of the CNCs showed absorption
bands that are typical for cellulosic materials. The presence of
signals at 1429 cm<"1> , 1 163 cm<"1>, 1 1 1 1
cm<"1> and 897 cm<"1> indicated that the CNCs are
primarily in the form of cellulose \[beta], except for
persulfate-treated bacterial cellulose in which characteristic
absorption bands at 3241 cm<"1> and 753 cm<"1>
confirms the high degree of cellulose type l0 in bacterial
cellulose. Persulfate treatment of bacterial cellulose resulted in
the reduction of the mass fraction of cellulose l0 to l^. The
absorption patterns of the CNCs remained unchanged after treatment
with persulfate, indicating that there are no significant changes
to the conformation of the cellulose structure, i.e. mercerization
did not occur. The ratios between crystalline absorption at 1430
cm<"1> and amorphous absorption at 895 cm<"1> of CNCs
was higher than that of its parental counterpart (Nelson and
O'Connor, 1964). Although this FTIR ratio has been used to
calculate CRI, its applicability is somewhat limited, e.g., not
applicable for mercerized cellulose (cellulose II) and provides
rather poor correlations with the PXRD data. The IR spectra of
CNCs prepared from various materials displayed a peak at 1735
cm<"1> , which was absent from CNCs obtained by acid
hydrolysis. This peak could be attributed to oxidation of the C6
primary hydroxyl groups on the cellulose fibers to form carboxylic
acids. The degree of oxidation (DS) of the CNCs was determined
using a conductometric titration method to be about 0.08 (da Silva
Perez et al., 2003). Conductometric titration experiments on
different samples showed that the degree of oxidation can be in a
range of from about 0.08 to 0.19. The increase of CRI of flax and
hemp after their treatment with ammonium persulfate also
correlated well with an increase in cellulose content as observed
by NIR analysis (Kramer and Ebel, 2000; Poke and Raymond, 2006).
Cellulose content for flax fibers increased from 70% to 79% after
persulfate treatment, whereas the cellulose content for hemp
increased from 75% to 83%. CNCs with higher cellulose content may
be prepared using fibers treated with pectate lyase, with
cellulose content of 84% and 93% for flax and hemp, respectively.
The observed increase in cellulose content illustrated that the
present process is effective in the removal of non-cellulosic
content from the natural fibers.
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Other advantages that are inherent to the structure are obvious to
one skilled in the art. The embodiments are described herein
illustratively and are not meant to limit the scope of the
invention as claimed. Variations of the foregoing embodiments will
be evident to a person of ordinary skill and are intended by the
inventor to be encompassed by the following claims.
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