Exploring the Possibilities of Producing Pulp and Paper from Discarded Lignocellulosic Fibers

ABSTRACT The main objective of this work was to explore the prospects of producing pulp and paper from leftover lignocellulosic fibers. In this study discarded Cocos nucifera fibers were collected from an abandoned site and were washed thoroughly. FTIR analysis, chemical composition and fiber morphology studies were conducted. FTIR showed the presence of holocellulose and lignin in the fibers. Chemical analysis showed the holocellulose content as 37.8 wt%. Fiber length, fiber diameter, lumen diameter and cell wall thickness were observed using microscope and derived fiber indices that determine the possibility of producing paper were evaluated. The derived indices such as Runkel index, slenderness ratio, co-efficient of rigidity, flexibility co-efficient, Luce’s shape factor and Solids factor of the fibers were evaluated as 67.9%, 44.11, 58.83%, 0.199, 0.49 and 278.53x103 respectively. All these indices are in good agreement with fibers recommended and used for pulp and paper production. High lignin content present in the fibers is a limitation and it can be removed through appropriate delignification techniques. Thus the study showed that discarded fibers can be used for producing pulp, paper and allied products.


Introduction
Paper is an indispensable source for preserving, maintaining important documents for future need since time immemorial. Literacy growth, industrialization, need for communication, population growth have accelerated the usage of paper and its allied products. Even in this present era of digitalization, the need for paper is felt almost in every industry. The global paper consumption was production, 30% for Copra production and the remaining for domestic consumption. The mesocarp of the coconut fruit has fibers that are used for a variety of purposes like rope making, floor coverings, garden articles, ply, organic manure, water absorbant, carpet, sustainable composites and handicrafts. (NagarajaGanesh and Rekha 2019a). After using coir fibers for producing useful items, the remaining fibers are discarded and left unused. These fibers were collected and the study was conducted.

Collection of materials
Cocos nucifera fibers were collected from an abandoned place in the outskirts of Madurai city, India. These fibers are light brown in color and may be the remnants left after appropriate use. They were washed many times using tap water to remove mud and unwanted dirt. Then they were washed using distilled water and sun dried for 3 days and then used for study.

FTIR
FTIR analysis of the discarded Cocos nucifera fibers was done using powdered fibers mixed with Potassium bromide (KBr) and they were pressed to form pellets. A Shimadzu (FTIR 8400S, Japan) spectroscope was used for the analysis (NagarajaGanesh & Muralikannan 2016a). The analysis was conducted in the wave number region from 4000 to 500 cm −1 with a scan rate of 32scans/min and at 2 cm −1 resolution.

Chemical composition
The fibers were initially kept in an oven maintained at 105°C for 4 hours to remove moisture content. The chemical composition such as Klason lignin and holocellulose of the moisture free fibers was determined according to D1106-96 (2007) and D1107-96 (2007) standards respectively. Initially a unit mass of the discarded Cocos nucifera fiber was placed in sulfuric acid bath maintained at 15°C. After blending, the content was transferred to beaker containing water at 20°C. Then it was washed and diluted by adding distilled water. Then it was heated for about four hours. Insoluble content was filtered and dried in an oven maintained at 105°C and weighed. The ratio of the dry weight lignin to the initial moisture free mass represents the Klason lignin content in weight%.
Sodium acetate, sodium chloride and ethanoic acid were mixed with 180 ml of distilled water and kept in a conical flask. The fiber sample of 2gms was added to this mixture and the flask was covered. The flask was kept in a fuming chamber with its contents at 60°C for four hours. Change in color of the solution and sample was noticed. The whitish sample was washed, filtered and kept in oven for four hours at 105°C. The ratio of weight of the sample to the initial dry weight represents the weight% of holocellulose. Ash content present in these fibers was found in accordance with ASTME1755-01 standards.

Determination of fiber dimensions
The fiber dimensions or the characteristics that influence the properties of paper are fiber length, fiber diameter, fiber lumen width and fiber cell wall thickness (Zobel and Buijtenen 1989). Fiber architecture plays a vital role in determining a fiber's potential applications and mechanical behavior . Twenty samples of fibers were randomly taken and cut with a steel razor. The morphological characteristics of fibers were observed under Coslab (India) light microscope. Fiber Diameter (FD), Lumen Diameter (LD) and Fiber Wall Thickness (FWT) were measured at 400x magnification while the Length of the Fiber (FL) was measured at 10x magnification as per International Association of Wood Anatomists (IAWA) standards (1989). The derived indices were calculated using the fibers dimensions observed from the microscope.

Determination of derived indices of fibers
The derived indices such as Runkel ratio, Co-efficient of flexibility, Slenderness ratio, Rigidity ratio, Luce's shape factor and Solid's factor govern the suitability of fibers for pulp production were computed from the Equations (1) to (6) (Rana et al. 2009).

FTIR
The FTIR spectrum of the discarded Cocos nucifera fibers seen in Figure 1 shows the presence of the typical hydroxyl stretching peak ascribed to the presence of cellulose at 3745 cm −1 , in addition two minor peaks at 3870 and 3610 cm −1 are also seen 2016b. These hydroxyl groups are capable of making strong hydrogen bonds which are related to the quality of paper (Laftah and Rahman 2016). Similar peaks were obtained for bamboo fibers (Guan, An, and Liu 2019). The peak at 2940 cm −1 is related to the asymmetric stretching mode of C-H. Peak at 1720 cm −1 is related to the presence of carbonyl group specifying the presence of lignin and hemicellulose. Also, the aromatic stretching vibration due to C-O confirms lignin is seen at 1590 cm −1 . The bending vibration of C -H and C -O group at 980 cm −1 corresponds to lignin. The tiny protruding peak at 618 cm −1 is associated with the glycosidic linkages between the monosaccharides present in cellulose. Thus the presence of cellulose and lignin was confirmed from the FTIR spectrum.

Chemical composition
The chemical constituents of the natural fibers, such as cellulose, hemicellulose and lignin influence the pulp production. The total content of these carbohydrate materials is termed as holocellulose and a high holocellulose content is directly related to high pulp yield and it is highly desirable (Estudillo and Mabilangan 1996). Cellulose content positively influences the strength and makes the fiber strand liable to natural and synthetic dye binding while hemicelluloses is responsible for the water absorption by plant fibers and reduces internal fiber stress (NagarajaGanesh and Rekha 2019b). From a chemical composition point of view, plant fibers that have holocellulose content greater than 33% and lignin content less than 30% are considered as promising candidates for the production of paper Nieschlag 1960). In this study, the holocellulose content present in the fibers was 37.8%, thus qualifying for the pulp and paper production. Lignin content in the fibers was found as 46.8% and its content greater than 30% produces an inter-fiber bond with holocellulose. It increases the time consumption and chemicals necessary for beating, pulping and bleaching process (Azizi et al 2010). Hence it should be removed before pulping and bleaching which is possible using delignification techniques. The ash content in Cocos nucifera was 2.22% which is lesser than ash content in Eucalyptus grandis (2.87), a fiber generally used for pulp and paper production. Ash content refers to the presence of carbonates, silicates, oxalates in the fibers and they should be removed during paper and pulp production. More amount of ash content requires more energy and processing time. In addition the dark color of lignin reduces the brightness of paper. Hence these fibers can be used for pulp production after reducing the lignin content through appropriate delignification techniques or thermo-mechanical pulping. Delignification may be done using chemicals like sodium hydroxide and sodium sulfide and this process is called as chemical pulping. The extracted lignin though not desirable, but can be used as a binder in particleboard or adhesive for linoleum.

Fiber length
The fiber length is the number of bonding sites that is available on an individual fiber to form an interwoven network of fibers and is measured from one end to another end. To manufacture paper, long fiber length is desirable as they provide a more open and less uniform sheet structure. The tearing strength of paper is controlled by fiber length and a lengthy fiber resists tearing of paper (Oluwadare and Sotannde 2007;Wimmer et al. 2002). Fibers are classified into three groups based on their length. The first group comprises hardwood fibers having lengths lesser than 0.9 mm. Short fibers having fiber length between 0.9 mm and 1.933 mm belong to the second group. This includes fibers derived from coir, bagasse and oil-palm. The third group contains fibers having lengths greater than 2 mm (Hamzeh et al. 2012). In this study, the fiber length of Cocos nucifera fiber was 939 ± 45 μm (0.939 mm) and is classified under short fibers.

Fiber diameter and lumen diameter
Fiber diameter is another parameter that is usually measured from side to side end and across the fiber length. Fiber lumen is a tiny opening or a cavity in the fiber cells through which water and other nutrients are inducted by the plant. This is measured in transverse direction. Lumen diameter is an important fiber parameter that determines the type of paper. Pulp beating is an important sub-process in the pulp production which is affected by fiber lumen. The beating of pulp will be better when the fiber lumen width is large, since through capillary action the fluids can easily penetrate into the empty spaces of the fiber. Fiber lumen of negligible width becomes inaccessible and hence the fluids may not enter into the lumen resulting in poor beating. This is associated with more power consumption. Such fibers do not collapse easily and papers produced from them would be poor in tensile, burst and compressive strengths. Such fibers can used for the manufacture of base paper for printed circuit boards (Dutt et al 2003), seed germination paper (Dutt et al. 2005) and tea bag paper (Dutt et al 2007). In this study, the diameter of the Cocos nucifera fiber is 21.29 ± 2.28 µm. Lesser the diameter, better is the mechanical strength on account of higher effective contact area for fiber matrix adhesion. The lumen width of Cocos nucifera in the present study is 12.53 µm and standard deviation of 2.07 µm with the values ranging between 8.96 µm and 16.4 µm. The lumen diameter of Eucalyptus grandis, Sanseviera zeylanica, Jacitara palm, and Sanseviera cylindrica fibers are 14.32, 14.2 ± 5.7 µm (Krishnan & Rajadurai 2016), 9.2 ± 3 µm (Fonseca et al 2013), 8.8 ± 0.4 µm (Sreenivasa 2016) respectively.

Fiber cell wall thickness
Fiber cell wall thickness is also an important parameter that is determined by the age of the tree and its proportion varies in trees. It was reported that matured wood are thick walled while juvenile wood fibers are thin walled (Kiaei, Tajik, and Vaysi 2014). Thick wall fibers adversely affect the bursting strength, tensile strength and folding endurance of paper and the paper manufactured from thick walled fibers will be bulky having a coarse surface and contain a large amount of void volume. So, to produce dense papers pulp is made from thin walled fibers. Cell wall thickness of the Cocos nucifera fibers was 4.25 ± 0.34 µm which is similar to the cell wall thickness (4.194-5.766 µm) as reported by Shakes et al (2011) suitable for pulp and paper production. The observed fiber parameters are shown in Table 1.

Derived indices of FIBERS
The derived indices such as Runkel ratio, co-efficient of flexibility, slenderness ratio, rigidity ratio and Luce's shape factor are the indices that govern the suitability of fibers for pulp production. The aforementioned indices of the fibers were found.

Runkel ratio
The Runkel ratio is the ratio of fiber cell wall thickness to its lumen that determines the suitability of a fibrous material for pulp and paper production. Fibers with high Runkel ratio are stiff, less flexible and possess poor bonding ability; while high Runkel ratio fibers produce bulkier paper (Xu et al. 2006, Enayati et al. 2009). In addition low Runkel ratio fibers have positive effect on the tensile, bursting and folding endurance (Ma et al. 2011). A fiber having a thick cell wall when compared with lumen diameter will have a high Runkel ratio (greater than 1), making it unfit for pulp and paper production. When the Runkel ratio is equal to 1, it means that the fiber is quite good for paper production. In this study, the Runkel index of the Cocos nucifera fibers is 67.9%., a value lesser than 1 making it good for paper production. Fiber with thin cell wall has cellulose that can be used for paper production. The Runkel values of weeds Crotolarea pallida, Scoparia dulcis, Sida cordifolia and Urena lobata are 0.60, 0.66, 0.62 and 0.69 respectively as reported by Sharma et al (2013) for paper production. These values agreed with the value obtained in this study.

Slenderness ratio
Slenderness ratio, a ratio between fiber length to fiber diameter is an important measure that determines the tearing property of pulp. Slenderness ratio is associated with paper's strength. High slenderness ratio provides resistance against sheet break and bursting. Fibers with slenderness value greater than 33 alone are considered suitable for making paper. However, to produce high-quality papers slenderness value should be greater than 60 (Ververis et al 2004). The fibers are classified as high elastic fibers, elastic fibers, rigid fibers and high rigid fibers, when the slenderness value is greater than 75, 50-75, 30-50 and less than 30 respectively. In this study, the slenderness value was 44.11, which is greater than 33. This shows that the fiber is slightly rigid but quite suitable for paper production. The slenderness value of hardwood Eucalyptus tereticornis used for paper production was reported as 39.07 in line with the present research (Basu et al. 2021).

Coefficient of flexibility
Coefficient of flexibility usually expressed in percentage is derived from the ratio of lumen diameter to its fiber diameter which determines the elasticity or rigidity nature of the fibers. Coefficient of flexibility gives the bonding strength of individual fiber and by extension the tensile strength and bursting properties (Wangaard 1962). Fibers with coefficient of flexibility greater than 50% are considered fit for pulp production (Afrifah, Osei and Ofosu 2020). The Coefficient of flexibility of the Cocos nucifera fibers is 58.83% corroborating with the sunflower stalk fibers Chrysophyllum albidum reported as 57% (Hassan Mehdikhani, Torshizi, and Ghalehno 2019) and 64% respectively (Ofosu, Boadu, and Afrifah 2020). The value obtained in this study shows that the fibers are best suited for producing printing and writing papers.

Rigidity coefficient
Rigidity coefficient is obtained by dividing cell wall thickness and fiber diameter. It is a measure to find the suitability for pulp making in terms of conformability and energy requirements. Value less than 0.5 is considered appropriate for pulping process. The rigidity coefficient of Cocos nucifera fibers is 0.199, a less value which means good conformability and less energy requirement. The rigidity values are comparable with Solanum lycopersicum (0.23) and Capsicum annuum var. grossum reported as 0.23 and 0.24 respectively (Sharma, and Lama 2015).

Luce's shape factor and solid's shape factor
Luce's shape factor is an important fiber index and derived from fiber diameter and lumen diameter and it is directly related to paper sheet density. Luce's shape factor values less than 0.5 are considered good for paper and pulp making. The Luce's shape factor of Cocos nucifera fibers is 0.49 which is marginally less than 0.5. Hence the fibers are deemed fit for paper production. Solid's shape factor is inversely associated with the resistance of pulp to beating and resistance of paper to bending, and breaking length of paper. The Solid's shape factor of Cocos nucifera fibers is 278.53x10 3 μm 3 .This value yields high resistance of paper to bending and low resistance of pulp to beating. Papers with good strength were obtained from fibers with low Luce's Shape Factor and Solids Factor (Sharma et al. 2018).
The Luce factor and Solid's shape factor of the Chrysophyllum albidum fibers were reported as 0.41 and 346 μm 3 respectively (Samuel Ofosu et al. 2019). These values are quite agreeable with the Luce factor and Solid's shape factor of Cocos nucifera fibers found as 0.49 and 278.53x10 3 μm 3 . The Solid's factor of Eucalyptus tereticornis was 256 × 103 μm 3 (Monga et al. 2017) and Luce factor of Setaria glauca was reported as 0.41 (Sharma et al. 2015) similar to the present study. The derived indices of some fibers recommended for paper making are shown in Table 2.
The fiber morphology and the derived indices obtained show that the discarded fibers can be effectively used for the production of pulp and paper. The derived indices obtained are acceptable and comparable with the respective derived indices of some other wood and non-wood fibers that are considered suitable for pulp and paper production. The high lignin content is the challenging factor found in the Cocos nucifera fibers and that too can be overcome adopting appropriate delignification techniques such as Kraft or even soda pulping. However, the extracted lignin may be used as a raw material for processing vanillin.

Conclusions
The potential behind the discarded lignocellulosic fibers in the production of paper and pulp was explored. The study revealed that the discarded fibers possess adequate chemical composition, fiber morphology and the derived indices that are desirable for pulp and paper production. Pulp and paper can be produced through appropriate techniques from discarded Cocos nucifera fibers and can help people in generating income to improve their livelihood. Less Runkel index, moderate slenderness ratio, less rigidity coefficient, high flexibility ratio, low Luce's Shape Factor and Solids Factor make the discarded Cocos nucifera fibers as a potential source for producing paper and pulp.