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The objective of this paper is to model the energy consumed in generating cellulose microfibres, 1 μm in diameter, as reinforcing agents, by refining bleached softwood kraft pulp in a PFI mill. An average initial fibre diameter of 13 μm was assumed. 125,000 revolutions in a PFI mill was found to produce a high yield of fibres 1.3 μm in diameter, and the minimum refining energy needed to reduce the fibre diameter to 1.3 μm was estimated as 1875 kJ for each 24 g charge in the PFI mill. Since elastic deformation of the fibres was found to be negligible, the size reduction was assumed to follow Rittinger’s Law. This gave a Rittinger’s constant of 28 J.m/kg for the given system. Using this value of Rittinger’s constant, the energy required to generate microfibres 1 μm in diameter was predicted as 2480 kJ for each 24 g charge in the PFI mill. It was deduced that microfibres generated in this way would cost a minimum of $2.37 per kilogram. Hence even this relatively inefficient method of grinding would not be prohibitively expensive, provided the resulting microfibres can be used as high quality reinforcements.
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Chakraborty et. al. (2006). “Microfiber refining energy use,” BioResources 2(2), 210-222. 210
Ayan Chakraborty,a Mohini M. Sain,a,b*, Mark T. Kortschota and Subrata B.Ghoshb
The objective of this paper is to model the energy consumed in
generating cellulose microfibres, 1 µm in diameter, as reinforcing agents,
by refining bleached softwood kraft pulp in a PFI mill. An average initial
fibre diameter of 13 µm was assumed. 125,000 revolutions in a PFI mill
was found to produce a high yield of fibres 1.3 µm in diameter, and the
minimum refining energy needed to reduce the fibre diameter to 1.3 µm
was estimated as 1875 kJ for each 24 g charge in the PFI mill. Since
elastic deformation of the fibres was found to be negligible, the size
reduction was assumed to follow Rittinger’s Law. This gave a Rittinger’s
constant of 28 J.m/kg for the given system. Using this value of Rittinger’s
constant, the energy required to generate microfibres 1 µm in diameter
was predicted as 2480 kJ for each 24 g charge in the PFI mill. It was
deduced that microfibres generated in this way would cost a minimum of
$2.37 per kilogram. Hence even this relatively inefficient method of
grinding would not be prohibitively expensive, provided the resulting
microfibres can be used as high quality reinforcements.
Keywords: cellulose microfibres, reinforcing agents, refining, PFI mill, Rittinger's Law
Contact information: a: Department of Chemical Engineering and Applied Chemistry, University of
Toronto, 200 College Street, Toronto, Ontario, Canada M5S 3B3; b: Faculty of Forestry and Centre of
Biocomposites and Biomaterials Processing, University of Toronto, 33 Willcocks Street, Toronto Ontario,
Canada M5S 3E5; *Corresponding author:
There is widespread interest in the production and use of micro- and nano-scale
fibres for use as reinforcing agents. This trend originated with the production of nano-
scale clay particles by researchers at Toyota, and has been accelerated because of the
more recent introduction of carbon nano-tubes. There is also a great deal of interest in
producing nano-scale cellulosic fibres from wood pulp and agricultural by-products;
however, reliable and economical processes for doing this have yet to be developed.
In the present study, we examine the possibility of using refining as a method to
generate microfibres 1 µm in diameter from wood pulp fibres of approximately 13 µm
diameter. Such microfibres with high aspect ratio (length/diameter) would have the
potential to act as excellent reinforcing agents in polymers. Refining is a specialized
method of grinding fibres used in the pulp and paper industry; it is used in mechanical
pulping, and also in developing fibre properties of pulp in papermaking. In a refiner, a
dilute pulp suspension is forced through a gap between two surfaces moving rapidly
relative to each other. Of these two surfaces, at least one has bars with sharp edges.
Chakraborty et. al. (2006). “Microfiber refining energy use,” BioResources 2(2), 210-222. 211
Although this attempt to generate microfibres through refining is novel,
production of fines during mechanical pulping has been studied and reviewed extensively
by various researchers (Brecht, Klemm 1953; Mohlin 1977; Corson 1980; Luukko and
Paulapuro 1997; Luukko 1998; Courchene et al. 2002). Fines are defined by the
Technical Association of the Pulp and Paper Industry (TAPPI) as “particles that will pass
a round hole 76 µm in diameter or a nominally 200 mesh screen”. Therefore, fines
include both fibrillar materials of submicron diameter and hundreds of microns long, as
well as chopped fragments of fibres having diameters in the same range as the dimension
of the mesh opening.
Fibre development during mechanical refining, and its effect on the fracture
energy of paper sheets of different pulp mixes, has been studied in detail (Lidbrandt et al.
1980; Mohlin et al. 1995; Mohlin 1997; Hiltunen et al. 2000; Hiltunen et al. 2002).
However, these studies looked into the effect of refining on the strength of the paper
structure directly, rather than focusing on the generation of microfibres.
The process of delamination of fibre walls by beating and refining has been
studied in considerable detail (Page et al. 1967). As elucidated by Karnis (1994), the
forces acting on the fibres in refining are assumed to act along the fibre length. As a
result, the fibres are peeled off the surface in refining, as opposed to being chopped off
perpendicular to the fibre length. This mechanism suggests that fibre length remains
unchanged. The paper further noted that microscopic observations substantiated the
above assumption. However, in practice, there is some fibre shortening associated with
refining. Models on such comminution of fibres have been developed by Roux and
Mayade (1997), who examined the change of the mean fibre length during refining. In
this model, the potential of fibre cutting under given conditions was predicted to be a
function of the energy consumed by the solid phase and of the average impact intensity,
i.e., the ratio between the net machine power and the “cutting” length of bars per unit
time. Corte and Agg (1980) used a comminution model to compare the shortening rate of
fibres in a disc refiner and a laboratory beater. They found that the disc refiner cuts
longer fibres more rapidly than the short fibres, while the laboratory beater was found to
cut long and short fibres at the same rate. Olson et al. (2003) found that the probability of
a fibre being selected for cutting during refining is proportional to the applied energy and
fibre length, and was independent of pulp consistency. Corson (1972) mathematically
modeled the refining of wood chips into individual fibres using a comminution approach.
This approach was more recently expanded by Strand and Mokvist (1989) to model the
operation of a chip refiner employed in mechanical pulping of wood chips.
However, these studies all used comminution models to predict the cutting of
fibres perpendicular to the fibre axis, rather than the peeling action assumed to be
predominant in refining (Karnis, 1994). The objective in this present study, therefore, is
to model the energy consumed in peeling of a cellulose fibre to yield microfibres of
smaller diameter
To study the effects of refining of pulp on papermaking in a laboratory scale, a
PFI mill is commonly used. The PFI mill is used to pulp beat fibres to increase fibre
flexibility and improve the properties of the resulting handsheet. In the process, the
refiner causes fibrillation generating fibres of smaller diameter, and also produces fines.
Chakraborty et. al. (2006). “Microfiber refining energy use,” BioResources 2(2), 210-222. 212
The components of a PFI mill are shown in Fig. 1. During the operation of the PFI
mill, the head containing the bars is pushed to one side of the casing, as shown in Fig. 1.
As elaborated by Murphy (1962), the stock in a PFI mill is centrifuged against the wall of
the mill house. It is carried around in a narrow band toward the beating gap where it
converges with the moving bars of the roll. The mill house at that point forms a smooth
bedplate. The fibres are subjected to impact by rotating bars against the bedplate. The
action of any refiner is determined by shear and compression forces in the refining zone,
and by their distribution on single fibres. These forces are more evenly distributed in a
laboratory beater such as the PFI mill than in industrial refiners. In over-simplified terms,
the beating action resembles a plunger moving down into the pulp mass (Watson, Phillips
1964). The primary effects of the beating process are:
(a) intra-fibre bond breaking (internal fibrillation)
(b) external fibrillation
(c) fibre cutting and the production of fines
Studies of handsheet properties and microscopic examination of fibres have
shown that all these effects take place in a PFI mill (Stationwala et al. 1996).
The dynamics of a single bar are shown pictorially in Fig. 2. The forces applied to
the fibres in the beating gap can be resolved into two components – the compression
force normal to the bar surface directly due to the load, and a tangential stress due to the
resistance of pulp to shear imposed by the relative motion between the roll and the
housing. Although the terms “shear” and “compression” are used to describe two
components, the compression of the fibres in the gap in itself may involve a small
component of shear.
Fig. 1. Components of a PFI mill
Chakraborty et. al. (2006). “Microfiber refining energy use,” BioResources 2(2), 210-222. 213
Fig. 2. Parameters related to the beating gap in a PFI mill
The aim of this work was to develop a quantitative model for microfibre
generation from bleached kraft pulp through refining in a PFI mill. Theoretical studies on
refining action noted earlier for other kind of refiners have estimated only the energy
needed for fibre shortening. Thus, no work done so far provides any quantitative measure
of the energy needed to generate microfibres from fibres by the reduction of diameter
without any fibre shortening.
The goal of the refining modeled in this work is not merely to achieve better
fibrillation on the cell wall surface, but also to separate microfibres as discrete entities
with high aspect ratios. Therefore, the total energy imparted to the fibres needs to be
much greater than what is usually employed in refining. This is substantiated by the
conclusion drawn by Stationwala et al. (1996) that the peeling-off mechanism described
earlier demands a relatively high amount of energy. Variations in the operating
conditions of the PFI mill (load, consistency, temperature, nature of fibre, mill speed,
etc.) may lead to an increase in the energy obtained in refining. But these variations alone
may not be sufficient to substantially increase the generation of microfibres.
One simple way to substantially increase the energy imparted to the fibres is to
keep the operation running for a much longer period of time. This implies that the
number of revolutions employed has to be far greater than commonly used in laboratory
conditions. As Kerekes (2001) noted, the extent of refining is most commonly expressed
in terms of the number of revolutions. This forms the basis of the theory presented in this
work, where bleached softwood kraft pulp was subjected to 125,000 revolutions in a PFI
mill, as opposed to about 1,000 revolutions commonly used to study the effect of refining
on fibre properties.
Developing a model for the energy consumption in generating microfibres
requires some experimental data. These data form the basis for evaluating two of the
assumptions used in the model development, as described in this section.
Beating Gap
Roll House Speed, V2
Relative Speed = V2 – V1
Chakraborty et. al. (2006). “Microfiber refining energy use,” BioResources 2(2), 210-222. 214
Generating Refined Fibres
The experimental methods for generating these data are based on a process
detailed by Chakraborty et al. (2005) for producing wood microfibres from bleached
northern black spruce kraft pulp. In summary, 24 g of pulp fibres of 10% consistency was
charged in a PFI mill, which was then rotated for 125,000 revolutions. In the original
study, this refining step was followed by an additional stage of crushing the fibres under
liquid nitrogen to obtain high yield of microfibres 1 µm in diameter. For the purpose of
the present work, however, the fibres were characterized right after the refining stage at
the end of 125,000 revolutions of the PFI mill, before they were subjected to
Fibre Characterization
SEM studies: A Hitachi S2500 Scanning Electron Microscopy (SEM) instrument
was used to characterize the fibres after 125,000 revolutions. The refined fibres were
dispersed in water to form a suspension of 0.1% fibres in water. A drop of this suspension
was placed on the SEM stub, and allowed to dry before analyzing in the SEM. Each
sample was gold coated, and a voltage of 10 kV was used during imaging with the SEM.
Samples of the bleached kraft pulp (BKP) were also prepared in the same manner.
25 such samples were prepared for both BKP and the refined fibres generated
thereof, and the SEM images of 200 fibres of each were analyzed. Analysis of these
images (Fig. 3) showed an average BKP fibre diameter of 13 µm. Moreover, image
analysis revealed that more than 90% of the fibres generated in this way were in the range
between a few nanometers and 2 µm in diameter (Fig. 4). The distribution was centred
around a diameter 1.3 µm. Therefore, for modeling purposes, an average final fibre
diameter of 1.3 µm was assumed at the end of 125,000 revolutions.
(a) (b)
Fig. 3. Typical SEM images of (a) bleached kraft pulp (BKP) and (b) BKP after 125,000
revolutions in a PFI mill
10 µm 5 µm
Chakraborty et. al. (2006). “Microfiber refining energy use,” BioResources 2(2), 210-222. 215
Study of Tensile Property of Single Fibres
The energy spent in elastic deformation of the fibres is critical in modeling the
energy consumed in refining, for reasons described later. Therefore, the energy consumed
in elastic deformation needed to be calculated as a fraction of the total energy consumed
by the fibres before fracture. For this purpose, understanding of load-elongation (or
stress-strain) behaviour of single fibres was important.
Fig. 4. Number yield of microfibres generated after refining
Previous studies (Page and EL-Hosseiny 1983) on tensile strengths of single pulp
fibres indicated non-linear deformation behaviour especially with fibres having high
micro fibril angle. The initial linear elastic region (following Hooke’s law) was small
compared to the total area under the stress-strain curve. In a recent study, Gilani (2006)
also reported the tensile behaviour of isolated single pulp fibre. The study suggested that
the nature of stress-strain curve and the fracture strength is strongly dependent on the
isolation technique (mechanical or chemical) and on the micro fibril angle. While the
mechanically isolated fibres failed at comparatively low strain in a rather brittle manner,
chemically isolated (acid or alkali treated) fibres, which more closely resembles the BKP
fibres used in this study, had large non-linear deformation region associated with high
strain compared to small elastic deformation.
However, given the small length of ~ 2 mm of the BKP fibres, it was practically
impossible to clamp a single fibre in between the two clamps of a tensile tester. The
strong tendency to twist in chemically isolated pulp fibres also add to uncertainties in the
stress-strain results (Gilani 2006). Additional complications like were also reported in
handling the short length pulp fibres.
Therefore, some other natural fibre had to be chosen that had similar properties as
that of BKP fibres. The criteria that were matched were cellulose content and percentage
crystallinity. These predominantly dictate the chemical and mechanical properties of the
fibres. The cellulose content and crystallinity of BKP fibres were 90% and 55%
respectively (Chakraborty et al. 2006). It was noted that the cellulose content of hemp
fibres after acid and alkali treatment was 94%, and the crystallinity was 55% (Bhatnagar
Chakraborty et. al. (2006). “Microfiber refining energy use,” BioResources 2(2), 210-222. 216
Therefore, chemically treated single hemp fibres were used for tensile property
studies. The acid and alkali treatments are described briefly as follows:
Acid treatment was performed to remove pectin and hemicellulose from hemp.
Alkali treatment completes this removal process and also disrupts the lignin structure by
separating the linkages between lignin and carbohydrates. For this purpose, hemp fibres
were submerged in a 1M hydrochloric acid solution in a beaker at 80ºC ± 5oC for two
hours with constant stirring. Subsequently, 2% w/w sodium hydroxide solution was
added to the sample at 80°C ± 5oC for two hours with constant stirring for better
impregnation of alkali into the fibres. The treated fibres were then cooled and washed
with abundant distilled water until it became neutral, and vacuum filtered.
Single fibres were isolated from the chemically treated hemp. The mechanical
strength and modulus of the single fibres were analyzed with a Sintech-1 machine model
3397-36 in tensile mode with a load cell of 5 lb using a gauge length of 15 mm. The
tensile tests were performed at a crosshead speed of 2.5 mm/min. 20 chemically treated
fibres were studied in this manner.
A typical stress-strain curve for a chemically treated hemp fibre is shown in Fig.
5. Energy taken up during tensile stress is given by the area under the stress-strain curve.
The results indicated that the energy absorbed before breaking the fibres was
predominantly in the non-linear deformation zone. The energy spent in elastic
deformation, i.e., deformation up to the point where Hooke’s Law holds good, ranged
between 3% and 5% of the total energy.
Therefore, in view of the small proportion of energy transferred to the chemically
isolated BKP fibre during elastic deformation, the elastic component of energy supplied
to the fibres in tensile mode was neglected for modeling purposes.
0 0.005 0.01 0.015 0.02 0.025
Stress (MPa)
Fig. 5. Energy consumed during elastic and plastic deformations of chemically treated hemp
fibres in tensile mode
Plastic zone
Chakraborty et. al. (2006). “Microfiber refining energy use,” BioResources 2(2), 210-222. 217
In summary, the following assumptions were made in the model:
(i) Initial and final fibre diameters before and after refining are 13 µm and 1.3
µm respectively.
(ii) The deformation of the fibres in the elastic zone is minimal. Therefore, the
work of elastic deformation is negligible. This forms the basis of Rittinger’s
law, as discussed below.
(iii) A charge of 24 g of bleached kraft pulp of 10% consistency is used, which is
the standard charge for a PFI mill (Tam Doo, Kerekes 1989).
Although comminution theory has been applied to the refining of fibres
previously, there still exists substantial theory on comminution, correlating fibre size
reduction to energy consumption, that has not been used by researchers studying pulp
fibres. In this study, standard comminution theory has been used to determine the refining
energy required to produce microfibres from bleached softwood kraft pulp fibres. For this
purpose, some of the most common laws of comminution were considered.
Rittinger’s Law
This law states that the energy input into a comminution process is proportional to
the quantity of new surface produced. In the mathematical form
E = C (A2 – A1) (1)
where E = specific energy (i.e., energy per unit mass) input, J/kg
1 = initial specific surface (i.e., initial surface per unit volume), m-1
2 = final specific surface, m-1
C = Rittinger’s constant, J.m/kg
The value of C depends on the material being crushed. In this case, C is primarily
a function of the hydrogen bond density among the different microfibres of cellulose.
Kick’s Theory and Bond’s Theory
Rittinger’s formula only accounts for the decomposition of molecular bond
forces. In practice, most materials elongate elastically before breakage, but the work of
elastic deformation preceding fracture is neglected in Rittinger’s formula. It has,
however, been incorporated in other comminution laws, such as Kick’s “volume” theory
and by Bond’s theory, referred to as “third theory” (Beke 1981). Kick’s theory, however,
considers only this elastic deformation energy, and neglects the work done in breaking a
material into small particles. Bond’s theory, on the other hand, accounts for the energy
required in both elastic deformation, and the final breakage of the materials.
Chakraborty et. al. (2006). “Microfiber refining energy use,” BioResources 2(2), 210-222. 218
It is worth mentioning that Kick’s and Bond’s theory are more appropriate with
larger particles, while Rittinger’s law is applied to fine grinding (Holdich 2002) as in PFI
mill used in this study.
Rationale for Using Rittinger’s Law to Characterize Microfibre Generation
from Fibres
The energy imparted to the fibres during refining produces both elastic
deformation and plastic deformation before breakage. However, the experiments on hemp
discussed previously revealed that the energy for elastic deformation was negligible, and
BKP fibre fracture occurs primarily by the decomposition of molecular bond forces
(thereby creating new surfaces) in the plastic zone. Therefore, Rittinger’s Law
characterizes the process under investigation quite well. This is also substantiated by a
study by Wisconsin (1957) who demonstrated experimentally that Rittinger’s Law holds
almost perfectly for ball mill beating of pulp.
The average initial diameter of each fibre of the starting material (BKP) is
assumed to be 13 µm, as noted earlier. The refining action generated microfibres 1.3 µm
in diameter. Hence, the energy used in refining reduced the size of the fibres from 13 µm
to 1.3 µm.
Energy Consumed by Fibres according to Rittinger’s Law
We consider a fibre of length l, m. For a fibre of diameter d, A1 in equation (1)
has a value of [(πdl)/(πd2l/4)] = 4/d = 4/(13×10-6) m-1 = 3.1X105 m-1. Similarly, A2 =
4/(1.3×10-6) m-1 = 3.1×106 m-1.
Putting these values of A1 and A2 in equation (1) gives the specific energy to
generate microfibres of 1.3 µm diameter from fibres 13 µm in diameter as
E = C (3.1×106 – 3.1×105) = 27.9×105 C, J/kg (2)
For the present work, the energy expressed in equation (2) is provided by a PFI
mill. The charge in a PFI mill in this case consists of 24 g of fibres. Thus, for a charge of
24 g of fibres, equation (2) can be written as
= 27.9X105 C × (24/1000), J = 66960 C, J (3)
is the energy required for microfibre production for a sample size of 24 g of
Energy Consumed by Fibres in Refining
The operations of a PFI mill have been previously described by various
researchers, but theoretical modeling relating the product properties to the operating
parameters have been scarce. The most relevant studies in this direction have been
Chakraborty et. al. (2006). “Microfiber refining energy use,” BioResources 2(2), 210-222. 219
performed by Kerekes and coworkers (reference?). The specific energy (E) on pulp in a
PFI mill is expressed as
E = N I (4)
where N = number of bar impacts per unit mass of fibres, and I is the energy/impact, also
called refining intensity (Welch and Kerekes, 1994)
. Welch and Kerekes (1994) deduced that the specific energy consumption in PFI
refining of bleached softwood kraft pulp lies within the range of 0.59 to 0.68 J/g.rev. In
accordance with the approximate average value adopted by Kerekes (2001), a value of
0.63 J/g.rev was assumed in the present case.
For a 24 g charge of BKP, and a number of revolutions = Nr, this gives the value
of the total energy consumed as
= 0.63 J/g.rev × 24 g × Nr rev., i.e,
= 15 Nr, J (5)
Equating Refining Energy to Energy Consumed by Fibres According to
Rittinger’s Law
Equating equations (3) and (5) gives
66960 C = 15 Nr (6)
i.e., Nr = 4464 C (7)
Hence, with the knowledge of the value of Rittinger’s constant C, the number of
PFI revolutions to generate microfibres 1 µm in diameter can be estimated.
Net Energy Required for Microfibre Generation and Rittinger’s Constant
Considering 125,000 as the number of revolutions for generating fibres in the
range of 1.3 µm in diameter, equation (5) shows that 1875 kJ of energy is needed by a 24
g batch in a PFI mill to reduce the fibre diameter from 13 µm to 1.3 µm.
Putting 125,000 as the number of revolutions in equation (7) gives a value of 28
J.m/kg for the Rittinger’s constant.
Energy Required to Generate Microfibres 1 µm in Diameter
Knowledge of the value of the Rittinger’s constant for a given material makes it
possible to deduce the energy required to reduce its diameter from any initial diameter to
any final diameter. Therefore, if microfibres 1 µm in diameter are to be generated by
refining alone, the energy consumed in the process can be calculated by putting 1 µm as
the final diameter. This gives a value of A2 (i.e., 4/diameter) as 4/1×10-6 = 4×106.
Therefore equation (1) gives
E = C (4X106 – 3.1X105) = 28 (J.m/kg) X 36.9X105 m-1 = 103.3 MJ/kg (8)
Chakraborty et. al. (2006). “Microfiber refining energy use,” BioResources 2(2), 210-222. 220
Hence, if microfibres 1 µm in diameter are generated solely by refining in a PFI
mill, each 24 g charge would consume an energy of about 2480 kJ.
The number of revolutions needed for microfibre generation in a PFI mill may be
calculated by putting the value of the energy per 24 g charge (
) = 2480 X 103 J in
equation (5)
2480 X 103 = 15 Nr i.e., Nr = 165333 (9)
Therefore, each 24 g of pulp charged in a PFI mill should be rotated for 165,333
revolutions to generate microfibres 1 µm in diameter starting with fibres 13 µm in
It should be recognized that the value of 103.3 MJ/kg obtained in equation (8)
represents the energy requirement for microfibre generation from softwood bleached
kraft pulp fibres through any size reduction process, not only for refining in a PFI mill.
Cost of Microfibre Generation
Considering the cost of electricity as 6 cents per kWh, i.e., 6 cents per 3600 kJ,
the cost of supplying 103.3 MJ of energy to the pulp is calculated as $1.72. This implies
that the generation of microfibres 1 µm in diameter starting with bleached softwood kraft
pulp costs $1.72/kg. Moreover, the price of northern bleached softwood kraft pulp,
although variable, is roughly in the range of $650 per metric ton, i.e., 65 cents/kg. This
indicates that the total cost of the cellulose microfibres is about $2.37/kg, which is the
sum of prices of the bleached kraft pulp, and of generating the microfibres thereof. This
may be compared to the price of conventional polymers, most of which cost around
$1/kg. Since cellulose microfibres act as reinforcing agents, and contribute to an increase
in mechanical properties of the matrix, a price of $2.37/kg of the reinforcing agents may
be considered reasonable.
The energy required in generating microfibres 1 µm in diameter from bleached
kraft wood pulp was successfully modeled. The average diameter of each initial fibre
before refining was considered as 13 µm. Rittinger’s Law was used to characterize the
energy requirement in generating the microfibres. Considering the experimental evidence
that 125,000 revolutions in a PFI mill gave a high yield of fibres 1.3 µm in diameter,
Rittinger’s constant for the given system was found out to be 28 J.m/kg. Using this value
of Rittinger’s constant, the refining energy needed for generating microfibres 1 µm in
diameter was estimated as 103.3 MJ/kg. For unit electricity cost of 6 cents per kWh, this
corresponds to a cost of $1.72/kg, which brings the total cost of the microfibres to
$2.37/kg. Given that the price of conventional polymers is in the range of $1/kg, the price
of microfibres used as reinforcing agents may be feasible.
Chakraborty et. al. (2006). “Microfiber refining energy use,” BioResources 2(2), 210-222. 221
The authors would like to acknowledge BIOCAP/NSERC Strategic Projects and
Ontario Graduate Scholarship for providing financial assistance for this project.
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Article submitted: Dec. 3, 2006; Resubmitted after format change: Dec. 5, 2006; First
cycle of reviewing completed: January 9, 2007; Revised article submitted: April 20,
2007; Article accepted April 22, 2007; Published: April 24, 2007
... La refinación es un método mecánico que provoca la fibrilación de las fibras, se utiliza tanto en la fabricación de pulpa mecánica como en el desarrollo de propiedades de la fibra en la pulpa química (33). ...
... La pulpa ubicada contra la pared de la carcasa del PFI es empujada por acción centrifuga (33). ...
... Este mecanismo sugiere que la longitud de fibra se mantiene sin cambios. Sin embargo, en la práctica, hay acortamiento de las fibras asociada con esta operación (33). ...
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The objective of this work is to study the relative effect of the different components of the tensile strength of an industrial Eucalyptus grandis kraft pulp. We studied the components of the tensile strength of a brown pulp subjected to a change of base (to Na and Ca), and after a series of treatments O, Op and P, with and without addition of a phosphonated chelating agent. The resulting pulps (M) were subjected to the bleaching sequences OOpP and OQOpP, with or without addition of the chelating agent at each stage, generating 20 pulps: 10 in Na base and 10 in Ca base. The relative bonded area (RBA) of brown pulps and bleached pulps was calculated from the light scattering coefficient (S), as (S0-S)/S0, where S0 is the light scattering coefficient of unbonded fibers. For doing this, 15 handsheets of each pulp were subjected to three different pressures (20, 50 and 90 psi), and then tested. The RBA of the O and Op pulps was obtained from a value of S0 resulting from a regression equation involving the sodium and magnesium content of the pulps (R2 = 98.0). Bond strength per unit of bonded area values (b) were calculated using the Page equation. This work shows that there are differences in the bond strength per unit of bonded area values b that are due to the type and content of cations in the fibers. Since RBA and fiber strength (represented by ZI) tend to increase with bleaching, the large decrease in bond strength per unit of bonded area b would be responsible for the decline in tensile strength of the TCF bleached pulp.
... Rotating bars also have an effect on fibres. As a result, fibrillation both internally and externally, thus reducing the fibre volume [152]. Importantly, generated nanofibres bear a considerable quantity of water through processes like PFI mills (or valley beaters). ...
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Nanocellulose products derived from different forms of biomass have significant importance in the modern era. This is due to its extraordinary physical characteristics, wide surface area as well as its biodegradability, which lead to being promising reinforcements as a nanomaterial. The nanomaterials which represent the cellulosic structures comprise nanocellulose reinforcements with biodegradable characteristics and tremendous ability to be used in eco-friendly applications to supplant fossil-based products. Meanwhile, the syntheses approach of such nanoscale structures still possesses challenging tasks at nanoscales. In addition, the virtuous distribution of nanocellulose in the hydrophobic polymer matrix has still difficulties to produce high-performance nanomaterials. Consequently, this study concludes many approaches and techniques to structural alteration of cellulosic materials to improve the distribution of nanocellulose to enhance the characteristics and features of nanocomposites. The macroscale and nanoscale cellulosic structures get popularity because of their high strength, stiffness, biodegradability, renewability, and use in the preparation of nanocomposites. Application of cellulose nanofibres for the production of nanocomposites is a relatively recent research field. Cellulose macro- and nanofibres can be used as insulation nanocomposite materials because of the improved mechanical, thermal, and biodegradation properties of nanocomposites. Cellulose fibres are hydrophilic, so it became important to improve their surface roughness for the production of nanocomposites with improved properties. This article includes the surface modifications of cellulose fibres by different methods as well as production processes, properties, and various applications of nanocellulose and cellulosic nanocomposites. A high thermal conductivity of cellulosic nanocomposite material for electronic devices can be obtained by combining cellulose nanofibrils (CNF) as the framework material with carbon nanotubes, graphene, and inorganic nitrides. Additionally, the research developments in this field with prospective applications of CNF-based materials for supercapacitors, lithium-ion batteries, and solar cells are emphasized. Moreover, the emerging challenges of different cellulosic nanofibrils-based energy storage devices have been discussed in this review paper. Graphical abstract
... These result in internal and external fibrillation and, consequently, fiber size reduction. 139 Importantly, produced nanofibers through processes that include a PFI mill (or a Valley beater) contain a considerable amount of water. Consequently, it causes troubles during storage and handling because of the bulk volume and contamination concerns. ...
Nanocellulose, as a promising natural material, has recently received much attention because of its remarkable features including recyclability, biocompatibility, low risk of toxicity, tunable surface properties, etc. This review article first introduces three types of nanocellulose (NCC, NFC, and BNC) and evaluates their production processes. In addition, contemporary research is discussed in the formulations of nanoparticles, tablets, hydrogels, aerogels, etc. As reported in the literature, the release time of nanocellulose-based systems varies from a few minutes to several days as they provide a controlled and sustained release. Thus, such systems have shown considerable potential for developing a novel generation of controlled drug delivery for different routes of administration (oral, transdermal, etc.). This review facilitates the selection of proper source and processing techniques for nanocellulose production while addressing opportunities and challenges ahead. This would allow identifying sustainable ongoing research directions into its applications in drug delivery.
... The estimated values of parameters obtained for the second order polynomial model are presented in Table 3. Optimization of the dependent responses was carried out to obtain the best conditions for sprouting soybeans which will be used produce to produce functional soymilk that will serve as suitable intermediate for other soy-based products. Response plots are used to show the effect of processing parameters on responses (Chakraborty, Sain, Kortschot, & Ghosh, 2007). The effect of soaking and germination time on the responses investigated are shown in the individual 3 dimensional plots (Figure 1). ...
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This study aimed at producing functional soymilk by optimizing the sprouting conditions of soybeans using response surface methodology. Soaking (12–24 h) and germination times (48–96 h) were optimized using central complete randomized design. Responses obtained from experimental runs were fitted into second order polynomial regression model. Significance of model parameters was tested using ANOVA and R² was evaluated. The optimum sprouting conditions of soybeans were 12 h soaking and 52 h germination using desirability concept. Soymilk made from optimized conditions had 17% increase in total proteins, 50% reduction in phytic acid, 1.7% increase in total phenolics and a color change (∆E) of 4.89 compared with the control. There was a significant reduction in trypsin inhibitor activity (0.03 mg/g TI), with increase in total amino acids and similar rheological properties in optimized soymilk. Optimized conditions obtained are adequate in the production of soymilk with improved nutritional and quality attributes.
... Cellulose nanofibrils production involves a repeated mechanical disintegration of cellulose fibers that results in high energy consumption (over 25,000 kWh/Mg), restricting the commercialization of CNFs Chakraborty et al. (2007). To address this problem, several chemical pretreatments have been developed, such as carboxymethylation and TEMPO (2,2,6,6tetramethylpiperidine-1-oxyl radical) oxidation (Saito, Kinura, Nishiyama, & Isogai, 2007;Wågberg et al., 2008). ...
The thermal stability of cellulose nanofibrils (CNFs) can be improved by converting cellulose crystalline structure to cellulose II using an alkaline treatment method. The conventional method requires around 20 wt.% NaOH solutions and causes the cellulose interdigitation and aggregation, making CNFs production difficult. The objective of this study is to develop a new pretreatment method using a low-concentration alkaline solution to produce well-dispersed CNFs with improved thermal stability. CNFs with 90 nm diameter were successfully prepared by treating cellulose powder with 2 wt.% NaOH solution below 0 °C, followed by homogenization through a French pressure cell press. The CNFs had relatively high thermal stability with the mean onset and maximum thermal decomposition temperature of 305 °C and 343 °C, respectively, compared with the CNFs prepared without the NaOH pretreatment (283 °C and 310 °C). The increased thermal stability can create new opportunities for the development of CNF-based bio-composites and electronics.
... If the pulps are refined in a PFI mill for 20,000 revolutions prior to the mechanical disintegration process as described by Sharma et al. (2015), the PFI mill would consume approximately 3600 kWh/ton. Also, the refining energy required by a PFI mill was estimated as high as 21,700 kWh/ton to generate cellulose microfiber 1.3 μm in diameter from bleached softwood kraft pulp 13 μm in diameter ( Chakraborty et al. 2007b). Therefore, the development of low-energy mechanical pretreatment technique can contribute to the reduction in the overall energy use for CNFs production. ...
Cellulose nanofibrils (CNFs) are one type of nanostructured cellulosic materials with a width below 100 nm and a length of several micrometers. CNFs have many desirable characteristics, such as a unique rheological behavior, high mechanical and barrier properties, and lightweight. They are produced from cotton, wood, grasses, and other lignocellulosic biomass. Thus, CNFs are abundantly available and can be a cheap alternative to petroleum-based polymers. Manufacturing of CNFs consists of pretreatment process and mechanical disintegration process. The pretreatment process makes cellulose fibers more responsive to be fibrillated, and pretreated fibers are mechanically disintegrated into nano-sized fibers in the next stage. Moreover, the type of raw materials can be a principal factor that affects CNFs production and properties. In this chapter, we reviewed the production, characterization, and the current applications of nanocellulose for food industries, such as food additives, food packaging, and coating.
... The mechanical pretreatment process with the PFI mill can be performed at very high revolutions, consuming a much higher amount of energy. For example, Chakraborty et al. (2005) refined bleached kraft pulp in a PFI mill at 125,000 revolutions to produce submicron sized cellulose precursors, and the estimated energy consumption was 21,700 kWh/Mg (Chakraborty et al., 2007). Consequently, three-cycle shear cutting of cellulose fluff pulp has a potential to reduce energy consumption for the mechanical pretreatment process. ...
Cellulose nanofibrils (CNFs) are typically produced via wet milling pretreatment method to facilitate efficient disintegration of cellulose fibers by pre-fibrillation and fiber size reduction. However, the use of high energy and presence of water during wet milling methods leads to the increase in overall energy consumption during CNFs production and pose challenges during storage and handling. The objective of this study is to investigate the dry grinding of cellulose fluff pulp using the shear cutting method to determine the specific energy required to produce cellulose precursors and their characteristics for manufacturing CNFs. The specific energy required to grind cellulose fluff pulp with a screen size of 0.25 mm in three cycles was measured. The ground sample received after each cycle was sampled to characterize its properties and its potential to produce CNF hydrogels. As the number of grinding cycles increased, the specific energy required per cycle decreased with an overall net energy consumption of 894 kWh/Mg for three grinding cycles. The cellulose powder from the third grinding cycle was successfully disintegrated into cellulose nanofibrils with an average diameter of 119 nm without any fiber clogging. In conclusion, the three-cycle shear cutting process was sufficient to produce dry cellulose precursors for CNFs production, while reducing the overall energy consumption and handling and storage problems.
... Papirindustriens Forskningsinstitutt (PFI) mills are a specialty type of shear mill developed for laboratoryscaled paper pulp testing that have proven valuable for mechanical pretreatment studies. A PFI mill consists of bars and a smooth bedplate [48]. The bars are pushed to one side of the bedplate, which provides compression and shear forces to fibers. ...
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Production of advanced biofuels from woody and herbaceous feedstocks is moving into commercialization. Biomass needs to be pretreated to overcome the physicochemical properties of biomass that hinder enzyme accessibility, impeding the conversion of the plant cell walls to fermentable sugars. Pretreatment also remains one of the most costly unit operations in the process and among the most critical because it is the source of chemicals that inhibit enzymes and microorganisms and largely determines enzyme loading and sugar yields. Pretreatments are categorized into hydrothermal (aqueous)/chemical, physical, and biological pretreatments, and the mechanistic details of which are briefly outlined in this review. To leverage the synergistic effects of different pretreatment methods, conducting two or more pretreatments consecutively has gained attention. Especially, combining hydrothermal/chemical pretreatment and mechanical refining, a type of physical pretreatment, has the potential to be applied to an industrial plant. Here, the effects of the combined pretreatment (combined hydrothermal/chemical pretreatment and mechanical refining) on energy consumption, physical structure, sugar yields, and enzyme dosage are summarized.
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The gel point is the lowest solids content at which a fibre suspension forms a continuously connected network and is related to the fibre aspect ratio. In this paper we firstly investigated the conditions required to accurately measure a gel point using sedimentation. We found that very heavily treated cellulose nanofibres can produce anomalous sedimentation data, due to the electrostatic repulsion from fibre surface charges dominating the gravity driven sedimentation. Screening the anionic surface charges by adding high levels of Na+ or Ca2+ ions reduced the electrostatic interactions between the fibres and allowed them to settle normally. Gel point measurement was then used to probe the development of nanofibre quality with increasing energy input for three different feedstocks: eucalypt kraft pulp, commercial microfibrillated cellulose and de-lignified, bleached spinifex pulp. By combining the data of the aspect ratio and average diameter, determined from SEM and TEM, we were able to compare the differences in feedstock processability. The aspect ratio of all three feedstocks increased with increasing homogenisation energy, showing that the fibre delamination dominated over fibre shortening. The slope of the aspect ratio versus energy consumption showed the ease of processing of each sample. The spinifex fibres had the fastest rate of aspect ratio increase and therefore were the most processable. Gel point is an excellent tool to track quality development of nanocellulose through processing and to compare the potential of different feedstocks for nanocellulose production.
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High- and low-yield pulp fibers were subjected to small-amplitude cyclic bending (flexing) in the PAPRICAN fiber flexibility tester. Fiber flexibility was measured in situ during the course of the flexing. It was found that 2×104 cycles of transverse bending with a maximum deflection equal to 3.5% of beam span reduced fiber stiffness by about 50%. A similar decrease was found after beating in a PFI mill when compared on the basis of equal numbers of bending cycles. The specific energy required to flexibilize pulp fibers to this level was estimated to be less than 1% of the energy used in typical beating and refining equipment.
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The PFI mill continues to be the most commonly used laboratory refining device, despite much evidence that its refining effect differs from disc and conical refiners. The potential causes of this difference are evident in its unique geometry, high consistency, and mode of operation. But precisely how do these differences affect its refining action relative to industrial refiners? This study has examined the features of the PFI mill that affect its refining action and compared these to those of a laboratory conical refiner in terms of refining energy, refining intensity, and other factors governing action on pulp. Comparisons were also made of pulp property changes at equivalent values of energy and refining intensity and to theoretical values of refining intensity and energy. Compared to the conical refiner, the PFI mill is a very low intensity and high energy refining device. This accounts in part for differences between the refiners in normal operation, but even at the same energy and intensity, the PFI was found to produce a differing refining effect, namely a smaller reduction in freeness and an increase in tensile strength. This suggests that the PFI action is mainly one of causing internal fibrillation, a finding in agreement with conclusions of earlier authors. Comparisons of the magnitude of the PFI mill intensity with intensities estimated from earlier work on a roller device and theoretical predictions are consistent with cyclic compression and internal fibrillation.
A study was conducted to optimize the-refining of softwood reinforcement pulp in TMP-based paper. The study was concentrated on analyzing in-plane fracture energy and fracture toughness. In addition, microscopic fracture behavior was also analyzed in order to determine the effect of kraft pulp refining on sheet structure. The results indicate that the fracture toughness of TMP-kraft pulp mixtures was not linearly dependent on component toughness values.
The effects of grinding and refining on the development of pulp fibres and on the quality of the fines were studied. Seven softwood species in which fibre properties were different were selected and studied. Wood quality was defined in terms of coarseness and length of fibres in the native wood. Pulping processes used were groundwood (SGW), pressure groundwood (PGW), atmospheric refining (RMP), and thermomechanical pulping (TMP). It was found that refining makes fibres more slender and flexible than grinding because the fibre cell wall is peeled off during refining. The amount of fines generated during grinding depends on both specific energy and wood species. Refiner fines have a greater specific surface than groundwood fines. The differences between the properties of fibres and fines produced in refining and grinding are explained on the basis of these different mechanisms.
The development of fibre bonding ability during refining is discussed from the point of earlier published work and some new results from pulps sampled in commercial refiners. Special attention is given to summerwood fibres. Fibre bonding ability, an important property for printing papers of good runnability and printability, is influenced by many factors. Cross-sectional dimensions of the fibre and fibre wall stiffness determine its ability to collapse. Chemical and structural differences of the fibre surface are also important. The development of fibre properties also indirectly influences the properties of the other fractions, as the finer fractions are composed of material removed from the fibres by the mechanical treatment. In the experimental work described in this paper, the change in fibre properties did not occur uniformly for all fibres. Fibre shortening happened primarily for thin-walled springwood fibres, and unravelling of the S2 layer was more marked for thick-walled summerwood fibres.
The mechanism for the development of the papermaking properties of refiner mechanical pulp is explored. While fibres are broken in the separation stage from the wood, subsequent breakage in the development stage appears to be insignificant. The development proceeds through delamination and subsequent stripping away of material from the cell wall as more energy is added. This results in a decrease in coarseness, which at higher energy levels can reach that of kraft pulp fibres. While the weight of fibres in the long Bauer-McNett fraction decreases with increasing specific energy, the number of fibres in the fraction increases because of the loss of weight in each fibre as a result of the decrease in coarseness. The short fibre fraction and fines originate mainly from the middle lamella and cell wall material of the longer fibres. Both fibre width and thickness decrease with the increasing slenderness. Experimental evidence suggests that these changes in fibre dimensions result in an increase in fibre flexibility. For a given input of specific energy, an increase in refining intensity increases the slenderness.
The fracture energy of a mechanical-chemical mixture sheet is maximize using softwood kraft refining. The microscopic fracture behavior of paper during kraft refining is investigated. Results show that the kraft and mixture sheets are quite different. The fracture energy of mixture sheets decreases during softwood kraft refining, while that of pure kraft sheets increases. It is shown that the fracture work of mixture sheets of thermomechanical pulp (TMP) and unrefined kraft is larger than in pure TMP or unrefined kraft.
Fines can be regarded as one of the most important fractions in mechanical pulps because of their significant effect on most paper properties. Fines have been studied extensively over the past decades, but the approach to fines characterization has been diffuse and unsystematic. The characterization of fines is more problematic than that of the whole pulp because of the small particle size. Isolation, treatment and measurement of fines are laborious and usually require special equipment. In the same way as pulp characterization, also fines characterization must have an objective. It is suggested that one objective in fines characterization could be to determine the papermaking potential of the fines. A profound understanding of the effects of fines on the sheet structure and of the factors determining fines properties should also be acquired. In this review, characteristics of mechanical pulp fines, their generation, research methods and effects on paper properties are reviewed. A characterization strategy for fines is proposed.