Abstract

Wood samples from each of Leucaena leucocephala, Moringa peregrine, Ceiba pentandra and Calotropis procera were macerated using Franklin method to evaluate their suitability as sources of cellulosic fibers (CFs). Chemical and anatomical characterizations of wood as well as its specific gravity (SG) were performed. The lignocellulosic resources (LRs) examined differed significantly in relation to all the properties studied. L. leucocephala wood is the best fibrous crop among the species studied due to it had the highest SG, holocelluloses content (HC) and fiber yield (FY) as well as the lowest lignin content (LC) and ash content (AC). M. perigrina had the highest LC, the shortest and the widest fibers. C. pentandra had the lowest total extractives content (TEC) and the longest fibers. Although C. procera possessed the lowest HC and FY and the highest TEC and AC, its utilization as a cellulosic precursor is not closed due to it has the lowest LC. The macerated fibers produced from the four species had low aspect ratio. Vessels of the four lignocellulosic resources are characterized by scalariform pitting system and L. leucocephala vessel has simple perforated plates.

1. Introduction

Wood is the most important precursor for the pulp and paper industry. The global wood fibers are the original source of over 98% of the fibrous constituent of paper in the world. Pulp is a product that is derived from wood and other cellulosic plant materials by mechanical or chemical treatment. The continued growth and economic considerations of the pulp and paper industry depends on availability of wood with propitiate quality and cost ¹.

Cellulosic fibers (CFs) macerated from lignocellulosic natural resources have been attracting much attention due to their biodegradability, availability, eco-friendliness and ease handling and manufacturing ². Leucaena leucocephala is a multi-purposes tree that can be adapted to various environments. It has gained a great attention for its utilization commercially as a precursor for pulp and paper as well as packaging industries ³.

Moringa peregrina is widely grown in Saudi Arabia, Iran and India. It has a wide range of utilization in agriculture as animal fodder, and human nutrition and health as a medicinal plant ⁴ as well as industry such as fixed oil extraction from seeds. Saudi Arabia is one of the main habitats of M. peregrina in the Middle East ⁵. The Moringa is a fast-growing evergreen or deciduous multipurpose tree species comprised of 13 species. Due to its numerous economic importance, easy propagation and sustainability, the plant is suitable for cultivation in Saudi Arabia. The plant is highly tolerant to drought and is widely cultivated in arid and semiarid regions ⁶. It was indicated by Hindi ⁷ that M. peregrina can be used to as a cellulosic resource to fill the gap between fiber production and demand.

Ceiba pentandra known as kapok or silk-cotton tree is ranked among the largest tropical trees in America and Africa. Importance of its wood has increased commercially due to its suitability for light constructions and plywood, easy for processing. Its wood is diffuse-porous, light brown, soft, straight-grained, and light in weight. Its specific gravity was ranged from 0.09 to 0.3 ⁸. In Jeddah, at the campus of King Abdullaziz University, it gave good growth and pods yield when cultivated depending on treated sewage water as an ornamental tree.

Calotropis procera (Family: Asclepiadaceae) is a desert plant known as calotrope, usher or milkweed and is adapted to a wide range of different environments. Its wood is termite-proof and can be used for roofing, building huts and as a cellulosic fibers precursor. Chemical constituents of the calotrope wood showed high contents of ash (5.4%), lignin (18.5%) and low contents of total extractives (11.9% and holocelluloses (61.2%). Accordingly, the quality of its fibers is lower than some of other conventional fibers such as cotton and kapok. This is due to its high content of ash that negatively impacted the chemical recovery process and, therefore, could constitute a serious drawback ⁹.

The presence of permeable structure in the cellulosic resources was very important for the chemical reagents penetration required for the various chemical industries. The mean value of pore diameter for the crude cellulosic resources were estimated to be 4.01 μm for macerated woody fibers of Leucaena leucocephala ¹⁰.

The objectives of this study were therefore as follows:

a. Determination of the specific gravity as well as chemical and anatomical properties of wood among the four lignocellulosic resources.

b. Comparisons of the different wood properties between the species studied.

2. Materials and Methods

Four lignocellulosic resources, namely Leucaena leucocephala (Lam) de wit (Figure 1), Moringa peregrine Forssk. ex. Fiori (Figure 2), Ceiba pentandra (L.) Gaertn. (Figure 3) and Calotropis procera (Ait). Ait. (Figure 4) were used as sources of cellulosic fibers (CFs). The selected species are adapted to the arid and semi-arid conditions in the Western region of Saudi Arabia and are suitable for afforestation programs as multipurpose species. In addition, these species have different physical, chemical and anatomical characteristics and are expected to give good qualities for fibers production.

Species selection and samples processing were done during May-June 2015 based on the trees and shrubs grown in the Agricultural Research Station, King Abdulaziz University (KAU), Hada Al-Sham (about 120 km Southeast Jeddah and at a latitude of 21° 46’.839N and a longitude of 39° 39’.911E and above the sea level by 206 m), except for kapok trees that were selected from the KAU-campus at Jeddah Governorate. The chosen sites have a sandy soil. Reference samples were identified through flora of KSA ¹¹. The age of the chosen species were about 12 years-old except for milkweed that was about 3 years-old. The diameter of branches outside bark of the selected species ranged from 15-45 cm. Four healthy trees/shrubs were chosen from each species. Accordingly, 16 plants were selected to represent the four species specified for this study. From each of the selected species, two healthy primary branches were taken randomly to represent its tree/shrub.

For each of the selected trees/shrubs, the selected branches were cut at height of 10 cm above its base level at which the main stem is branched. The height between the branch base and ground level was about 40 cm for Leucaena sp. and Calotropis sp., and ranged between 140-170 cm for Moringa sp. and Ceiba sp. From each cut branch, one disc of about 20 cm along the grain was cut beginning from its base. The removed disc was sawn into a diametric strip in dimensions of 10 cm radially and tangentially and 20 cm longitudinally. The resulted strip was split toward its center into two diametric strips (4 4 20 cm³ each) excluding the pith. The obtained strips were longitudinally sawn into sticks (1 120 cm³ each), air dried, oven-dried at 100°C for 24 h and stored until used for determinations of wood properties and production of CF’s (Figure 5).

About 100 g of air-dried wood from each of the four trees/shrubs selected from each species were converted into meal by a suitable grinder. Then, wood meal was screened using different sieves depending on the standard methods for wood properties determinations as explained at each test.

Samples of about one gram each were taken randomly from the screened wood meal and used to determine the wood properties as follows:

a. Three samples from each tree were used for the determination of ash content (3 g/per tree).

b. Three samples for the determination of total extractives content (3 g/tree). Then, after extraction, each of them was used to determine the lignin content.

c. Three samples for the determination of holocelluloses content (3 g/tree).

For specific gravity (SG) of wood, five cubic samples (1×1×1 cm³) from each tree were used.

About fifty grams of wood chips from each of the four species was delignified separately by applying Franklin method. The digestion agent was a mixture of hydrogen peroxide (35%) and glacial acetic acid in a ratio of 1: 1. The mixture was kept, with a compacted cotton stopper, in an oven at 60 °C for 24 h or until clear fiber-separation that featured by the white aspect ¹². The macerated fibers were removed, washed, air-dried and characterized.

The SG of wood was determined by using Pycnometric displacement of water ¹³ based on the oven-dry weight and saturated volume.

The TEC of wood is the chemical constituents of wood that are soluble in each of benzene, a mixture of benzene and 95%-ethanol and thirdly, in hot deionized water. Accordingly, the TEC of wood is a measure of the waxes, fats, resins, oils, tannins and water-soluble components. The TEC was determined based on the ASTM ¹⁴. About one gram of an air-dried wood meal (250/180 um) was extracted in a Soxhlet apparatus with ethanol-benzene mixture (1:2) for 4 hours, followed by 95% ethanol for 4 hours and finally with hot distilled water for 3 hours with changing of water every one hour. The TEC) content was calculated as follow:

Where:

W₁ = Weight of air-dried wood sample (g),

W₂= Extractives free-weight of the oven-dried sample (g), and

M.C = Moisture content in the air-dry sample determined in analogous samples (%).

The LC was determined according to the method described by the ASTM ¹⁵ and applied by Hindi ^{7, 12}. The extractives-free wood sample was primarily hydrolyzed with a sulfuric acid (72 %) at 35C. After one hour, the sample was secondary-hydrolyzed for half-hour by diluting it with 200 ml distilled water and boiling. The material was filtered using Whattman filter paper No. 44, air-dried, oven dried and weighed. The LC was estimated as follow:

Where:

W₁= Weight of air-dried extractives-free wood (g),

W₂ = Weight of Klasson lignin (g).

The HC of wood is the sum of alpha cellulose and hemicelluloses together. It was determined according to Viera ¹⁶ and Hindi ¹² as follow: a 5 wt % fiber solution was prepared and mixed with 0.5 mL of glacial acetic acid and 0.75 g of sodium chlorite. The flask was top sealed to prevent the loss of gas released during the reaction process and the temperature was maintained at 75 °C for one hour. Changing the chemical reagents was done twice. The HC of wood was calculated as follow:

Where:

W₂ = Weight of holocelluloses (g).

The AC of wood was determined depending on the ASTM ¹⁷. About two grams of wood meal (425um) was ignited at 600C until all carbon was eliminated. The AC was calculated as follow:

Where:

W₂ = Weight of minerals residue after ignition (g).

An optical speculation unit was used to measure the length and width of the macerated fibers of the four lignocellulosic resources studied. The speculation system is consisted of a light microscope (CE- MC200A) with suitable vision system (OPTIKA PRO 5 Digital Camera- 4083.12) using a Vision PRO 4 software.

For the determination of the length, width, and aspect ratio of the macerated fibers produced from the four species, a drop of fibrous solution was mounted on a glass slide and was stained with 1% aqueous safranine. One slide was specified for the three properties of the fibers to represent each of the four replicates designed for each of the four species examined. Twenty five fibers were specified randomly and measured from each slide. The aspect ratio of a cellulosic fiber is calculated as a ratio of its length to width.

The SEM study was performed to investigate the anatomical structure of the four selected species according to Hindi ¹⁸. A thin wood chip (0.5×0.5×0.1 cm) was placed on a carbon tape on an Al-stub, air-dried, and was sputtered with a 15-nm-thick gold layer (JEOL JFC- 1600 Auto Fine Coater, JEOL, Tokyo, Japan) in a vacuum chamber. The samples were examined with a Quanta FEG 450 SEM (FEI, Netherlands). The microscope was operated at an accelerating voltage ranging from 5 to 20 kV.

The FY of a certain wood species was determined as a percentage of the resultant weight of the macerated fibers based on the oven-dried weight of the parent wood as follow:

Where:

W₁ = Weight of air-dried wood sample (g),

W₂ = Weight of fibrous product (g).

A randomized complete block design (RCBD) was used to test each of the wood properties, namely specific gravity, total extractives, lignin, holocelluloses and ash contents of wood, fiber length, fiber width, and aspect ratio as well as the fiber yield of the four species. Statistical analysis of the recorded data was done according to Steel and Torrie ¹⁹ using the analysis of variance procedure and least significant difference test (LSD) at P≤0.05.

3. Results and Discussion

Results of the specific gravity (SG) and chemical characterization, namely total extractives content (TEC), lignin content (LC), holocelluloses content (HC), and ash content (AC) of wood for the four species are listed in Table 1. In addition, Fiber length (FL), fiber width (FW), aspect ratio (AS) and fiber yield (FW) are presented in Figure 11 - Figure 14, respectively. It was indicated from the statistical analyses the four materials tested were significantly different among the properties determined.

The SG of wood was affected significantly by species. The mean values of the SG increased from 0.392 for Ceiba pentandra to 0.597 for Leucaena leucocephala. Woods with higher SG values have more cell wall materials that can be used industrially comparing to those with lower SG. In addition, lower SG is inversely correlated with wood permeability. Porous materials permit to pulping reagents to penetrate more easily into cell walls of lignocellulosic tissues. However, the quality of a final fibrous product and the cost of production specify the required SG value of a certain wood precursor ⁸.

However, the woody materials studied had low specific gravity and subsequently had high permeability. Accordingly, they can be forced towards such industries such as particleboards or pulp manufactures ⁷.

The lignocellulosic resources examined differed significantly due to their of TEC. As shown in Table 1. Calotropis procera wood had the highest TEC value (11.9 %) followed by Leucaena leucocephala (9.74 %), and Moringa perigrina (8.52 %). On the other hand, Ceiba pentandra wood possessed the lowest TEC value (Table 1). The highest TEC content for Calotropis procera wood may be attributed to its open anatomical structure that easily accessible for the chemicals ²⁰. Industrially, accumulation of high amounts of extractives into a lignocellulosic tissue is un-preferred due to their interference with the chemical reagents used for delignification and separation of the fibers ^{7, 18}. In addition, there is another defect for materials possessing high TEC as reported by Lopez et al. ²¹ in which pulp yield is negatively correlated with ethanol-benzene and water soluble fractions present in wood. Accordingly, species with lower TEC are expected to yield more fibers than those with higher TEC. For the species studied other than Ceiba pentandra, one of three options may be selected: a) organic solvent-extracted before maceration, b) changing the maceration reagents after a suitable treatment period, and c) increasing strength of the reagents used for the chemical process ^{7, 18}.

Statistical analysis revealed a significant difference between the four species in relation to the LC. It can be seen from Table 1 that Moringa perigrina had the highest LC (28.26 %), approaching to that for softwoods as well as to typical contents presented in annual plants, non-woods and hardwoods, while quite higher than the other resources studied. On the other hand, Leucaena leucocephala and Calotropis procera woods contained the lowest LC (18.86 % and 18.5 %, respectively) which is below that for hardwoods. It is worth mentioning that low LC in lignocellulosic material reduces pulping time and amounts of chemical reagents compared to those resources with high LCs ^{21, 22}. In addition, higher LCs are expected to consume more chemical reagents upon the pulp industry ²⁰. The results are in agreement with Lopez et al. ²¹, Megahed et al. ²³, Hindi ^{7, 12, 18}.

The HC results showed that the natural resources studied were significantly different. Leucaena leucocephala and Ceiba pentandra had higher HC values (70.82 % and 69.69 %, respectively). This shows their importance as fibrous crops for many applications such as cellulose derivatives, fiber-reinforced composite materials, and papermaking ⁹. On the other hand, although Calotropis procera possessed the lowest HC value (64.2 %) as shown in Table 1, its utilization as a cellulosic precursor is not closed.

The lignocellulosic resources examined were significantly different in their AC. It is clear from Table 1 that Ceiba pentandra and Leucaena leucocephala contained lower ash (0.646 and 1.22 %, respectively) than the other resources. It is worth to mention that lignocellulosic materials with high AC such as Calotropis procera (5.4 %) will negatively impact the chemical recovery process after fiber maceration and consequently may constitute a serious drawback ⁹. However, the results were in agreement with those arising from other literatures ^{7, 18, 21, 22, 24, 25, 26}.

The wood species examined have more suitable properties for fiber products than the Calotropis procera. Furthermore, Ceiba pentandra and Leucaena leucocephala are the best resources due to their high contents of HC, and their lower TEC, LC and AC comparing with the other resources examined. Irrespective of their higher contents of LC and AC as well as relatively lower HC, wood of Calotropis procera could become important sources for fibers, chemicals and other industrial products.

The morphogical aspects of the cellulosic fibers macerated from the four lignocellulosic resources are presented in Figure 6. The statistical analyses indicated that the four lingocellulosic materials examined were significantly different among the dimensional properties (fiber length, width and aspect ratio) of the species studied. Thess difference may be attributed to the genetic difference between these species ²⁷.

A statistical difference was found between the species studied in relation to their FL. It is obvious from Figure 7 that Ceiba pentandra fibers had the highest FL value (1.188 mm), while the FL of Moringa perigrina was the lowest (0.57 mm).

Since pulp strength is greatly affected by fiber length ²⁸, paper manufactured from Ceiba pentandra is expected to give higher paper quality than the others with shorter fibers, especially when blended with other softwood fibers in papermaking purposes ¹⁸. The FL results agree with the previously published by other researches using other wood species such as Kherallah and Aly ²⁹, Megahed et al. ²³, Diaz et al. ²², Hindi ^{7, 18, 30}.

The macerated fibers obtained from the four species were statistically different due to their FW. Regarding Figure 8, Moringa perigrina and Calotropis procera had the highest FW values (203.63 and 203.93 μm, respectively). On the other hand, Leucaena leucocephala had the lowest FW value (173.32 μm).

The four precursors examined exhibited different behavior regarding to the AS. However, the macerated fibers produced from the four species had low AS with a range from 2.8 for Moringa perigrina to 6.02 for Ceiba pentandra (Figure 9). Using cellulosic fibers possessing high aspect ratio means that the fibers have a high tendency for clumping together when are suspended in water. To avoid fiber flocculation that may be arisen in this case, the fiber concentration in the pulp solution must be less than about 0.01% ³¹.

The anatomical features of the four lignocellulosic resources are presented at Figure 10 (Leucaena leucocephala), Figure 11 (Moringa perigrina), Figure 12 (Ceiba pentandra), and Figure 13 (Calotropis procera). It is obvious that all the wood tissues belong to the hardwood anatomy in which they are porous tissue due to possessing vessel elements. It is worth mentioning that hardwood contains two types of prosenchyma cells, namely vessels and fibers as longitudinal elements ³². In addition, the ray parenchyma, the transvers element, can be seen from the radial sections presented at Figure 11a and Figure 12a,b. Furthermore, the fusiform ray are obvious in the tangential section of Calotropis procera wood (Figure 14). The border pitting system for Leucaena leucocephala, Moringa perigrina, Ceiba pentandra and Calotropis procera is clear (Figure 10-Figure 13) and well-known to act as a transverse pathway for water and gases between prosenchyma cells within the woody tissue. Furthermore, the semi-border pits that connect parenchyma cells with adjacent prosenchyma cells can be noticed for Moringa perigrina, Ceiba pentandra and Calotropis procera (Figure 11 -Figure 13). In addition, the parenchyma cells characterized by their thin walls are connected transversely by simple pits.

The SEM results of the Leucaena leucocephala wood tissue is presented at Figure 10. For the transverse section presented in Figure 10a for crude wood tissue, vessels and fibers cells are clear with a noticeable border pits. In addition, for the macerated fibers, a vessel with obvious scalariform pitting system and a clear noticeable simple perforated plate is shown (Figure 10b). These openings, cavities, and the amorphous regions within the cell wall microfibrils exhibit their own permeable structure for each cellulosic resource ¹⁰.

The radial section of Moringa perigrina wood investigated by SEM (Figure 11a and Figure 11b) shows vessels and fibers cells that characterized by the scalable-border pitting system that increases its wood permeability. In addition, ray parenchyma can be seen in Figure 11a.

The micro-anatomical characteristics of Ceiba pentandra wood as a member of hardwood architecture design is shown in Figure 12. The radial section shows the longitudinal vessels (Figure 12a) and the ray parenchyma (Figure 12a and Figure 12b). The pitting system responsible for transversal movements of fluids within wood tissue is also clear in Figure 12b.

The microstructure of Calotropis procera investigated by SEM-tangential section is presented at Figure 13. It is obvious that it lies within the anatomical scope of hardwood species, since it is characterized by presence of prosenchyma cells of both vessels and fibers. It was indicated by Hindi ¹⁸ that fiber cells of Calotropis procera had thin walls and wide lumens occupying most of the transverse section area ¹⁸.

Furthermore, its wood is a diffuse porous in which the vessels (pores) are even-sized, so that the water conducting capability is scattered throughout the ring instead of collected in the earlywood. In addition, parenchyma cells and fusiform ray are also included in the micrograph. The ray is characterized by its narrow width, differed from uniseriate to tetra-seriate with different lengths as well as its heterogeneity in which containing both upright and procumbent cells. Furthermore, it can be seen from Figure 13 presence of white flakes from dried latex when it flooded from pores to the outer atmosphere adjacent to the cut surface ¹⁸. A high broad multiseriate ray contacting with dried latex flakes.

The FY values obtained from the four speciess were gradually decreased from Calotropis procera (39.2 %) up to Leucaena leucocephala (55.46 %) as indicated from Figure 14. This finding can be attributed to the logic relationship between the LC, TEC and AC contents of the wood precursor and their FY. The higher fiber productivity of the lignocellulosic materials will increase the interest in such materials for fiber production.

4. Conclusions

• The lignocellulosic resources examined differed significantly in relation to all the properties studied.

• Leucaena leucocephala wood is the best fibrous crop among the species studied for due to it had the highest specific gravity, holocelluloses content and fiber yield as well as the lowest lignin and ash contents.

• Moringa perigrina had the highest lignin, the shortest and the widest fibers.

• Ceiba pentandra had the lowest total extractives content and the longest fibers

• Although Calotropis procera possessed the lowest holocelluloses content and fiber yield and the highest total extractives and ash contents, its utilization as a cellulosic precursor is not closed due to it has the lowest lignin content.

• The macerated fibers produced from the four species had low aspect ratio.

• The four species belong to the hardwood that contains two types of prosenchyma cells (vessels and fibers) as longitudinal elements.

• The vessels and fiber cells of the four lignocellulosic resources are characterized by scalariform pitting system and Leucaena leucocephala vessel has simple perforated plates.

References