New approach to use of kenaf for paper and paperboard production

Abstract

This study sought to determine the suitability of fractionation and consequence-selective processing (separation of long fiber and short fiber, beating long fiber, and remixing with short fiber to target freeness) as a new approach to use of kenaf whole stem pulp for paper and paperboard production. A laboratory Bauer-McNett Classifier with screen 18 mesh was used to separate short fibers and long fibers of the unbeaten kenaf whole stem soda-anthraquinone high kappa and low kappa pulps. For comparison, the initial unbeaten pulps were beaten in the PFI mill to the same freeness (300 mL CSF). Results of our patented method showed that the fractionation process was able to provide a good opportunity to beat the long fiber portion at higher PFI revolutions and to achieve better fibrillation, significantly improving all paper properties of kenaf pulps except for tear index and producing sheets with better drainage and strength properties compared to conventionally beaten pulps, especially in the case of kenaf high kappa pulp.

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Azizi Mossello et al. (2010). “Kenaf fiber processing,” BioResources 5(4), 2112-2122. 2112
NEW APPROACH TO USE OF KENAF FOR PAPER AND
PAPERBOARD PRODUCTION
Ahmad Azizi Mossello,a, d Jalaluddin Harun,a Hossein Resalati,b Rushdan Ibrahim,c
Seyeed Rashid Fallah Shmas,d and Paridah Md Tahir a
This study sought to determine the suitability of fractionation and
consequence-selective processing (separation of long fiber and short
fiber, beating long fiber, and remixing with short fiber to target freeness)
as a new approach to use of kenaf whole stem pulp for paper and paper-
board production. A laboratory Bauer-McNett Classifier with screen 18
mesh was used to separate short fibers and long fibers of the unbeaten
kenaf whole stem soda-anthraquinone high kappa and low kappa pulps.
For comparison, the initial unbeaten pulps were beaten in the PFI mill to
the same freeness (300 mL CSF). Results of our patented method
showed that the fractionation process was able to provide a good
opportunity to beat the long fiber portion at higher PFI revolutions and to
achieve better fibrillation, significantly improving all paper properties of
kenaf pulps except for tear index and producing sheets with better
drainage and strength properties compared to conventionally beaten
pulps, especially in the case of kenaf high kappa pulp.
Keywords: Kenaf; Fractionation; Beating; Drainage time; Strength properties
Contact information: a: Institute of Tropical Forestry and Forest Products, University Putra Malaysia,
Malaysia; b: Faculty of Forestry and Wood Technology , Gorgan University of Agricultural and Natural
Resources, Iran; c: Forest Research Institute Malaysia, Malaysia; d : Department of Desert Region
Management, College of Agriculture, Shiraz university, Shiraz, Iran;
*Corresponding author: aziziahmad99@yahoo.com / azizi1353@gmail.com
INTRODUCTION
Kenaf (Hibiscus cannabinus L.) is a herbaceous annual plant of the Malvaceae
family. Being a dicotyledon, kenaf stem contains two distinct fiber components the bast
fiber and inner woody core that are significantly different in chemical and morphological
properties. Generally, the bast part accounts for about 35% of stem mass, and a woody
core part comprises the remainder. The bast fibers are long and slender with higher
cellulose content, while the core fibers are much shorter and wider with higher lignin
content (Abdul Khalil et al. 2010; Azizi Mossello et al. 2009). Because of their different
nature and structure, the two types of fiber show different behavior during the
papermaking process. Core pulp due to its low proportion of fiber and presence of
components with a high surface area to mass ratio coming from pith (Villar et al. 2009)
has low freeness and high susceptibility to refining action, so that the pulp rapidly attains
freeness values that are quite prohibitive for practical purposes (Calabro 1992). These
shortcoming restrict the use of core pulp, which is probably better used in an unrefined
condition (Kaldor 1989). On the contrary, bast pulps refine easily and develop strength
properties (Villar et al. 2009; Calabro 1992). Due to the difference in the quality of bast

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Azizi Mossello et al. (2010). “Kenaf fiber processing,” BioResources 5(4), 2112-2122. 2113
and core fiber, some researchers have proposed separation of core and bast, pulping, and
using each pulp separately (Villar et al. 2009; Nezamoleslami et al. 1997; Ren et al. 1996;
Calabro 1992; Kaldor 1989) or adjusting the ratio of their blending based on the final
product properties that are required (Villar et al. 2009). On the other hand, there are
advantages in using kenaf as a whole stem (bast and core together) for technical and
economical reasons (Ververis et al. 2004; Khristova et al. 2002). It is critically important
to find ways to overcome to these issues.
On the other hand, fiber fractionation processes, which utilize maximum potential
of the fiber, are becoming increasing important in the paper industry (Sood et al. 2005).
Fractionation is a process that separates a blend of fibers in pulp according to some
physical property of the fibers such as length, flexibility, coarseness, etc. (Sood et al.
2005; Gooding and Olson 2001). Fractionation and consequent selective processing of
furnish components offers potential to achieve raw material and energy efficiency
(Branvall et al. 2005; Vomhoff and Grundstr 2003). The idea is to direct the right kind of
fiber furnish, and only the right kind of fiber furnish, through the required processing,
depending on its end use, so that it will generate the highest customer value. Many
different scenarios have been practiced for fractionation in the pulp and paper industry
(Sood et al. 2005; Olson 2001). One general approach is to fractionate fiber into long and
short fiber portions, beat the long fiber portion, and then recombine the refined long
fibers with the unrefined short fibers. The main objective of this study was to determine
the effect of fractionation and consequent selective process on kenaf whole stem soda-
anthraquinone (soda-AQ) pulp properties.
EXPERIMENTAL
Materials
Unbeaten kenaf whole stem soda-AQ pulps, i.e. kenaf high kappa (KHK) and
kenaf low kappa (KLK) pulps (Table 1), were prepared from our earlier study (Azizi
Mossello et al. 2010).
Table1. Soda-AQ Pulping Result for Kenaf Whole Stem
† Lignin content =0.15× Kappa no. (Ren et al., 1996)
Cooking condition Pulp quality
Pulp A.A.
(%)
Time
(min)
Kappa no.
Lignin†
(%)
Reject
(%)
Total yield
(%)
Kenaf high Kappa 12 60 49.4 7.41 1.8 58.0
Kenaf low Kappa 15 60 25.4 3.82 0.2 54.9
A.A.- active aclkali as Na2O; Cooking temperature, 160 ºC; liquor to raw material ratio, 7:1;
AQ, 0.1% ( on oven dried chips basis)

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Methods
Fractionation and consequent selective process
Based on length of the kenaf bast (2.39 ± 0.43) and core (0.72 ± 0.17) fibers
(Azizi Mossello et al. 2009), a laboratory Bauer-McNett Classifier (BMC) with screen 18
mesh size (1mm) was used to separate long and short fibers of kenaf whole stem pulps
according to TAPPI T 233 cm-95 at the pulp and paper laboratory in Forest Research
Institute Malaysia (FRIM). For each run, about 3500 mL of 0.3% pulp suspension was
poured into the first chamber. A continuous flow of water passes through this chamber.
The short fraction passed the screen and was collected on a very fine screen (200 mesh).
After 20 min, the flow was stopped, the chamber was drained, and the long-fiber fraction
was collected. After that, each fraction was collected and span-dried, and the moisture
contents were determined according TAPPI T210 cm-93. The long-fiber fractions were
beaten with a PFI mill according TAPPI T 248 sp-00 and remixed with unbeaten short-
fiber fractions (recreation the initial pulps) to reach a freeness of 300 mL CSF and termed
recombined pulps, i.e. recombined kenaf high kappa (RKHK) and recombined kenaf low
kappa (RKLK). The kenaf high Kappa (KHK), and low Kappa (KLK) pulps were beaten
separately in a PFI mill at the same freeness level i.e. 300 mL CSF and designated as
beaten kenaf high Kappa ( BKHK) and beaten kenaf low Kappa (BKLK) pulps
respectively.
Fiber dimensions and derived values
The pulp samples designated as KHK, KLK, BKHK, BKLK, RKHK, and
RKLK, were boiled in water in separate beakers to remove air from the fibers and then
placed in separate test tubes containing an equal amount of glacial acetic acid and 35%
hydrogen peroxide, an approach similar to that followed by Franklin (1945). For this
study, fiber length (L), fiber diameter (D), lumen diameter (d), and cell wall thickness (w)
were measured based on an average of 50 measurements by using a Quantimeter Image
Analyzer equipped with a Lecia microscope and Hipad digitizer (from Quantimet 520,
Cambridge Instruments). Three derived values: slenderness ratio (L/ D), flexibility
coefficient( 100 × d/ D ), and Runkel ratio ( 2 × w/d), were calculated using the measured
data.
Fiber classification
The fiber classification for each pulp was carried out with a Bauer-McNett
Classifier (BMC) with 4 screens (18, 40, 70, and 140 mesh) according to TAPPI T 233
cm-95 with 3 replications.
Paper characterization
The drainage times of the pulps were determined according to T 221 cm-99. The
handsheets with basis weight of 150 g/m2 were made in a British handsheet former
according to TAPPI T 205 sp-02 and tested for tear index (TAPPI T 414 om-98 ), tensile
index (TAPPI T 494 om-01 ), burst index (TAPPI T 403 om-97), and ring crush
test(RCT) (TAPPI T 822 om-02). Light scattering coefficient was measured using a
Color TouchTM model ISO (Technidyne Corporation) spectrophotometer according to
TAPPI T425 om-01. The surface morphology of pulp fibers was observed using a

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Philips XL30 ESEM at the electron microscopy unit in the Institute of Bio-Science (IBS)
at University Putra Malaysia (UPM).
Statistical analysis
The fiber and paper properties experiments were done as a Completely
Randomized Design (CRD) with 6 pulp types as treatment. Analysis of variance and
Duncan multiple Range tests were done to show difference between treatments.
Statistical procedures were carried out using SPSS software.
RESULTS AND DISCUSSION
Fractionation and Recombination
Table 2 shows the fractionation results of kenaf whole stem soda–AQ pulps. It
can be seen that KLK had higher long fiber (47.59 vs. 41.19 %) and lower short fiber
(52.41 vs. 58.81) than KHK. According to Calabro (1992), at low soda concentration,
yield is greater for core, but with higher concentration of soda it inverts and become
greater for the bast. Also, Mittal and Maheswari (1994) based on their experience with
commercial kraft pulping of whole kenaf stalk in Thailand (Phoenix Paper Mill) reported
that during the process of pulping, a higher portion of bast fibers remain with the final
pulp, which resulted in higher average fiber length and good strength properties of the
final pulp sheets. Again, in contrast to short fiber, long fiber due to higher freeness, had
higher potential for beating
Table 2. Fractionation Results for Kenaf Whole Stem Soda-AQ Pulps
KHK KLK
Parameter
unit Long fiber Short fiber Long fiber Short fiber
Mass split ratio % 41.19 58.81 47.59 52.41
Length mm 2.25 0.63 2.43 0.60
freeness mL CSF 590 346 568 319
Figure 1 illustrates the pulp freeness development as a function of the number of
PFI mill revolutions for the kenaf pulps. In comparison to beaten pulps, recombined
pulps required more beating to reach a same freeness. For instance, RKHK required
higher PFI revolutions than KHK ( 2500 vs. 700) to reach a freeness of 300 mL CSF,
and RKLK took higher beating than KLK (2000 vs. 950 ) to reach a freeness level of
300 mL CSF. These results show that fractionation can provide a good opportunity to
beat the long fiber portion at higher PFI revolutions and remix those refined fibers with
short fiber pulps to reach the same freeness. Also, RKHK pulp, due to its higher lignin
content than RKLK (7.41 vs.3.82%), had lower swellability and beatabillity (Steh 2001)
and required more beating to reach a target freeness.

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Azizi Mossello et al. (2010). “Kenaf fiber processing,” BioResources 5(4), 2112-2122. 2116
200
250
300
350
400
450
500
0 500 1000 1500 2000 2500 3000 3500 4000
PFI revolutions
Freeness, mL CSF
KHK
KLK
RKHK
RKLK
Figure 1: Variation of kenaf soda-AQ pulps freeness with PFI mill revolutions
Fiber dimension and derived values
The fiber dimension results are given in Table 3. In comparison to KHK, KLK
probably due to higher percentage of long fiber (Table 2) had a higher average fiber
length, but lower fiber diameter, as well as lumen and cell wall thickness. Beating
decreased all fiber dimensions (Rushdan 2003) (Figure 2). BKHK, because of lower
beating, showed a smaller decrease in fiber dimensions. In comparison to beating pulps
(BKHK and BKLK), recombined pulps (RKHK and RKLK), and due to higher beating,
had larger decreases in fiber dimensions except for lumen diameter. Again, RKLK
showed a higher fiber length (1.39 vs. 1.27 mm) and lower cell wall thickness (3.05 vs.
3.21 µm) than RKHK.
Table 3. Fiber Dimensions of Kenaf Whole Soda-AQ Pulps
Pulp Length, mm Diameter, µm Lumen, µm Thickness, µm
KHK 1.50ab† 21.03a 13.20a 4.16a
BKHK 1.38b 19.95ab 12.97a 3.97a
RKHK 1.27b 18.75bc 13.10a 3.21b
KLK 1.69a 19.42b 12.70a 3.90a
BKLK 1.46b 18.01c 11.96a 3.44b
RKLH 1.39b 17.59c 12.51a 3.05b
† Means within a column followed by different letters differ significantly at α = 0.05
Derived values of kenaf whole stem soda-AQ pulps are shown in Table 4. In
comparison to KHK, KLK pulp had a higher slenderness ratio (87.02 vs. 71.33) but the
same flexibility coefficient and Runkel ratio. Beating with fibrillation and fiber
shortening (Table 2 and Figure 2) decreased the slenderness ratio and Runkel ratio but
increased the flexibility coefficient of pulps. In comparison to beaten pulps, recombined
pulps showed higher decreases in Runkel ratio (about 19 vs. 5%) and had the lowest
Runkel ratio (0.49), which can be explained as due to better fibrillation of long fiber at
higher PFI revolutions (see Figs. 1 and 2).

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Figure 2. ESEM micrographs kenaf soda-AQ pulps fibers with magnification 1000x: (a) unbeaten
kenaf high kappa pulp( KHK), (b) unbeaten kenaf low kappa pulp ( KLK), (c) beaten kenaf high
kappa pulp ( BKHK), (d) beaten kenaf low kappa pulp ( BKLK), (d) recombined kenaf high kappa
( RKHK), (e) recombined kenaf low kappa ( RKLK)

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Table 4. Derived Values for Kenaf Whole Soda-AQ Pulps
Bauer-McNett Fiber classification
The results for Bauer-McNett fiber classification of kenaf whole stem soda-AQ
pulps are shown in Table 5. It can be seen that KLK contained significantly more R18
fiber and less R40 fiber mass, but almost the same amounts of R70, R140, and P140
fibers, which means that KLK contained higher long fiber, in good agreement with
fractionation results that showed a higher percentage of long fiber for KLK (see Table2).
Beating significantly decreased R18 fiber. Recombined pulps showed higher decreases in
R18 fiber than beating pulps. Again, RKHK showed the highest decreases in R18 fibers.
Beating caused decreases in R40 and R70 fibers of beaten pulps, but caused increases in
R40 and R70 fiber of recombination pulps. Also, beating, due to fines generation,
significantly increased the amount of P140 fibers. From Table 4 it can be seen that there
were no significant differences between recombined pulps and BKHK for P140 fibers,
but BKLK had a significantly higher amount of P140 fibers. It can be explained that
beating, in case of kenaf whole pulps, had a greater effect on short fiber, but in case of
recombined pulps, beating long fibers provided a good opportunity to achieve better
fibrillation (Fig. 2), which is clearly documented by the lower Runkel ratio in Table 4.
Table 5. Bauer-McNett Fiber Classification of Kenaf Pulps
Pulp R18, %
R40, %
R70, %
R140, %
P140, %
KHK 39.01b† 22.17b 12.63a 5.15ab 21.04c
BKHK 36.76c 18.22c 11.90ab 6.26a 26.83b
RKHK 27.29d 24.83a 13.10a 5.74ab 29.01ab
KLK 45.19a 18.80c 11.89ab 4.60b 19.52c
BKLK 39.13b 13.10d 10.39c 6.53a 30.83a
RKLH 35.43c 20.16c 12.38a 4.79b 27.25b
† Means within a column followed by different letters differ significantly at α = 0.05
Handsheet properties of kenaf whole stem soda-AQ pulps
Table 6 shows the properties of handsheets made from unbeaten (KHK and
KLK), beaten (BKHK and BKLK), and recombined (RKHK and RKLK) kenaf whole
stem pulps. It can be seen that beating, due to fiber shortening and fines generation
(Tables 3 and 5), significantly increased the drainage time of pulps. There was no
Pulp Slenderness ratio Flexibility coefficient Runkel ratio
KHK 71.33 62.77 0.63
BKHK 69.17 65.01 0.61
RKHK 67.73 69.87 0.49
KLK 87.02 65.40 0.61
BKLK 81.07 66.40 0.58
RKLH 79.02 71.12 0.49

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significant difference between recombined pulps and BKHK, but BKLK had significantly
higher drainage time than other pulps. It is a very interesting point that except for the
higher beating of long fiber of the recombined pulps, they showed almost the same or
lower drainage time than beaten pulps.
Apparent density is among the most important properties of paper, since is a good
indicator of fiber flexibility and fiber bonding (Seth 2001; Brandon 1981) and is used by
some as a predictor of paper strength, since bonding in the sheet increases both strength
and density (Kline 1982). In the case of unbeaten pulps, KLK had significantly higher
density than KHK due to more flexible fibers and easier collapsability that is associated
with lower lignin content. Beating significantly increased the density of the pulps (Table
6). Recombined pulps showed higher density than beaten pulps, which showed more
flexible fibers (see Fig. 2), as indicated by a lower Runkel ratio (Table 4).
Table 6. Handsheet Properties Made from Kenaf Soda-AQ Pulps
† Means within a row followed by different letters differ significantly at α = 0.05
The light scattering indirectly indicates unbonded area between the component
fibers, thereby providing an inverse estimated degree of bonding (Seth 2001; Clark
1985). KLK exhibited more flexibility of the fiber due to lower lignin higher fiber
bonding than KHK, as indicated by a lower light scattering coefficient (Table 6). Beating
significantly decreased the light scattering coefficient. Recombined pulps showed lower
light scattering coefficient than beaten pulps. BKHK and KLK showed almost as same
light scattering coefficient.
Tear strength depends on fiber length, fiber bonding, and the total number of
fibers that are involved in the sheet rupture. The number of fibers participating in the
sheet rupture is determined by the flexibility of the sheet. A rigid sheet will concentrate
the force on a few fibers in a small area; a flexible sheet will distribute the force over a
much larger area and, therefore, a larger number of fibers. The force required to tear
paper is much less than the force necessary to break a strip of the paper. Moreover in a
weakly bonded sheet, since more fibers pull out than break in the tear zone, the tearing
resistance is controlled more by the number of bonds that break along the length of the
fibers; thus tearing resistance depends strongly on the fiber length (Institute of Paper
Chemistry Staff 1944). According to Table 6, KLK due to higher fiber bonding, as
indicated by higher density and lower light scattering coefficient, had higher tear index
Parameter KHK BKHK RKHK KLK BKLK RKLK
Drainage time, s 7.75d† 10.40b 10.82ab 8.49c 11.05a 10.55b
Apparent density, g/cm3 0.590e 0.651c 0.690ab 0.620d 0.672bc 0.705a
Light scattering
coefficient, m2/kg
24.90a 22.56b 19.13cd 22.83b 20.06c 18.05d
Tear index, mN.m2/g 13.38c 14.35b 13.09c 15.11a 14.36b 13.91b
Tensile index, N.m/g 71.03d 78.23c 87.52b 78.84c 86.10b 93.11a
Burst index, kPa.m2/g 4.18d 5.21c 6.10b 5.12c 5.90b 6.59a
RCT, kN/m 1.85e 2.25d 2.87b 2.25d 2.68c 3.11a

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Azizi Mossello et al. (2010). “Kenaf fiber processing,” BioResources 5(4), 2112-2122. 2120
than KHK. With the exception of BKHK, beating decreased tear index of pulps. This can
be explained, since under the condition of tightly bonded fibers, more fiber are ruptured
through the initial cut, and fiber rupture requires less energy than pulling fibers out from
network, so tear strength is reduced (Institute of Paper Chemistry Staff 1944).
Tensile strength and burst index are dependent on fiber bonding (Dutt et al. 2009;
Jahan and Rawshan 2009) and showed similar trends in this study. KHK showed
significantly lower tensile index and burst index than KLK. Beating increased the tensile
index and burst index of the pulps. At the same freeness, recombined pulps showed
higher tensile index and burst index than beaten pulps (Table 6). BKHK and KLK
achieved the same tensile index and burst index. Again, BKLK and RKHK reached
almost the same tensile index and burst index. RKLK showed significantly higher tensile
index and burst index than other pulps, which can be explained as due to better
fibrillation and improvement of fiber bonding. This was indicated by lower light
scattering coefficient and higher sheet density, as explained earlier.
The ring crush test (RCT) is used extensively for quality control during
production of linerboard and corrugating medium. Fiber with better bonding can produce
paper with higher ring crush strength (Parker et al. 2005). From Table 6 it can be seen
that KLK had higher RCT than KHK. Beating significantly improved the RCT of the
pulps. Again, recombined pulps achieved significantly higher RCT than beaten pulps at
same freeness. BKHK and KLK had the same RCT. RKLH had a significantly higher
RCT than the other pulps considered.
CONCLUSIONS
The fractionation results showed that kenaf low kappa (KLK) had higher long
fiber and lower short fiber than kenaf high kappa (KHK) pulp. Long fibers, because of
their higher freeness, have potential to be beaten and improve strength properties. In
comparison to beaten pulps, recombined pulps required more beating to reach the same
freeness. So, fractionation provides a good opportunity to beat the long fiber more
extensively and remix them with short fiber pulps to reach the same freeness. Also, the
recombined high kappa (RKHK) pulp, due to its higher lignin content than RKLK, had
lower swellability and beatabillity and required more beating to reach a target freeness.
Beating with shorting and fibrillation decreased all fiber dimensions. Recombined
pulps (RKHK and RKLK), due to higher beating, had larger decreases in fiber
dimensions, with the exception of lumen diameter. Moreover, recombined pulps showed
lower Runkel ratio than beaten pulps.
Beating affected the fiber distribution of pulps. Recombined pulps showed greater
decreases in R18 fiber than beaten pulps. Beating caused decreases in R40 and R70 fiber
fractions of beaten pulps, but caused increases of R40 and R70 fiber fractions in
recombined pulps. Beating, due to fines generation, significantly increased P140 fibers,
but there were no significant differences between recombined pulps and BKHK for P140
fibers; BKLK had a significantly higher amount of P140 fibers.
Beating with fiber shortening, fibrillation, and fines generation improved fiber
bonding, as indicated by lower light scattering and higher paper density. In comparison to

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Azizi Mossello et al. (2010). “Kenaf fiber processing,” BioResources 5(4), 2112-2122. 2121
beaten pulps, recombined pulp produced paper with significantly higher strength
(exception for tear index) and better drainage properties. RKLK yielded the highest paper
properties. RKHK gave paper with higher strength properties than BKLK.
Finally, fractionation, due to the possibility of beating the long fiber portion of the
pulp more extensively, provides a good opportunity to achieve better fibrillation, enhance
fiber bonding, and produce paper with higher strength properties, especially in the case of
kenaf high kappa pulp. This can be considered as a new approach to enhance kenaf whole
stem pulp properties.
ACKNOWLEDGMENT
The authors wish to thank the Economic Planning Unit of the Prime minister’s
Department Malaysia for financial support.
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Article submitted: June 23, 2010; Peer review completed: July 25, 2010; Revised version
received and accepted: August 9, 2010; Published: August 9, 2010.