J. Pack. Sci. Technol. Vol.6 No.6(1997)

Preparation and Properties of Retted Kenaf Bast Fiber
Pulp and Evaluation as Substitute for Manila Hemp Pulp
Abdolreza NEZAMOLESLAMI* , Kyoji SUZUKI* , Takashi KADOYA**

Using a retted Chinese kenaf bast fiber (CBF) as raw material, various pulps were prepared by using three cooking
methods, i.e. soda-AQ pulping, kraft pulping and non-pressurized soda pulping. The capabilities of these pulps, as
substitutes for manila hemp pulp which is used mainly as specialty paper applications, were investigated.
The soda-AQ and kraft pulps of retted CBF had lower Kappa number and ash content with greater pulp yield thanthose of unretted Japanese kenaf bast fiber (JBF). The CBF pulps prepared by the three cooking methods were
bleached to about 80% Hunter brightness, by a two-stage hypochlorite process, and the relationship between strength
properties and CSF was studied. Tensile index and burst index of those pulps at CSF 400ml were in the following
order: non-pressurized soda> soda-AQ> kraft. The strengths of the non-pressurized soda pulp were superior to those ofcommercial manila hemp pulp. The folding endurance of the three pulps were lower than that of manila hemp pulp,
while, that of the non-pressurized soda pulp became equal by 300ml CSF. Therefore, the non-pressurized soda pulp
from retted CBF was found to be hopeful as substitute for manila hemp pulp.
Keywords : Kenaf, Retted bast fiber, Non-pressurized soda cooking, Soda-AQ cooking, Kraft cooking, Strength
properties, Manila hemp pulp

Malvaceae, has attracted attentions as a potential
non-wood raw material for the pulp and paper-

1. Introduction
The utilization of non-wood plant fibers instead

making over the last decade 1 ) , 2 ) . Kenaf reaches

of wood fibers in the production of different kinds

4-5m high with a basal diameter of 25-35mm

of pulps and papers is growing in many devel-

during a growth cycle of 5-7months. The stem ofoping and developed countries. Since 1970 to

kenaf is composed of two main fiber types ," bastpresent time the non-wood plant fiber pulping

fiber and inner woody core fiber. The average
length of the bast fiber is 3-4mm, and that of thecapacity has increased, on a global basis, two or
three times as fast as the capacity for producing

core fiber is 0.5-0.7mm4) . The core fibers gener-

papermaking wood pulp .

ally are short and wide with thin wall while the

Kenaf (Hibiscus cannabinus L. ), a fast growing warm-season annual plant in the family ofbast fibers are long and stiff. Generally, on an
oven-dry basis, the bast part accounts for about

1)*Department of Forest Resources Sciences, Faculty of Agriculture Shizuoka University, 836 Ohya, Shizuoka-shi, Shizuoka, 422 Japan**Kanagawa University, 2946 Tsuchiya, Hiratsuka-shi, Kanagawa, 259-12 Japan- 339 -

Preparation and Properties ofRetted KenafBast Fiber Pulp

2. Materials and methods

1/3 of the stem mass and the woody core part for
the remainder. To date, the whole kenaf stem has

2.1 Materials

been commercially pulped by kraft method,

The commercially available retted kenaf bast

however, most of the conventional and some
non-conventional methods have given the required
properties5)-7).

fiber, imported from China, was used. This will be
simply referred to as Chinese bast fiber and

Because the kenaf bast and core fibers havedifferent nature and structure, they show different

designated as CBF. The epithelial layer has been

behavior during pulping processes3),6) . Nowadays,

fiber and the woody core part, which had been

researches on the kenaf pulping are mainly

harvested in Japan, were used in comparison withdirected on the pulp and paper characteristics of

the CBF. Those will be referred to as Japanesecore or bast part of kenaf in a separate system ofbast fiber and Japanese core part, and designatedprep aration.

as JBF and JCP, respectively. Some properties ofRecently some mechanical methods for separating the kenaf parts have been described and

CBF, JBF, and JCP are given in Table 1. JCP waspractically performed 3) . However, in some of theused for analytical comparison.

developing countries due to less expensive wages,

The commercial manila hemp pulp was compared to the kenaf pulps prepared in this study.

almost removed in CBF. Also the unretted bast

not subjected to any pulping method and only was

kenaf fibers are separated manually rather thanmechanically at present. One of the practicalmethods which facilitates the manually separationof bast fibers from core part and decreases thelignin content of the fibers is a method called"retting process" through which the kenaf stalks
are immersed in marsh and undergo the lignin
biodegradation by the enzymatic activities in themarsh during the treatment.
In the present paper, the pulping and papermaking characteristics of retted and unretted
kenaf bast fibers under various cooking conditions
are investigated. The obtained results are compared with those of commercial manila hemp pulp
which is used mainly as specialty paper applications.

2.2 Methods
2.2.1 Pulping
Both of the bast fibers were cut into pieces of
2-3cm length. The kraft and soda-AQ methods
were employed for them as conventional pulping
processes. For CBF, due to having some certain
characteristics such as low lignin content, a- 3 4 0 -

J. Pack. Sci. Technol. Vol.6 No.6(1997)

so-called non-pressurized soda method was em-

fibers. Considering the lignin content and also theployed. All the soda-AQ and kraft cooks wereamount of chemicals charged in soda-AQ cooks,

carried out with minidigester (capacity of 60ml,
heated in a temperature-controlled oil bath sys-

the conditions applied for kraft cooks were astem) and autoclaves (capacity of 1 and 2 ℓfollows :,Liquor to solid ratio : 5/1 ( ℓ/kg) ; Time toequipped with h eater regulated by electricalcooking temperature : 1 hr ; Cooking temper-transformer) in our laboratory. Charges of 100-

ature : 170°C ; Time at cooking temperature : 2200g bast fiber were charged in autoclave for

hr ; Active alkali charge : 13% and 16% for CBF

cooking and quantities of 8g were charged for

and JBF, respectively ; Sulfidity : 10% for bothbast fibers.

cooking in minidigester. For all the cooking tests,
the chemical charges were based on percentage tooven-dried bast fiber and calculated as Na2O base.

(3 ) Non-pressurized soda cooking(1) Soda-AQ cooking

soda cooking. At first the bast fiber was cut into

The CBF was subjected to non-pressurized

For pulping of the CBF, to achieve a proper

the lengths of 3-4cm. For each cooking test, l00g

condition, the cooking temperature and chemical

bast fiber and the necessary amounts of sodium

charge were examined as independent variableshydroxide in water were charged to a 2 ℓusing minidigester. The cooking temperature wasbeaker and heated to the boiling point at atmo-

examined within the range of 160-170℃ , and thespheric pressure. To retain constant volume of thetotal active alkali applied was within the range of

cooking liquor, required amount of hot water was

11-14%. In all of those soda-AQ cooks, the time

gradually added into the beaker. The cooking

at maximum cooking temperature was kept con-conditions were as follows :glassstant for 2 hr, and also the amount of anthraquinone addition was fixed at a same amount foring temperature : 100 ℃ ; Time at cooking tem-

all conditions, 0.2% (based on o.d. weight of bast

perature : 5hr ; NaOH charge : 21%.

Time to cooking temperature : 20 min ; Cook-fiber) .

In all pulping methods, the chemicals charged

The followings were the final conditions

were based on the oven-dried weight of bast

employed for the soda-AQ cooking of both bast
fibers :fibers. All of the cooking tests were carried out

Liquor to solid ratio : 5/1 ( ℓtwice or more. At the end of cooking, the system/kg) ; Cooking

was cooled and the recovered pulps were washedtemperature : 170℃ ; Time to cooking temper-by tap water until no residual chemicals found andature : 1 hr ; Time at cookingtemperature : 2hr ;then passed over a laboratory flat screen. Then the

Anthraquinone charge : 0.2% ; NaOH charge :13% for CBF and 16% for JBF.

screened pulps were dewatered and their yield and(2) Kraft cooking
The sulfate cooking was applied for both bast

Kappa number were determined for every cooking
conditions.

- 3 4 1 -Preparation and Properties of Retted Kenaf Bast Fiber Pulp

2.2.2 Conventional two-stage hypochlorite

according to the TAPPI T 452. Handsheets were

bleaching
A commercial sodium hypochlorite solution

prepared using a TAPPI Standard Sheet Machine.

(containing about 5% active chlorine) was used

determined with a Hunter-type reflectometer,as bleaching reagent. In order to reach theaccording to TAPPI T 452 and JIS P 8138,

handsheet brightness of about 80% (near to thatof manila hemp pulp sheets), the required amountrespectively. Handsheet strength properties were
determined according to TAPPI T 220.

The brightness and opacity of the handsheets wereo f a ctiv e chlo rine (C l) wa s add ed t o ea ch

3. Results and discussion

unbleached pulp sample. The necessary active
chlorine for each sample was estimated from the
Kappa number of each sample and about 70% of

3.1 Preparation and properties of unbleachedkenaf pulps

the chlorine demand was added as NaCIO solution

Figs. 1 to 3 show the changes in Kappa

in the first stage and 30% in the second stage. For

number, screened pulp yield, and brightness as aeach bleaching stage, the initial pH was adjusted
to about 10. The consistency of the pulp slurryfunction of alkali charge in the soda-AQ cookingof CBF at three different cooking temperatures;

was controlled to be 10%, and the pulp suspen-

160℃, 165℃, and 170℃. The Kappa number

sion was kept in a polyethylene bag and placed invalues decrease with increasing the soda concen-a water bath. The samples were heated at 60'C for

tration and cooking temperature. On the other4 hr. At the end of each bleaching stage, the slurryhand, the screened pulp yield increases withwas filtered and washed with hot tap water.

increasing the soda concentration, mainly up to

Finally, the bleached sample was dewatered and13%, and at larger concentration no significant
variation is observed. The maximum screened

kept in the refrigerator for the next procedure.

2.2.3 Pulp beating
The bleached pulps were beaten by a Tappi
Standard Niagara beater in our laboratory. The

three measurements. However, alkali charge ofamount of pulp charged for each beating trial was

13% is preferable from economical point of view.

90 g, pulp consistency is 1.2%.

The brightness is also improved with increasing

2.2.4 Analytical methods
The chemical compositions of materials, i.e.,
ash, alcohol-benzene extractives, holocellulose,

the soda concentration and cooking temperature,

Klasson lignin and pentosans, were determined

results indicate that an alkali charge of 13% andaccording to JIS P 8003, 8010, 8007, 8008 andcooking temperature of 170℃ is the most suitable

8011, respectively. The basic density of bast

condition for the 2 hr soda-AQ cooking of CBF.
Some properties of the unbleached soda-AQ

yield value (about 63%) was obtained at 170 ℃
and alkali charge of 13-14% from the average ofbut there is not very much change for thebrightness at the concentration of 13-14%. The

fibers and core part was calculated according to
JIS P 8014.The Kappa number was determined- 342 -

J. Pack. Sci. Techno!. Vol.6 No.6(1997)

ences in the chemical and fibrous compositions of
two bast fibers. As shown in Table 1, the lignin
content of CBF (8.2%) is lower than that of JBF(12.5%). Lower values of ethanol-benzene extractives and lignin content in the CBF, comparing tothose of JBF, are considered to be mainly causedby the retting pretreatment of CBF done in China.
Table 2 shows that the Kappa number values ofunbleached kraft and soda-AQ pulps of CBF (6.3
and 6.2, respectively) are significantly lower thanthose of unbleached JBF pulps (19.9 and 21.0, forthe soda-AQ and kraft pulps, respectively),
although the alkali charges were higher in JBF
cooking (16%) than that of CBF (13%). Furthermore, the unbleached CBF pulps have lower
ash content with a greater screened pulp yieldthan those of JBF. The Kappa number values ofand kraft pulps of CBF and JBF are compared inTable 2. The properties of the bast fiber pulps
obtained were highly influenced by the differ-

the soda-AQ and kraft pulps from both bast fibers
were not significantly influenced by the type of
cooking. On the other hand, the non-pressurized- 3 4 3 -Preparation and Properties of Retted Kenaf Bast Fiber Pulp

soda pulp of CBF has higher Kappa number andering the big difference existed in the Kappa

yield than those prepared by the soda-AQ andkraft methods.

number of unbleached samples (Table 2) , it would

3.2 Properties of bleached pulps and hand-be reasonable to have higher bleached yield forthe non-pressurized cooking.
3.2.2 Handsheet propertiess h e e t s
3.2.1 Pulp properties
All of the unbleached bast fiber pulps were
bleached to a brightness level of manila hemp
sheets (about 80%) by using a two-stage hypochlorite bleaching process. Some properties of thebleached pulps are given in Table 3. The yields ofboth kraft and soda-AQ pulps were decreased
almost to the same extent after bleaching, butthere exist a big difference between the yieldvalues for CBF and JBF. However, among thethree CBF pulps, at a similar handsheet brightnesslevel, the non-pressurized soda pulp of CBF
having 60% yield is superior to others. Consid-

To evaluate the effect of beating on different
bleached pulps obtained, the strength properties of
bleached handsheets were determined at different
degrees of beating. Figs. 4 to 7 show various
strength properties of handsheets plotted against
the CSF (Canadian Standard Freeness) for thesoda-AQ, kraft, and non-pressurized soda pulps of
CBF and the commercial manila hemp pulp. As
shown, the tensile, folding, and burst strengths of
CBF pulps improve as beating continues, but withdifferent tendencies. The beating process is more
effective for the non-pressurized soda pulp thanfor the soda-AQ or kraft pulp of CBF. Those threestrength properties of non-pressurized soda pulp

- 3 4 4 -

J . P a c k. S c i . T e c h n o l . Vol . 6 N o. 6 ( 199 7 )smoothly increase with decreasing the CSF andcomparable to those of manila hemp pulp.

then leveled off, except the folding endurance.

However, the tear index value of the non-Comparisons of the strength properties of variouspressurized soda pulp at the CSF of 400ml islower than that of manila hemp pulp, but it
become nearly equal at 300ml CSF (Fig. 7).

bleached handsheets at 400ml CSF are given in
Table 4. As indicated, the strength properties of
the non-pressurized soda pulp are sufficiently- 3 4 5 -Preparation and Properties of Retted Kenaf Bast Fiber Pulp

4. Conclusion
5 ) On the evaluation of strength properties ofFrom this research the following conclusions
can be given;handsheets of all bleached pulps, the non-

1 ) The contents of Klasson lignin and ethanol-

was superior to other pulps produced bysoda-AQ or kraft from both bast fibers.

pressurized soda bleached pulp from CBF

benzene extractives of CBF were lower, andthe amounts of holocellulose and pentosans6) In comparison with commercial manila hemp

were a little higher than those of JBF. These

pulp, the handsheets prepared from non-differences are considered to be caused by theretting pretreatment of CBF done in China.

pressurized caustic pulp of CBF showed quite

2) On the kraft and soda-AQ pulping, it ishemp pulp, at similar levels of freeness anddetermined that the CBF can be pulped withbrightness. Therefore, the non-pressurized

lower cooking chemical charges than for JBF,
and with greater pulp yields.

soda pulp of retted CBF can be used a
hopeful substitute for the manila hemp pulp.

3) An alkali charge of 13% and cooking temper-

comparable strengths to those of manila

Acknowledgment

ature of 170℃ is the optimum for the 2 hr
soda-AQ cooking of CBF.
4) The pulping properties of either bleached orunbleached soda-AQ pulps of both bast fibers
were satisfactorily similar to those of kraft
pulps, and this indicates that the soda-AQ
method can be a substitute for kraft methodin pulping of the kenaf bast fibers.

The authors thank to Dr. T. Ueno, former
professor of Shizuoka University, for his valuableadvice.

References
1) E.A.Joseph,TappiJ.,79 (10) ,87 (1996)
2) R. L. Cunningham,etal.,TappiJ.,53 (9) ,- 3 4 6 -

J. Pack. Sri. Technol. Vol.6 No.6(1997)

1697(1970)

6) A. F. Kaldor, C. Karlgren, H. Verwest, Tappi

3) T. Clark, R. L. Cunningham, I. A. Wolff,
TappiJ.,54 (1) ,63 (1971)

J.,73 (ll) ,205 (1990)
7) M. O. Bagby, T. Clark, TappiJ.,58 (7) , 121

4) A.F.Kaldor,TappiJ.,72 (9) ,137 (1989)( 1 9 7 5 )5) W. K. Moore, R. Trotter, S. Corkern, M. O.
Bagby,TappiJ.,59 (l) , 117 (1976)(Received 6 August 1997)(Accepted 4 December 1997)

精錬発酵ケナフ靭皮繊維パルプの製造と特性およびマニラ麻パルプ代替品としての評価
アブドレザ・ネザモルエスラミ*

鈴木恭治*

門屋

卓*

精錬発行処理を受けた中国産ケナフ靭皮繊維(CBF)を用いて、異なる蒸解方法により得られたパルプの特性を評価するとともに、特殊紙用に用いられているマニラ麻パルプ代替品としての可能性を検討した。蒸解方法は一般に行われているソーダAQ法、クラフト法に加えて蒸圧ソーダ蒸解法も行った。
ソーダAQ法、クラフト法ともにケナフ靭皮繊維に対し良質なパルプを与えたが、CBFから調製されたパルプは精錬発酵処理されていない日本産ケナフ靭皮繊維（JBF）より低活性アルカリ条件にも拘らず高パルプ収率、低カッパ価、低灰分量であった。
各パルプ化方法により調製したパルプを漂白した後ピーターで叩解し、濾水度と強度特性の関係を調べた結果、CSF４００での引張りおよび破裂指数は常圧ソーダ、ソーダAQ、クラフトの順に高い値となり、常圧ソーダパルプはマニラ麻を上回る強度を示した。耐折強さは3種パルプともマニラ麻パルプ
より低い値となったが、常圧ソーダパルプはCSF３００付近でマニラ麻と同様の値となった。したがって常圧ソーダ蒸解したCBFパルプはマニラ麻パルプの代替品として有望であることが分かった。
キーワード：ケナフ、精錬発酵靭皮繊維、常圧蒸解、ソーダAQ蒸解、クラフト蒸解、強度特性、
マニラ麻パルプ- 3 4 7 -