Sudanese agro-residue as a novel furnish for pulp and paper manufacturing

Abstract

Sudan has rich sources of lignocellulose materials from agricultural waste that have potential to be used as a papermaking furnish following adequate chemical compositions, elemental analysis, fibre dimensions, and morphology of millet stalks and date palm leaves. Paper sheet properties from the various pulps made were investigated, and it was found that there was no difference in the polysaccharide (cellulose and hemicelluloses) content between millet stalks and date palm leaves, although millet stalks had a high lignin content of 18.20% relative to date palm leaves’ content of 15.34%. Moreover, millet stalks showed a high pulp yield (42.04%) with a viscosity of 665 mL/g compared to that (34.43%, 551 mL/g) and (38.50% and 534 mL/g) of date palm leaves and the blend, respectively. Papers produced from date palm leaves and millet stalk blends showed better physical properties compared to that of pure millet stalks and date palm leaves. The Scanning Electron Microscopy (SEM) analysis showed that fibres in the blend were more closely packed than that of the pure millet stalks and date palm leaves fibers. Based on their physical and chemical composition properties, millet stalks and date palm leaves have a high potential as a furnish for pulp and papermaking.

Full Article

Sudanese Agro-residue as a Novel Furnish for Pulp and Paper Manufacturing

Haroon A. M. Saeed,^a,b,* Yu Liu,^a,* Lucian A. Lucia,^a,c and Honglei Chen ^a

Sudan has rich sources of lignocellulose materials from agricultural waste that have potential to be used as a papermaking furnish following adequate chemical compositions, elemental analysis, fibre dimensions, and morphology of millet stalks and date palm leaves. Paper sheet properties from the various pulps made were investigated, and it was found that there was no difference in the polysaccharide (cellulose and hemicelluloses) content between millet stalks and date palm leaves, although millet stalks had a high lignin content of 18.20% relative to date palm leaves’ content of 15.34%. Moreover, millet stalks showed a high pulp yield (42.04%) with a viscosity of 665 mL/g compared to that (34.43%, 551 mL/g) and (38.50% and 534 mL/g) of date palm leaves and the blend, respectively. Papers produced from date palm leaves and millet stalk blends showed better physical properties compared to that of pure millet stalks and date palm leaves. The Scanning Electron Microscopy (SEM) analysis showed that fibres in the blend were more closely packed than that of the pure millet stalks and date palm leaves fibers. Based on their physical and chemical composition properties, millet stalks and date palm leaves have a high potential as a furnish for pulp and papermaking.

Key words: Sudanese agro-residue; Millet stalks; Date palm leaves; Cellulose; Pulp and paper

Contact information: a: Key Lab of Pulp and Paper Science and Technology of Ministry of Education (Shandong Province) Qilu University of Technology, Jinan, 250353, Shandong, P.R. China; b: Center of Fibers, Papers and Recycling, Faculty of Textiles, University of Gezira, Box 20, Wad Medani, Sudan; c: Department of Forest Biomaterials, North Carolina State University, Box 8005, Raleigh, NC 27695-8005, USA; ٭ Corresponding author: haroonsaeed75@gmail.com, leoliuyu@163.com

INTRODUCTION

An increase in pulp and paperboard consumption, stricter environmental and sustainability regulations, and the increased use of wood materials for furniture production have prompted scientists and researchers to seek additional lignocellulosic material for pulp and papermaking (Danielewicz and Surma-Slusarska 2011; Xing et al.2016). Globally, non-wood cellulosic materials are a major part of raw material inventories for pulping and paper. Sudan is rich in non-wood cellulose materials, such as bagasse, cotton linters, sorghum, sunflower, millet and sesame stalks, and date palm rachis and leaves, which can be used in pulp and paper (Elzaki et al. 2012).

The use of agricultural residues in pulp and papermaking has many benefits for farmers and the environment, such as reducing the need for waste disposal, which currently increases farming costs and sustainability by reducing environmental pollution, fires, and pests (Hammett et al. 2001; Ashori 2006). Compared with other classical pulping processes, soda pulping is the most economical, efficient, and simple for non-woody feedstock. Soda-anthraquinone pulping has been applied to various agricultural residue non-wood materials, such as bagasse (Samariha and Khakifirooz 2011; Sánchez et al. 2016), banana (Rosal et al. 2012), and cotton stalks (Khider et al. 2012), but no work on Sudanese millet stalks and date palm leaves has been reported to date.

Pearl millet (Pennisetum glaucum) known as “Dukhun,” is the most important cereal crop in Sudan (Kordofan and Darfur States). The average total area annually cultivated is approximately 2.5 million ha. However, millet stalks are used as feed for animals and mostly used as building material or fuel (Abuelgasim 2011). Their application in pulp and paper manufacturing can be more beneficial because they are abundant, inexpensive, and can provide economic and environmental benefits (Elzaki et al. 2012; Dulermo et al. 2016). Harinarayana et al. (2005) found that the millet stalks were rich in cellulose content (39.4%), hemicellulose (23.9%), and relatively low in lignin (12.8%), thus representing a promising feedstock.

Date palm (Phoenix dactylifera) is common in Northern Sudan along the Nile (El Amin 1990). Khristova et al. (2005) investigated alkaline pulping with additives of date palm rachis and leaves from Sudan. Date palm rachis gave the best yields and displayed the best mechanical properties. The leaves were best pulped with soda at a low yield with very good strength properties. Nevertheless, date palm is a promising material for the paper industry. Khiari et al. (2010)studied the Tunisian date palm rachis as an alternative source of fibres for papermaking applications. It was found that the physical properties of the prepared handsheets were like those displayed by other papers made of common lignocellulosic fibres. Moreover, the pulps displayed good drainability together with excellent mechanical properties. No similar previous study was conducted, and only a few studies have evaluated the potential use of Sudanese date palm leaves for pulp and paper manufacturing (Khristova et al. 2005).

The authors herein study millet stalks and date palm leaves to create a novel furnish for pulp and paper manufacture with resources available in Sudan.

EXPERIMENTAL

Materials

Millet stalks (Pennisetum glaucum) were collected from a farmer in north Kordofan state, Sudan, January 2016. They were harvested from a tropical and subtropical area of low soil fertility and limited moisture where rainfall ranges from 200 mm to more than 1000 mm. The date palm leaves (Phoenix dactylifera) were collected from 7 to 9-year-old trees from Khartoum state, Sudan, January 2016. The air-dried samples were cut 3 to 5 cm in size and a part of each individual sample was ground in a star mill. The 40-mesh to 60-mesh fraction was analyzed according to TAPPI standards. Hydrogen peroxide (H₂O₂), sodium hydroxide (NaOH), and anthraquinone were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China).

Methods

Chemical composition

Holocellulose was determined by Wise’s chloride method (Wise and Jahn 1952), and cellulose (Rowell 2005), as well as lignin and ash, were determined per TAPPI T222 om-06 (1996) and TAPPI T211 om-93(1993), respectively.

Pulping processing and testing

The pulping process of millet stalks, date palm leave, and their blend (50% of each raw material) was performed in a 10-L capacity rotating autoclave according to Khiari et al.(2010) with a total alkali solution with 20% (w/w with respect to oven dried (o.d). material) expressed in NaOH, in the presence of an anthraquinone concentration of 0.1% (w/w with respect to o.d. material) at a constant temperature for 150 min. The liquor to solid ratio was 5:1 and the cooking temperature was 160 °C. After cooking, the pulp was washed, disintegrated in a laboratory disintegrator, and then screened on a 0.15-mm laboratory slot screen. The pulp yield was calculated and the kappa number and freeness of pulp were determined according to TAPPI T236 om-13 (2013) and TAPPI T227 om-99 (1999), respectively, while the pulp viscosity was determined according to TAPPI T230 om-08 (2008). Pulp bleaching was performed in two stages for 2 h at a 10% pulp concentration by hydrogen peroxide (H₂O₂) at 4% in sodium hydroxide (NaOH) at 80 °C.

Papers making and testing

Sets of hand-sheets paper (60 g/m²) were made from the pulp of the samples using a laboratory hand-sheet former (PTI laboratory Equipment, Vorchdorf, Austria) according to the TAPPIT 205 sp-95 (1995) standard method. All of the hand-sheets tests were based on the following standards: thickness, TAPPI T411 om-97 (1997); bulk, TAPPI T500 cm-98 (1998); tensile resistance, TAPPI T494 om-96 (1996); burst resistance, TAPPI T403 om-97 (1997); tear resistance, TAPPI T496 sp-99 (1999); opacity TAPPI T1214 sp-98 (1998); and brightness TAPPI T1216 sp-98 (1998). The reported results represent the average values of 5 tested hand-sheets.

Papers surface morphology

The fiber morphological properties were observed under a scanning electron microscope (SEM) (OCTANE 9.88/1114658 AMETEK^®, Mahwah, USA). Images were taken under several magnifications to observe the content, arrangement, and compactness of the fibers. Furthermore, C, O, N, Si, Ca, Mg, and Al were also identified by the same machine. A suspension of materials pulp was used for detailed anatomical features including fibre length, fibre width, curl index, and kink index, by using a Fiber Quality analyzer (LDA02128, OpTest Equipment Inc., Ontario, Canada).

RESULTS AND DISCUSSION

Chemical Composition

Table 1 shows the chemical properties of millet stalks and date palm leaves. The holocellulose content of millet stalks and date palm leaves was 61.89% and 61.63%, respectively. Their cellulose content was 40.99% and 39.00%, respectively, which is suitable for pulp and paper manufacturing (close to or above 40%) (Samariha and Khakifirooz 2011). However, millet stalks had a similar content of 39.4%, and date palm leaves have 39.37% as observed by Harinarayana et al. (2005) and Nasser et al. (2016), respectively. Compared to other non-woods, millet stalks and date palm leaves were higher than that of sorghum 35.87% (Cardoso et al. 2013), corncob residues 38.8% (Liu et al. 2010), rice husk 36%, and cotton stalk 38% (Singh and Chouhan 2014). Hemicellulose content from millet stalks and date palm leaves ranged from 20% to 22%. However, higher hemicellulose values resulted in higher paper strength (especially tensile, burst, and fold). The pulp yield may have a negative effect during pulping because hemicellulose is the cell wall polymer component with the highest water sorption (Syed et al.2016). The Klason lignin of millet stalks and date palm leaves was 18.20% and 15.34%, respectively, which was lower than that of bagasse at 20.35% (Samariha and Khakifirooz 2011), rice husk at 23%, cotton stalk at 20.88%, bagasse at 19%, and sesame at 23.3% (Singh and Chouhan 2014). Lignocellulose material with low lignin content needs a lower temperature and high pulping chemical charges, as well as bleaching to achieve a satisfactory kappa number. When more lignin can be removed from pulp, the paper that will be made is of higher quality compared to paper made from other lignocellulosic materials. The ash content of date palm leaves was low, 2.06%, while the ash content for millet stalks was high, 5.96%. The high ash contents, however, were unfavorable for pulp and papermaking, because they increase alkali consumption, incur recovery problems for cooking chemicals (evaporation, combustion, and lime mud reburning), and result in operational problems for further pulping (Agnihotri et al. 2010). Moreover, millet stalks showed relatively high hot and cold water extractives 13.00% and 5.20% respectively, compared to that 12.12% and 3.02% of date palm leaves. This is evidence of solubility of carbohydrates as well as lignin and dyes. The solubility in 1% NaOH was 15.30% and 12.20% of millet stalks and date palm leaves reflected the degradation of the cell wall material by weak alkali in pulping and bleaching process.

Table 1. Chemical Composition of Millet Stalks and Date Palm Leaves

Elemental Analysis

Low mineral content in the lignocellulose materials is desirable in pulp and paper production (Plazonić et al. 2016). In Table 2, the millet stalks had elemental proportions of carbon (C) 45.21%, oxygen (O) 20.90%, sodium (Na) 2.09%, calcium (Ca) 0.50%, silica (Si) 0.48%, magnesium (Mg) 0.38%, and aluminium (Al) 0.17%, respectively. The composition of date palm leaves was oxygen (O) 52.98%, carbon (C) 43.29%, sodium (Na) 1.86%, silicon (Si) 0.37%, aluminium (Al) 0.30%, and magnesium (Mg) 0.26. The millet stalks and date palm leaves contained relatively little silica, which should not therefore induce issues in the chemical recovery. Moreover, elemental analyses showed that the minerals components, which represent the inorganic constituents, were relatively low. However, this lower silica content is expected to reduce the amounts of alkali required during pulping and bleaching processing.

Table 2. Elemental Analysis

Pulp Yield, Kappa Number, and CSF

In Table 3, the pulp yield of millet stalks, date palm leaves, and their 50% blend was 42.04%, 34.43%, and 38.50% with kappa numbers of 12.43%, 13.50%, and 11.96%, respectively. Considering the pulping yields and kappa numbers, it could be recommended that the simple soda process be chosen for the pulping of millet stalks/date palm leaves blend. Moreover, the blend of millet stalks and the date palm leaves produced better quality pulp as indicated by a lower kappa number. However, the combination of high yield with good papermaking properties suggested that date palm leaves could be used in blends with other cellulosic material, or pulped together with the rachis to give bleachable pulp grades (Khristova et al. 2005). Millet stalks, date palm leaves, and their blend showed viscosities of 665 mL/g, 551 mL/g, and 534 mL/g, respectively (Fig.1). Moreover, date palm leaves obtained a higher freeness value at CSF 720 mL compared to that of the millet stalks and the blend, 555 mL and 600 mL, respectively.

Fig. 1. Pulp yields and viscosities of millet stalks, date palm leaves, and their 50% blend

Table 3. Pulp Properties

Fibre Properties

The fibre length is considered an important parameter for pulp and paper properties because it has a significant impact on the paper mechanical properties (Jahan et al. 2010). As shown in Table 3, the average fibre length of millet stalks and date palm leaves was 0.41 mm and 0.51 mm, respectively. These results were in the range of those associated with common agriculture plants such as date palm rachis (Khiari et al. 2010). Moreover, these materials showed fibre thicknesses similar for millet stalks and date palm leaves, 27.10 mm and 28.80 mm, respectively. Fibre curl strongly affects the tensile strength and bonding ability of fibres within a network (Robertsén and Joutsimo 2005). However, millet stalks showed higher fibre curl and kink indices (0.12 mm and 1.67 mm) compared to that of date palm leaves (0.09 mm and 1.33 mm). Fibre curl affected the tensile index such that a sheet formed from such fibres would have a low tensile index, but may have a high tear strength. This has been explained by the uneven distribution of stress along the length of a curled fibre in a fracture zone (Robertsén and Joutsimo 2005).

Table 3. Fibre Properties

Paper Properties

The physical strength properties of handsheets from millet stalks, date palm leaves, and their 50% blend are shown in Table 4 and Fig. 2. The paper made from the 50% blend of millet stalks and date palm leaves had better physical properties, such as tensile strength 45.23 N m/g, tearing index 11.04 mNm²/g, bursting index 2.34 KPa m²/g, and a folding endurance of 16 ( log₁₀ n). For date palm leaves and millet stalks, the values were for tensile strength (39.55 N m/g, 33.95 N m/g), tearing index (6.24 mNm²/g, 2.31 mNm²/g), bursting index (1.64 KPa m²/g, 1.96 KPa m²/g), and folding endurance (11, 5 (log₁₀n)), respectively. However, this data showed that millet stalks and date palm leaves were promising for papermaking applications. Date palm leaves showed the best bleachability and reached the highest brightness of 70.50%, while it was 66.05% and 63.60% of millet stalks and 50% blend, respectively. Moreover, the highest opacity was 88.90% and was achieved by the 50% blend, while it was 88.50% and 86.00% of millet stalks and date palm leaves, respectively. However, opacity is very important for printing and writing papers, while for tracing paper, lampshades, and some packing papers, brightness is considered very important (Tajik et al. 2016).

Table 4. Paper Properties

Fig. 2. Tensile strength and tearing index of the materials

Paper Morphology

The strength of the fiber matrix could be extrapolated based on its packing of the fibers. In Fig. 3, the millet stalks, date palm leaves, and 50% blend handsheets were magnified at 100 µm. (Fig. 3(A)) showed that the millet stalk fibers were more closely compacted compared to the date palm leaves (Fig. 3(B)). SEM images showed that the produced papers from the millet stalks were quite homogeneous and compact compared to that produced from date palm leaves; however, this will lead to smooth surface and good structure on produced paper. Alternatively, the blend handsheet in Fig. 3(C) showed a more closely packed arrangement than the millet stalks and date palm leaves fibers. Inevitably, the highly dense arrangement and compact packing imparted better mechanical properties and quality (Daud et al. 2013).

Fig. 3. Scanning electron microscope images of millet stalks (A), date palm leaves (B), and 50% blend (C)

CONCLUSIONS

1. The relatively high cellulose contents (40.99% and 39.00%) and low lignin contents (18.20% and 15.34%) of millet stalks and date palm leaves, respectively, led to high-quality pulp and paper.
2. The millet stalks and date palm leaves blend showed high mechanical properties (tensile strength, tear index, and fold test) similar to wood materials when compared to pure millet stalks and date palm leaves.
3. The SEM analysis showed a condensed arrangement of fibres that led to a stronger structure in the date palm leaves and the blend than in the millet stalks.
4. The papers produced from millet stalks, date palm leaves and 50% blend, could be used for writing and printing paper as well as in packaging applications.

ACKNOWLEDGMENTS

The authors are grateful for the financial support from the National Science Foundation of China (Grant Nos. 31470602, 31600472, and 31570566,), the Foundation for Outstanding Young Scientist in Shandong Province (BS2015SW011), the Project of Shandong Province Higher Education Science and Technology Program (No. J13LD03), the Foundation of Key Laboratory of Pulp and Paper Science and Technology of Ministry of Education (Shandong Province) of China.

REFERENCES CITED

Abuelgasim, E. H. (2011). “Pearl millet, pennisetum glaucum, production and improvement in the Sudan,” Sudanese Encyclopedia of Agricultural Sciences, (http://www.erails.net/SD/nectarforum/senassudan/research-and-studies/pearl-millet-pennisetum-glaucum-production-and-improvement-in-the-sudan/), Accessed February 2017.

Agnihotri, S., Dutt, D., and Tyagi, C. (2010). “Complete characterization of bagasse of early species of Saccharum officinerum-Co 89003 for pulp and paper making,” BioResources5(2), 1197-1214.

Ashori, A. (2006). “Nonwood fibers – A potential source of raw material in papermaking,” Polymer-Plastics Technology and Engineering 45(10), 1133-1136. DOI: 10.1080/03602550600728976

Cardoso, W. S., Tardin, F. D., Tavares, G. P., Queiroz, P. V., Mota, S. S., Kasuya, M. C. M., and Queiroz, J. H. d. (2013). “Use of sorghum straw (Sorghum bicolor) for second generation ethanol production: Pretreatment and enzymatic hydrolysis,” Química Nova 36(5), 623-627. DOI: 10.1590/S0100-40422013000500002

Danielewicz, D., and Surma-Slusarska, B. (2011). “Pulping and bleaching OCC. Part I: Delignification,” Appita Journal: Journal of the Technical Association of the Australian and New Zealand Pulp and Paper Industry 64(1), 62.

Daud, Z., Hatta, M. Z. M., Kassim, A. S. M., Awang, H., and Aripin, A. M. (2013). “Exploring of agro waste (pineapple leaf, corn stalk, and napier grass) by chemical composition and morphological study,” BioResources 9(1), 872-880.

Dulermo, T., Coze, F., Virolle, M. -J., Méchin, V., Baumberger, S., and Froissard, M. (2016). “Bioconversion of agricultural lignocellulosic residues into branched-chain fatty acids using Streptomyces lividans,” Oilseeds and Fats, Crops and Lipids 23(2), 1-8. DOI: 10.1051/ocl/2015052

El Amin, H. M. (1990). Trees and Shrubs of the Sudan, Ithaca Press, University of Minnesota, Minneapolis, MN, USA.

Elzaki, O. T., Khider, T. O., and Omer, S. H. (2012). “Pulp and papermaking characteristics of Cajanus cajan stems from Sudan,” American-Eurasian Journal of Agricultural & Environmental Sciences 12(2), 159-163.

Hammett, A., Youngs, R. L., Sun, X., and Chandra, M. (2001). “Non-wood fiber as an alternative to wood fiber in China’s pulp and paper industry,” Holzforschung 55(2), 219-224. DOI: 10.1515/HF.2001.036

Harinarayana, G., Melkania, N., Reddy, B., Gupta, S., Rai, K. N., and Kumar, P. S. (2005). “Forage potential of sorghum and pearl millet,”in: Biofortified Crops for Human Nutrition, West Lafayette, IN, pp. 292-321.

Jahan, M. S., Chowdhury, N., and Ni, Y. (2010). “Effect of different locations on the morphological, chemical, pulping and papermaking properties of Trema orientalis Nalita),” Bioresource Technology101(6), 1892-1898. DOI: 10.1016/j.biortech.2009.10.024

Khiari, R., Mauret, E., Belgacem, M. N., and M’henni, M. F. (2010). “Tunisian date palm rachis used as an alternative source of fibres for papermaking applications,” BioResources 6(1), 265-281.

Khider, T. O., Omer, S., Taha, O., and Shomeina, S. K. (2012). “Suitability of Sudanese cotton stalks for alkaline pulping with additives,” Iranica Jounal of Energy & Environment 3(2), 167-172. DOI: 10.5829/idosi.ijee.2012.03.02.0228

Khristova, P., Kordsachia, O., and Khider, T. (2005). “Alkaline pulping with additives of date palm rachis and leaves from Sudan,” Bioresource Technology 96(1), 79-85. DOI: 10.1016/j.biortech.2003.05.005

Liu, K., Lin, X., Yue, J., Li, X., Fang, X., Zhu, M., Lin, J., Qu, Y., and Xiao, L. (2010). “High concentration ethanol production from corncob residues by fed-batch strategy,” Bioresource Technology101(13), 4952-4958. DOI: 10.1016/j.biortech.2009.11.013

Nasser, R. A., Salem, M. Z., Hiziroglu, S., Al-Mefarrej, H. A., Mohareb, A. S., Alam, M., and Aref, I. M. (2016). “Chemical analysis of different parts of date palm (Phoenix dactylifera L.) using ultimate, proximate and thermo-gravimetric techniques for energy production,” Energies 9(5), 374. DOI: 10.3390/en9050374

Plazonić, I., Barbarić-Mikočević, Ž., and Antonović, A. (2016). “Chemical composition of straw as an alternative material to wood raw material in fibre isolation,” Drvna Industrija: Znanstveno-stručni časopis za Pitanja Drvne Tehnologije 67(2), 119-125.

Robertsén, L., and Joutsimo, O. (2005). “The effect of mechanical treatment on kraft pulps produced from different softwood raw materials,” Paperi ja Puu [Paper and Timber] 87(2), 111-115.

Rosal, A., Rodríguez, A., González, Z., and Jiménez, L. (2012). “Use of banana tree residues as pulp for paper and combustible,” International Journal of Physical Sciences 7(15), 2406-2413. DOI: 10.5897/IJPS11.1661

Samariha, A., and Khakifirooz, A. (2011). “Application of NSSC pulping to sugarcane bagasse,” BioResources 6(3), 3313-3323.

Sánchez, J. H., Fajardo, M. E., and Quintana, G. C. (2016). “Viscoelastic properties of pulp suspensions of bleached sugarcane bagasse: Effects of consistency and temperature,” BioResources11(4), 8355-8363.

Singh, S., and Chouhan, A. S. (2014). “Experimental studies on enhancement of bio-oil production using agro waste materials pre-treated with alkaline solutions,” African Journal of Basic & Applied Sciences 6(1), 19-24.

Syed, N. F. N., Zakaria, M. H., and Bujang, J. S. (2016). “Fiber characteristics and papermaking of seagrass using hand-beaten and blended pulp,” BioResources 11(2), 5358-5380.

Tajik, M., Resalati, H., Hamzeh, Y., Torshizi, H. J., Kermanian, H., and Kord, B. (2016). “Improving the properties of soda bagasse pulp by using cellulose nanofibers in the presence of cationic polyacrylamide,” BioResources 11(4), 9126-9141.

TAPPI T205 sp-95 (1995). “Forming handsheets for physical tests of pulp,” TAPPI Press, Atlanta, GA, USA.

TAPPI T211 om-93 (1993). “Ash in wood, pulp, paper and paperboard: Combustion at 525 °C,” TAPPI Press, Atlanta, GA, USA.

TAPPI T222 om-06 (1996). “Acid-insoluble lignin in wood and pulp,” TAPPI Press, Atlanta, GA, USA.

TAPPI T227 om-99 (1999). “Freeness of pulp (Canadian standard method),” TAPPI Press, Atlanta, GA, USA.

TAPPI T230 om-08 (2008). “Viscosity of pulp (capillary viscometer method),” TAPPI Press, Atlanta, GA, USA.

TAPPI T236 om-13 (2013). “Kappa number of pulp,” TAPPI Press, Atlanta, GA, USA.

TAPPI T403 om-97 (1997). “Bursting strength of paper,” TAPPI PRESS, Atlanta, GA, USA.

TAPPI T411 om-97 (1997). “Thickness (caliper) of paper, paperboard, and combined board,” TAPPI Press, Atlanta, GA, USA.

TAPPI T414 om-99 (1999). “Internal tearing resistance of paper (Elmendorf-type method),” TAPPI PRESS, Atlanta, GA, USA.

TAPPI T425 om-98 (1998). “Diffuse opacity of paper (d/0 paper backing),” TAPPI PRESS, Atlanta, GA, USA.

TAPPI T494 om-96 (1996). “Tensile properties of paper and paperboard (using constant rate of elongation apparatus),” TAPPI Press, Atlanta, GA, USA.

TAPPI T500 cm-98 (1998). “Book bulk and bulking number of paper,” TAPPI Press, Atlanta, GA, USA.

TAPPI T1216 sp-98 (1998). “Indices for whiteness, yellowness, brightness, and luminous reflectance factor,” TAPPI Press, Atlanta, GA, USA.

Xing, L., Xu, M., and Pu, J. (2016). “The properties and application of an ultrasonic wheat straw pulp having enhanced tendency for ash formation,” BioResources 12(1), 871-881.

Article submitted: February 10, 2017; Peer review completed; April 13, 2017; Revised version received and accepted: April 18, 2017; Published: April 24, 2017.

DOI: 10.15376/biores.12.2.4166-4176