Biomass Conv. Bioref. (2014) 4:53–58 DOI 10.1007/s13399-013-0095-1

ORIGINAL ARTICLE

Evaluation of mulberry plant as a pulping raw material M. Mostafizur Rahman & M. Sarwar Jahan

Received: 7 May 2013 / Revised: 19 August 2013 / Accepted: 23 August 2013 / Published online: 8 September 2013 # Springer-Verlag Berlin Heidelberg 2013

Abstract In this paper, a complete chemical analysis of the mulberry plant was performed to evaluate the differences in its chemical composition in relation to plant age. The mulberry plant is characterized with moderate cellulose, lignin, and pentosan. Plant age (from 8 to 12 months) increased cellulose content from 42.0 to 42.8 % and decreased total lignin content from 26.0 to 23.3 %. Extractive contents also decreased with plant age. Kraft and soda-AQ pulping processes were also evaluated for 12-month-old mulberry plant. The mulberry plant was very difficult to delignify and needed drastic cooking conditions. To achieve kappa number 25, 18 active alkali charges was needed for 3 h of cooking. At this kappa number, the kraft process showed slightly better pulp yield. The physical properties of the mulberry plant pulp were similar to hardwood pulp. Kraft process showed a slightly better tear index. Blending of the mulberry plant pulp with longer jute fiber pulp increased the tear index significantly. Keywords Mulberry plant . Soda-AQ and Kraft pulping . Pulp reinforcement . Papermaking properties

1 Introduction The paper industry is always looking for alternative materials that can be used as a source of fiber. The forest land in Bangladesh is very low and population density is very high. Therefore, the forest in Bangladesh can no longer meet the demands for forest industry. Thus, the use of cellulose fiber from annual plant is inevitable. Nonwood fiber is an important source for papermaking in many developing countries. Most M. M. Rahman : M. S. Jahan (*) Pulp and Paper Research Division, BCSIR Laboratories, Dr. Qudrat-i-Khuda Road, Dhaka 1205, Bangladesh e-mail: [email protected]

papermakers and many consumers are already concerned for the environment and they campaign vociferously against the use of primary wood fibers in papermaking. It should also be kept in mind that fibers harvested annually have a high biomass production potential. Some nonwood plants even give more pulp per hectare than wood. From a marketing viewpoint, the environmental awareness related with paper containing nonwood pulp is encouraging nowadays. Jute is one of the most important natural fibers in Bangladesh. Jute has a long historical role in the socio-economic development of Bangladesh. Once, it was known as the “Golden fiber of Bangladesh.” The export of jute and related products accounts for a significant portion of total export. In addition, it provides considerable employment opportunities to the country’s work force. The chemical and morphological characteristics of jute favor it as pulping raw material [1]. Utilization of jute in pulp production may create a new horizon for farmers. With this aim in view, many studies have been done on the pulping of jute in home and abroad [2–6]. Jute pulp was comparable to softwood pulp [2]. But the price of jute fiber cannot compete with wood fiber. Therefore, cheap fibrous raw materials need to be reinforced with the jute fiber pulp. Recently, the Bangladesh Government is keen to reopen the North Bengal Pulp and Paper Mills (NBPM) with jute fiber. The mulberry plant is an important fibrous raw material within the region of NBPM. The northern region of Bangladesh is well known for silk production. Mulberry leaves are fundamental for silk cultivation. After taking leaves by the worm, branches/stems are discarded. Presently, it is being used as domestic fuel in a rural area to a limited extent. China produces pulp from mulberry plant and reinforced it with Eucalyptus pulp. The mulberry plant can be a suitable source for pulping in Bangladesh also. The present government is keen to establish a pulp mill based on jute in the northern region. Jute is an expensive raw material. So it will not be viable to establish a

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pulp mill based on jute alone. It needs to blend with other short-fiber pulp. In this context, the mulberry plant can be an ideal raw material. Therefore, in this study, morphological and chemical characteristics of mulberry plant of 8 and 12 months old were carried out. Pulping of mulberry plant was done by soda-AQ and kraft processes and papermaking properties were also evaluated. Jute fiber pulp was reinforced with the mulberry plant pulp.

2 Material and methods 2.1 Materials Mulberry plants were collected at the age of 8 and 12 months from the Sericulture Research Institute, Rajshahi, Bangladesh. After air-drying, the mulberry plant was striked on a concrete wall to remove dirty materials. After taking to the laboratory, these were chopped in a hand chopping machine to 2–3 cm in length for subsequent cooking experiments. For chemical analysis, the mulberry plant was ground in Willey Mill and screened to 40/60 mesh. 2.2 Chemical analysis The chemical compositions were carried out according to TAPPI Test Methods (Tappi 2003–4) [7]: extractive (T204 om88), water solubility (T207 cm99), Klason lignin (T211 om83). Holocellulose sample was prepared by treating extractive free wood meal with NaClO2 solution [8]. The pH of the solution was maintained at 4 by adding CH 3 COOHCH3COONa buffer, and α-cellulose was determined by treating holocellulose with 17.5 % NaOH (T203 om 93). 2.3 Pulping Pulping of mulberry plant of 12 months old was carried out by the soda-anthraquinone (AQ) and kraft processes in an electrically heated oil bath containing four bombs of 1.5-L capacity [9]. The bombs were rotated at 1 rpm. In the sodaAQ process, alkali charge was varied from 14, 16, 18, and 20 % on od raw materials in 2 and 3 h of cooking. In the kraft process, cooking time was fixed for 3 h. For the evaluation of papermaking properties, the mulberry plant pulp of almost identical kappa number was cooked in 20-L capacity digester [4]. The following parameters were kept constant: – – – –

Anthraquinone charge 0.1 % on o.d raw material Fiber to liquor ratio 1:5 g/mL. Temperature 170 °C. Sulphidity 25 % (kraft process)

2.4 Evaluation of pulps Mulberry plant pulps were beaten in a Valley bleater at different times. The handsheets of about 60 g/m2 were made in a Rapid Kothen Sheet Making Machine. The sheets were tested for tensile, burst, and tear strength according to TAPPI Standard Test Methods [7]. 2.5 Pulp reinforcing The different proportion of jute pulp previously prepared with soda-AQ process was mixed with mulberry plant pulp and disintegrated for 30,000 revolutions in a standard laboratory British disintegrator. Handsheets were prepared and papermaking properties were determined as above.

3 Results and discussion 3.1 Chemical characteristics Studies on the chemical composition of the mulberry plant are important for evaluating as a pulping raw material. A complete chemical analysis was performed to evaluate the differences in its chemical composition in relation to plant age. Table 1 shows the chemical characteristics of 8- and 12month-old mulberry plant and compared the results with 6month-old dhaincha. The holocellulose and α-cellulose content are similar in both aged mulberry plant. Both acid soluble and Klason lignin content are higher in the 8-month-old mulberry plant than the 12-month-old plant. Similar trends are observed for dhaincha [10]. As shown in Table 1, the pentosan and lignin content in the mulberry plant are very similar to 6-month-old dhaincha. The cellulose, pentosan, and lignin content in the mulberry plant are also similar to hardwood [11]. The α-cellulose content is related with higher pulp Table 1 Chemical and morphological characteristics of mulberry plant 8 months

12 months

Dhaincha (8 months)

Holocellulose (%) α-Cellulose (%)

72.2±1.4 42.0±1.0

72.3±1.3 42.8±1.1

72.8 37.9

Klason lignin (%) Acid soluble lignin (%) Pentosan (%) 1 % Alkali solubility (%) Extractives (acetone) (%) Ash (%) Density (g/cm3) Fiber length, mm Fiber width, μm

23.0±0.4 3.0±0.06 18.8±0.3 23.1±0.4 2.57±0.02 2.0±0.03 0.39±0.01 0.65±0.01 22±0.4

21.3±0.3 2.0±0.04 18.3±0.3 21.7±0.3 2.37±0.02 2.01±0.02 0.41±0.01 0.73±0.02 21±0.5

21.9 – 19.7 35.2 1.6 – – 1.34 18.3

Biomass Conv. Bioref. (2014) 4:53–58

55 60

Process

Alkali Time at Pulp yield (%) Kappa charge (%) 170 °C (h) number Screened Reject Total

Soda-AQ 14

2

1.7

70.0

71.7

46.6

16 18 20 14 16 18 20 14 16 18 20

2 2 2 3 3 3 3 3 3 3 3

3.3 13.0 24.1 30.8 36.7 41.3 38.4 40.4 41.3 41.8 40.3

63.0 46.0 24.0 26.2 15.1 4.2 1.6 10.9 5.4 1.9 1.1

66.3 59.0 48.1 57.0 51.8 45.5 40.0 51.3 46.7 43.7 41.4

42.2 39.2 32.5 38.0 32.1 26.2 23.8 32.1 24.7 23.8 22.5

Kraft

yield, and obviously, α-cellulose content in mulberry plant is higher than common nonwood-like straw [12], dhaincha [11], and bagasse [13]. Acetone extracts of the mulberry plant is about 2.5 %, which is lower than agricultural residues [14]. Some substances including resins, wax, fat, and ethanol–benzene extracts can precipitate upon pulping and leave stains in the resulting paper sheets. Therefore, low extracts are expected in the raw material. Acetone extract at the 8month-old mulberry plants was slightly higher than that in the 12-month-old mulberry plant. One percent alkali solubility in the 12-month-old mulberry plant is 21.7 % only, while in the 8-month-old plant is 23.1 %. The mulberry plant can be expected to provide a medium pulp yield from its 1 % NaOH solubility data. Such content is lower than those of the agricultural residues [15], and also than those of pine and eucalyptus.

Total pulp yield (%)

Table 2 Effect of chemical charge and cooking time on the mulberry plant pulping

55 50 45

Soda-AQ Kraft

40 35 30 20

25

30

35

40

Kappa number

Fig. 2 Delignification selectivity of kraft and soda-AQ processes of mulberry plant

The ash content in mulberry plant is 2 %. Plant age had no effect on the ash content. High ash content can causes problem in chemical recovery. In the assessment of raw material quality for pulping, density is one of the most important parameters. High densities are advantageous for the better use of digester capacity. The wood density of 8-month-old mulberry plant was slightly lower than that of 12 months old (0.39 vs. 0.41 g/cm3). It was observed that wood density increased with the tree age [16]. The wood density of mulberry plant is slightly better than that of Trema orientalis, where density of stem was 0.368 g/cm3 and branch was 0.330 g/cm3 [16]. 3.2 Morphological characteristics Fiber length is recognized as an important parameter for pulp and paper properties. Fiber length influences paper strength, particularly, tear, and paper machine runnability [17, 18]. Table 1 also shows the fiber length of the mulberry plant. The average fiber length was slightly increased from 0.65 to 0.73 mm with the increase of tree age from 8 to 12 months. It was similar to shorter range of tropical hardwoods (0.7– 1.5 mm) considered as short fiber [18]. 60

60

TPY, TPY, R (%)

40 30

SY

20

R TPY

10

Tensile index (N.m/g)

55

50

50 45 40 35

Soda-AQ

30

Kraft

25

0

14

16 18 Alkali charge (%)

20

Fig. 1 Effect of alkali charge on total pulp yield, screened pulp yield and reject of mulberry plant in soda-AQ process (SY screened yield, TPY total pulp yield, R reject)

20 20

30

40

50

60

70

80

Drainage resistance (0SR)

Fig. 3 Tensile strength development of kraft and soda-AQ pulps from mulberry plant

Burst index (kPa.m2/g)

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Biomass Conv. Bioref. (2014) 4:53–58 5

Table 3 Comparison of physical properties of mulberry plant pulp with mixed hardwood pulp

4

Pulp

o

Tensile index N.m/g

Burst index kPa.m2/g

Tear index mN.m2/g

3

Mulberry plant Mixed hardwood*

42 42

45.0 50.1

4.1 4.7

8.7 9.3

Soda-AQ

SR

*Active alkali, 18 %, sulphidity 25 %, Time 2 h at 170 °C

2 Kraft

1 20

30

40

50

60

70

80

Drainage resistance (0SR)

Fig. 4 Burst strength development of kraft and soda-AQ pulps from mulberry plant

3.3 Pulping The effects of alkali charge and time on kappa and yield of the pulps are shown in Table 2. It can be seen that both alkali charge and time had a significant influence on the kappa number reduction. Thus, either an increase in alkali charge at a constant time or on the other hand, an increase in time at a constant alkali charge, resulted in a clear reduction in kappa number in the soda-AQ process. In the kraft process, kappa number was decreased from 32 to 22 with the increase of alkali charge from 14 to 20 % at 3 h of cooking at 170 °C, which is lower than the soda-AQ process. In the soda-AQ process, screened pulp yield was increased and reject and total pulp yield were decreased with increasing alkali charge and cooking time. Figure 1 shows that the screened pulp yield was decreased after 18 % alkali charge for 3 h of cooking. This point was considered for subsequent papermaking properties determination. At the 20 % alkali charge for 3 h of cooking, pulp yield was 40.0 %, but still 1.6 % rejects. At the 20 % alkali charge for 3 h of cooking,

Tear index (kPa.m2/g)

10

9

kraft process produced 1.4 % higher screened pulp yield and lower rejects. The chemical composition of the raw material is probably the primary variable in pulp yield. The higher the polysaccharide content (especially cellulose) and the lower the amounts of lignin and extractives, the higher the yield of pulp from wood. Aspen is a leading example—with lignin content often below 20 % and (acetone) extractives below 3 %, it cooks rapidly to the highest bleachable-grade kraft pulp yield in industrial practice, typically about 55 % at kappa 12. Western red cedar, with an unusually high extractive content, is at the low end of the spectrum, providing a bleachable-grade pulp yield in the low 40s at kappa 30 [19]. But the expected pulp yield from mulberry plant was slightly higher based on α-cellulose content (Table 1). The physical nature of wood also plays an important role in the yield. Large differences exist among wood species, especially in the percentage of “fibers” (the preferred cell type for papermaking) versus that of less desirable cells (e.g., ray parenchyma in softwoods, vessel elements in hardwoods). As shown in Fig. 2, the kraft process showed better delignification selectivity than the soda-AQ process with in the kappa number range 22–30. At kappa number 25, the pulp yield was 1.7 % higher in the kraft process than the soda-AQ process. Usually, the soda-AQ process is suitable for the nonwood pulping because of relatively low-average molecular weight of lignin [20]. But in this study, better pulping selectivity is observed in the kraft process. This may be explained by the morphological properties and topochemistry of the mulberry plant. Saka et al. [21] found that the addition of AQ and sodium sulfide resulted in transition from a slow initial to a rapid bulk delignification, particularly in the middle lamella, and in an enhanced bulk delignification in the

8

Table 4 Reinforcing potential of jute pulp with mulberry plant pulp 7

J:M

°SR

Tensile index N.m/g

Tear index mN.m2/g

Burst index kPa.m2/g

0:100 25:75 50:50 75:25 100:0

62 58 48 31 29

53.3 56.4 62.7 51.3 50.5

8.7 11.4 17.0 18.1 20.3

3.9 4.5 4.7 4.0 4.1

Soda-AQ 6

5 10

Kraft

20

30

40

50

60

70

80

Drainage resistance (0SR)

Fig. 5 Tear strength development of kraft and soda-AQ pulps from mulberry plant

Biomass Conv. Bioref. (2014) 4:53–58

secondary wall. But sodium sulfide addition improved the bulk delignification in the middle lamella significantly. 3.4 Papermaking properties The papermaking properties of mulberry plant pulps from soda-AQ and kraft processes are shown in Figs. 3, 4 and 5. The tensile and burst indexes of soda-AQ and kraft process are almost similar at °SR value 50, but at the higher °SR value, kraft pulp showed higher strength than soda-AQ pulp. The tear index of kraft pulp was higher than the kraft pulp at any ° SR value as shown in Fig. 5. At °SR 40, kraft pulp showed 13 % higher tear index than the soda-AQ process. Table 3 shows the comparison of physical properties of mulberry plant pulp from kraft process, which is used in the Kharnaphuli Paper Mill in Bangladesh with the mixed hardwood pulp [22]. It is seen that all properties of the mulberry plant pulp is slightly inferior to the mixed hardwood pulp. 3.5 Pulp reinforcing Table 4 shows the handsheet properties of pulp blends in longer jute fiber pulp and shorter mulberry plant pulp. Fiber length of jute fiber is about 2.5 mm [23]; therefore, reinforcement of jute pulp can enhance the tear strength of the mulberry plant pulp. It was also observed that jute fiber pulp from the soda-AQ process showed better pulp yield and papermaking properties than the pulp from the kraft process [24]. Therefore, soda-AQ pulp was chosen for reinforcement with mulberry plant pulp. It is important to understand how pulp properties change when jute pulp is mixed with mulberry plant pulp especially for packaging and kraft liner paper in Bangladeshi prospective. The initial drainage resistance of the mulberry pulp was 62, while the drainage resistance (°SR) of jute pulp was 29 only. Our main goal of blending jute pulp with mulberry plant was to improve the tear index of the mulberry plant pulp and also to reduce the cost of jute pulp by using cheap pulp-like mulberry plant pulp. Therefore, we used lower drainage resistance (°SR) jute pulp. The addition of jute pulp to the mulberry plant pulp showed a significant increase in tear index. Blending of jute pulp with mulberry plant pulp up to 50 % increased tensile, tear, and burst indexes significantly. Further increase of jute pulp reduced tensile and burst indexes. At the addition of 25 % jute to mulberry plant pulp tear index increased by about 31 %. This was expected since the longer pulp showed higher tear value. Reinforcing the potential of softwood pulp with different Eucalyptus was studied by Mansfield and Kibblewhite [25] and showed that influence of softwood fiber quality decreased with increasing Eucalyptus fiber. But 20 % eucalyptus fiber reinforcement did not change the tear-tensile strength of softwood fiber.

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4 Conclusions The plant age of mulberry reduced lignin content and extractives. It is characterized with moderate lignin and cellulose. Soda-AQ and kraft pulping were evaluated. Kraft process produced higher pulp yield than soda-AQ process at the kappa number range 22 to 30. Soda-AQ and kraft pulp showed almost similar tensile and burst indexes, while kraft pulp showed better tear index. Reinforcing of jute pulp with mulberry plant pulp up to 50 % increased tensile, tear, and burst indexes significantly. Finally, it can be concluded that mulberry plant pulp can be produced by kraft process and can be blended with jute fiber pulp up to 50 %, which can be used as kraft liner.

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