Anatomical and Physico-mechanical Characterization of ...philjournalsci.dost.gov.ph/images/pdf/pjs_pdf/vol... · v). Mechanical Properties The mechanical testing was done by using

Keywords: anatomical properties, branchwood, fiber indices, high value forest crops, physico-mechanical properties

Anatomical and Physico-mechanical Characterization of Narra (Pterocarpus indicus Willd.) Branchwood Collected

in Mount Makiling Forest Reserve, Laguna, Philippines

*Corresponding Author: [email protected]

The properties of narra (Pterocarpus indicus Willd.) branchwood collected from Mount Makiling Forest Reserve (MMFR), Los Baños, Laguna, Philippines were studied. These branchwood properties were also compared with narra stemwood’s experimental and published properties. Core samples of stemwood were taken for anatomical characterization while branchwood samples were taken for anatomical (area percentage of fiber and parenchyma cells, fiber morphology, and computation of fiber indices), physical (moisture content, density, specific gravity, and shrinkage), and mechanical (static bending and compressive strength) analysis. Results showed that narra branchwood exhibits similar anatomical features to narra stemwood. Branchwood, however, has smaller pores, abundant inclusions, and less distinct storied arrangement of wood rays than stemwood. The area percentage of fiber cells is higher in stemwood while parenchyma percentage is higher in branchwood. Fiber dimensions appeared to be statistically the same for stemwood and branchwood, except for fiber length. Computed fiber indices of narra branchwood are also within the standard value ranges. In terms of mean density and specific gravity at 12% moisture content of the branchwood were 0.70 g/cm3 and 0.66, respectively. Mean tangential, radial, and longitudinal shrinkages were 3.59%, 3.37%, and 0.6%, respectively; while volumetric shrinkage was about 6.35%. The static bending properties of branchwood were not different from stemwood. The mean modulus of elasticity and modulus of rupture were 9.92 GPa and 96.59 MPa, respectively. On the other hand, the compressive strength parallel to the grain was lower at 32.64 MPa. Thus, narra branchwood may be used as a substitute for narra stemwood in various uses such as for high-grade furniture and cabinetry, musical instruments, pulp and paper, production of novelty items, and wood parquet.

Philippine Journal of Science148 (4): 705-713, December 2019ISSN 0031 - 7683Date Received: 30 Jul 2019

Rosalie C. Mendoza1*, Vivian C. Daracan1, Ronniel D. Manalo1, Chelle Hennessy R. Batallones1, Kisses G. Jaurigue2,

Arlene D. Romano1, and Willie P. Abasolo1

1Department of Forest Products and Paper Science, College of Forestry and Natural Resources, University of the Philippines Los Baños,

College, Los Baños, Laguna 4031 Philippines2MSA Academic Advancement Institute, Esteban cor. Dela Rosa,

Legazpi Village, Makati City 1229 Philippines

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INTRODUCTIONWood is one of the most used renewable raw materials and the main source of livelihood of the people belonging to the forest products sector. As a structural material, wood can be used in many forms of construction. In the Philippines, the demand for wood and other timber products has increased over the years. Guiang (2001) stated that several studies show that the existing domestic wood supply from natural and plantation forests is insufficient to meet the increasing domestic demand.

The timber industry has a significant contribution to the national economy, but its operation is not sustainable. Material wastage is one of the major problems of the timber industry. It is reported that nearly 50% of the tree volume is left in the forest to decay. These are in the form of branches, crown wood, and stumps (Adam et al. 1993, as cited by Okai et al. 2003). Hence, there is a continuous effort to explore the utilization of other sources of wood to sustainably meet the soaring demand for timber and other forest products. The utilization of branchwood of high-value forest crops (HVFC) could be one of the potential sources of wood raw materials and could be used as an alternative to stemwood. Unfortunately, limited studies have been reported on the properties of HVFC utilized in the Philippines.

In general, stemwood and branchwood differ in terms of rate of growth, cell structure, and anatomical properties. The results of the study of Kiaei and Roque (2015) revealed that Alder stemwood, branchwood, and root wood had significant differences in the physical properties and fiber dimensions. Stemwood exhibited the highest wood density, volumetric shrinkage, and fiber length, whereas branchwood had lowest fiber dimensions compared to other wood sample parts. Furthermore, in the research conducted by Dadzie and Amoah (2015) on the comparison of the anatomical properties of stemwood of Entandrophragma cylindricum (sapele) and Khaya ivorensis (mahogany) to the branchwood using Ceiba pentandra (onyina) stemwood as control, fiber proportion was comparable in stemwood and branchwood of only sapele but it was significantly lower in branchwood than stemwood of mahogany (p < 0.01).

Narra (Pterocarpus indicus Willd.) is one of the HVFC and commercially-important timber species in the country. The high demand for narra wood has been increasing yet the supply for raw materials continues to decline due to the depletion of timber resources and loss of forest cover. Although narra stemwood anatomy is well-known (InsideWood n/d), very little is known about the anatomical, physical, and mechanical properties of its branchwood. Thus, this study aimed to characterize the anatomical and physico-mechanical properties of narra branchwood.

MATERIALS AND METHODS

Sample CollectionNarra branchwood and stemwood were collected from MMFR, Los Baños, Laguna, Philippines. Three mother trees of narra were randomly selected from two locations in the MMFR. The first major branch that is almost straight and has fewer knots was selected for the characterization of branchwood. Each major branch was cut into three sections, measuring 1 m each. Three stemwood core samples were also collected from the same trees using increment borer. A total of nine branchwood and nine core stemwood samples were collected, labeled, and transported to the wood machining shop of the Department of Forest Products and Paper Science, College of Forestry and Natural Resources (CFNR), University of the Philippines Los Baños (UPLB). The sample size was determined considering the findings of Eckblad (1991).

Anatomical Properties Structural observation and area percentage determination of fiber and parenchyma cells. Three branchwood samples (1 x 1 x 2.54 cm3) were prepared and softened by boiling them in water for 48 h. Thin sections (cross, radial, and tangential) were cut from the transverse surfaces of the samples with the sliding microtome at a thickness of 20 µm. The preparation of thin sections was prepared following the method designed by Franklin (1945). The sections were first washed in distilled water and stained in safranin in 50% ethanol solution for 5 min. Samples were washed again in distilled water and dehydrated in increasing concentration of ethanol from 30, 50, 70, 80, 90, and 95% for 5–10 min. Sections were then stained in fast green for 1 min and rinsed with 100% ethanol for 5 min. Sections were then soaked in xylene to remove little traces of water and mounted permanently on the slide. Optika light microscope with digital video camera was used to capture images of thin sections. In addition, the area percentages of fiber and parenchyma cells were calculated using ImageJ (Version 1.49).

Fiber morphology and fiber indices. Three replicates of narra stemwood and branchwood with nine samples from each wood type were taken for analysis. branchwood samples (1 x 1 x 2.54 cm3) were prepared and reduced into splinters while stemwood core samples were cut into smaller pieces. Samples were macerated according to Franklin’s method (1945) wherein a solution of glacial acetic acid and 50% by volume of hydrogen peroxide in a 1:1 ratio was used. Samples were cooked until they turned whitish in color and separated into individual fibers. A total of 30 fibers were measured for each sample. Optika light microscope with an image analysis program (Optika ISView) was used to measure fiber length (FL), fiber

Mendoza et al.: Anatomical and Physico-mechanical Characterization of Narra Branchwood

Philippine Journal of ScienceVol. 148 No. 4, December 2019

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diameter (FD), and lumen diameter (LD). Fiber cell wall thickness (CWT) was also calculated as the average of the difference between FD and LD. Derived values were calculated using the following formulas:

Flexibility ratio = (LD X 100) / FD (1)Runkel ratio = 2CWT / LD (2)Slenderness ratio of felting power = FL/FD (3)

Physical PropertiesFour wood blocks (2.54 x 2.54 x 2.54 cm3) were prepared from the sample discs taken from each branchwood. Initial weight and volume of each woodblock were determined before placing them in an oven at 103 ± 2 °C for 24 h. Wood samples were removed and cooled down in a desiccator before obtaining its final weight and volume. The computed physical attributes of wood were percent moisture content (%MC), density, specific gravity, percent shrinkage (%S), and volumetric shrinkage (%Sv).

Mechanical PropertiesThe mechanical testing was done by using an automated Universal Testing Machine (UTM) in Innovative Engineering Materials Laboratory at the Civil Engineering Department, College of Engineering and Agro-industrial Technology, UPLB.

Static bending test. Three different branchwood samples with three replicates were tested. Static bending tests were done to determine the modulus of rupture (MOR) and modulus of elasticity (MOE) of branchwood following the American Society for Testing and Materials (ASTM) D143-14 standard. The loading was made at the center span with a speed of 4 mm/min. The load-deflection curve was plotted to determine the load and deflection at proportional limits. The MOR and MOE were computed using the following formulas:

MOE = WL/481D (4) MOR = 3PL/2bd2 (5)where:

W = load at proportional limitP = maximum forceL = lengthD = deflection at midspanb = widthd = depth (thickness)I = moment of inertia (bd3 / 12)

Compressive strength. Nine branchwood samples were tested for compression strength parallel to the grain following ASTM D143-14. The compressive strength was computed using the formula:

CS = P/A (6)where:

S = compressive strengthP = maximum force applied to the sampleA = cross-section area of the sample

Statistical AnalysisTo analyze the variation in the anatomical and physic-mechanical properties of narra branchwood, one-way analysis of variance (ANOVA) using JMP Statistical Discovery, SAS was utilized.

RESULTS AND DISCUSSION

Anatomical PropertiesStructural observation and area percentage determination of fiber and parenchyma cells. Figure 1 shows the cross section of one of the branchwood used in this study. Just like the other collected branches, the heartwood and sapwood portions are easily recognizable. Sapwood is the living and outermost portion of the xylem, while heartwood is the dead and inner portion. Sapwood may be distinguished from heartwood by its lighter color. The difference is attributed to the presence of extractives. It can be observed that the amount of sapwood is lesser than the heartwood. The presence of more heartwood is

Figure 1. Cross-section of narra branchwood showing its sapwood and heartwood portions.

an indicator that the sampled trees are already matured.

The light microscopy showed the frequency of the four distinct tissues: vessels, rays, parenchyma, and fibers of both stemwood and branchwood (Figures 2–4). Baas and co-authors (1989) and Wheeler (2011) described in detail the anatomical features of narra wood. However, some of these features may vary because of the woodʼs



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inherent variability. It is expected that some features will be very distinct in some samples while absent on the other samples of the same species. Hence, a range of features was enumerated. Growth rings of narra may or may not be distinct. Its topography is classified as semi-ring or

semi-diffuse porous. Its vessel has simple perforation plates and its inter-vessel pits are polygonal in shape with a size of 7–10 µm and arranged in diagonal rows. It also has vestured pits. Vessel-ray pitting is also present where pits have distinct borders, similar to inter-vessel

Figure 2. Cross-section of narra (A) stemwood and (B) branchwood.

Figure 3. Radial section of narra (A) stemwood and (B) branchwood.

Figure 4. Tangential section of narra (A) stemwood and (B) branchwood.



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pits in size and shape throughout the ray cell. The mean tangential diameter of vessels ranges from 100–200 µm or even greater and a length of 350 µm and there are around 5 vessels per mm2. Usually, pore occlusions like gums and deposits are present in heartwood vessels. Fibers present are non-septate with simple to minutely bordered pits. Fibers are characterized to have thin to thick-walled cells, lumen diameter less than three times the double wall thickness, and distinctly open. Axial parenchyma present are diffuse, scanty paratracheal, vasicentric, aliform, winged aliform, and confluent that are arranged in narrow bands or lines up to three cells wide. There are two cells per parenchyma strand. All rays are procumbent, uniseriate, and storied. There are 4–12 rays per mm. Axial parenchyma, vessels, and fibers are also storied. Prismatic crystals are also present in chambered, axial parenchyma cells.

The area percentages of fiber and parenchyma cells for branchwood are 58 and 24%, respectively, compared with stemwood with values of 62 and 23% (Table 1). Based on the study of Stokke and Manwiller (1994), it can be expected that the stemwood will have higher percentage of axial parenchyma and lower percentage of fibers compared to branchwood.

studies on different species, branches have shorter fiber length than in stems. Furthermore, Kiaei and Roque (2015) reported that the fiber dimensions of branchwood were shorter than the stemwood. Branchwood has relatively shorter fibers and smaller vessel lumen diameter than its stemwood in most hardwoods (Bowyer et al. 2003, Samariha et al. 2011, Dadzie et al. 2016). These results may be attributed to the characteristics of longitudinal cells of branchwood, which is generally narrower in diameter and shorter in length (Antwi-Boasiako and Apreko-Pilly 2016). Shorter fibers in branchwood could also be explained by the faster growth rate during wood formation at the sapwood region and the extent of the intrusive growth of the tip of fibers during their differentiation (Wilson and White 1986, as cited by Antwi-Boasiako and Apreko-Pilly 2016). Moreover, the strength property of wood, according to Wiedenhoeft (2013), is directly affected by the thickness of the fiber cell wall. Low density and strength are attributed to the species with thin-walled fibers whereas species with thick-walled fibers have high density and strength. The thickness of the cell wall is also important in pulp refining process. The thicker the cell wall, the more flexible the fiber is in paper

Table 1. Area percentages of parenchyma and fiber cells in the total composition of narra.

Type of cellAverage values (%)

Stemwood Branchwood

Fiber 62.30 58.02

Parenchyma 23.17 24.23

Table 2. Fiber morphology of narra stemwood and branchwood.

Parameter (µm)

Average values (%) Significance levels of

differences(P-value)

Stemwood Branchwood

Fiber length 1307.33 1183.96 0.008*

Fiber diameter 23.76 23.28 0.6092

Lumen diameter 15.56 14.16 0.096

Cell wall thickness 4.10 4.56 0.0516

Fiber morphology and fiber indices. Table 2 shows the fiber morphology of the narra stemwood and branchwood fibers (Figure 5). It was observed that stemwood has longer fibers than branchwood. However, both exhibited comparable cell wall thickness. Similar results were previously reported by many researchers. Some studies revealed that fiber length of branchwood is more slender and shorter than its stemwood. Bhat and co-authors (1985, 1989), as cited by Yaman (2014), reported that in many

Figure 5. Isolated fibers of narra (A) stemwood and (B) branchwood.



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refining process (San et al. 2016). Nevertheless, the result of analysis showed no statistical difference.

Table 3 shows the computed fiber indices of narra stemwood and branchwood fibers. Flexibility ratio is used to determine the bonding strength of individual fiber, its tensile strength, and bursting properties. Samariha and co-authors (2011) classified fibers according to its flexibility ratio: high elastic (≥ 75), elastic (50–75), rigid (30–50), and highly rigid (≤ 30). According to this classification, the flexibility ratio of narra stemwood and branchwood fibers are 73 and 71, respectively; hence, they are classified as elastic fibers.

Similarly, in terms of fiber diameter, lumen diameter, and cell wall thickness, the highest values were observed in the second meter of the branch. However, the observed difference in values were found to be statistically insignificant.

Physico-mechanical Properties of Narra Branchwood Physical properties refer to density and moisture relations that affect wood utilization while mechanical properties refer to the strength, toughness, and capacity of the wood to withstand applied forces. Moisture content affects most of the properties of the wood. Hence, determining wood moisture is important prior to the conduct of any test. The computed average %MC of wood samples is 18.01%. Nevertheless, it is relatively close to the equilibrium moisture content of Los Baños, Laguna, where the study was conducted. The comparison between the experimental and published physico-mechanical properties of narra

Table 3. Fiber indices of narra stemwood and branchwood.

Fiber indices Stemwood Branchwood

Flexibility ratio 0.73 0.71

Runkel ratio 0.39 0.44

Slenderness ratio 37.81 38.90

Table 4. Variation in anatomical properties of narrra branchwood along the length.

Parameter (µm)

Branchwood (mean values) Significance levels of

differences(P-value)

1st meter 2nd meter

3rd meter

Fiber length 1078.67 1254.55 1218.67 < 0.0001*

Fiber diameter 22.73 24.43 22.67 0.2548

Lumen diameter 13.80 15.00 13.67 0.3802

Cell wall thickness 4.47 4.72 4.50 0.6772

Table 5. Comparison between the experimental and published physical and mechanical properties of narra (P. indicus) branchwood and stemwood.

Physical properties Stemwood1 Branchwood

Density (g/cm3)(OD, 12% MC) 0.52, 0.66 0.63, 0.78

Specific gravity (OD, 12% MC) – , 0.52 0.63, 0.70

Tangential shrinkage (%)* 4.5 3.59

Radial shrinkage (%)* 3.0 3.37

Longitudinal shrinkage (%)* – 0.60

Volumetric shrinkage (%)* 7.3 6.35

Modulus of elasticity (GPa)** 9.90 9.92

Modulus of rupture (MPa)** 95.60 96.59

Compressive strength parallel to the grain (MPa)** 86 32.6

*Moisture content at test to oven-dry condition**Air-dry condition 1Alipon and Bondad (2008)

Runkel ratio is used to determine the suitability of fibrous material for pulp and paper production (San et al. 2016). Fibers with Runkel ratio of greater than 1 are stiffer, are less flexible, and have poor bonding ability. Whereas, fibers with low Runkel ratio (< 1) produce good quality pulp and paper (San et al. 2016). It is reported that materials having a Runkel ratio less than 1 would be suitable for papermaking because they collapse (become ribbon-like) and provide a large surface area for bonding (Jang and Seth 1998). Therefore, the calculated Runkel ratios for narra stemwood (0.39) and branchwood fibers (0.44) make both fibers suitable for papermaking.

A high value of slenderness ratio provides better forming and well-bonded paper. Generally, the acceptable value for the slenderness ratio of papermaking is more than 33 (San et al. 2016). Hence, the computed slenderness ratio for narra stemwood (37.81) and branchwood fiber (38.90) implies its suitability for paper production.

Variation in anatomical properties of narra branchwood along the length. Table 4 presents the variation in anatomical properties of narra branchwood along its length and summary of statistical analysis using one-way ANOVA. The longest fibers are observed along the middle part of the branch (second meter) while the shortest fibers are observed near the main stem (first meter). This difference is statistically significant (p-value =< 0.0001*). Branchwood diameter could be the possible cause of the variation in the length of fibers. Based from the study of Hakkila (1989), as cited by Gurau et al. (2008), the length of branchwood cells increases with branch diameter, which is related to the proportion of juvenile wood present in the branches. Small branches have proportionally more juvenile wood.



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stemwood and branchwood were presented in Table 5.

Density is the simplest indicator of the strength of wood. It is reported that a great proportion of cells with thicker cell walls and small cavities gives greater density. The computed mean density of narra branchwood is 0.63 g/cm3 at oven-dry condition. However, in the study of Alipon and Bondad (2008), the oven-dry density of narra stemwood is 0.52 g/cm3. Several studies revealed that branchwood exhibits a greater density in comparison with the density of its stemwood (Okai et al. 2003, Amoah et al. 2015a). Cells found in branchwood have gradual maturation that results to shorter and thick-walled cells which, accordingly, shows a denser branchwood than stemwood (Bowyer et al. 2003, Amoah et al. 2015b). This correlated well with the result of the fiber morphological analysis. Moreover, branchwood exhibits greater number of rays and vessels (Tsoumis 1968, Vurdu and Bensend 1980, Bowyer et al. 2003) and a higher percentage of fiber volume and longitudinal parenchyma (Vurdu and Bensend 1980, Hakkila 1989). Amoah and co-authors (2015) also claimed that branchwood is denser than stemwood due to its variation in growth and development. Due its moderately high density, narra branchwood can be utilized in making the sides and back of guitars, violins, cellos, and other string instruments.

The value obtained for the specific gravity of narra branchwood and the reported stemwood value at 12% MC is 0.70 and 0.52, respectively. The experimental specific gravity of branchwood is higher than the reported specific gravity of narra stemwood. Hence, the result correlated well with the density and fiber morphology of the branchwood. Al-Sagheer and Devi Prasad (2010) reported that economically- important wood can be categorized based on its specific gravity: very light (> 0.28), light (0.28–0.42), moderately heavy (0.42–0.56), and heavy (0.56–0.70). Thus, the wood of narra is categorized as moderately heavy to heavy. When the specific gravity is being considered, the moderate specific gravity (0.4–0.6) is desired for veneer cutting (Cabangon 2006). Tangential shrinkage in branchwood (3.59%) is lower than the tangential shrinkage in stemwood (4.5%), whereas the radial shrinkage in branchwood (3.37%) is higher than the radial shrinkage in stemwood (3.0%). The difference between the tangential and radial shrinkage values of branchwood and stemwood may be attributed to the distribution of summerwood and springwood, arrangement of microfibrils at the S2 layer, and arrangement and number of pits in the transverse direction of the cells (Bowyer et al. 2003). Shrinkage in the longitudinal direction in branchwood is 0.60 while in the study of Alipon and Bondad (2008), longitudinal shrinkage was not given because it is usually negligible in normal wood. The longitudinal shrinkage in branchwood may be attributed to the presence of tension wood, which is reported

to cause excessive shrinkage in the longitudinal direction.

The volumetric shrinkage in branchwood and stemwood is 6.35% and 7.3%, respectively. In the study of Kiaei and Roque (2015), density and volumetric shrinkage are positively and significantly associated. However, volumetric shrinkage in branchwood was found out lesser than its stemwood. This reduction in the volumetric shrinkage could be attributed to many factors, among which are the microfibrillar angle in the S2 layer of the secondary wall, heartwood and sapwood ratio (Kiaei and Roque 2015), and chemical composition of the cell wall (Drozdzek et al. 2017). According to Alipon and Bondad (2008), the raw material belongs to Group V as its average volumetric value (7.3%) is below the category’s upper limit (7.8%). Because of this, the possible end-uses of the branchwood are for high-grade furniture and cabinetry and other end-products wherein shrinkage is the most important consideration.

The experimental MOE and MOR in static bending for narra branchwood correlated well with the published means for narra stemwood. The high MOE and MOR values obtained from branchwood can be attributed to its high density. The compressive strength parallel to the grain of narra branchwood (32.6 MPa) is very low compared to the reported stemwood’s value of 86–96 MPa. This could be due to numerous medullary rays and tension wood found in branchwood, which results in a lower compressive strength parallel to the grain of a wood (Tsoumis 1968, Bowyer et al. 2003, Gurau et al. 2008). Given its high MOE and MOR, the branchwood is suitable for medium-heavy construction where strength is considered in making a certain end-product such as heavy-duty furniture and cabinets, medium-grade beams, girders, rafters, and more (Alipon and Bondad 2008).

CONCLUSIONNarra branchwood has relatively similar anatomical and physico-mechanical properties with narra stemwood. The obtained fiber indices of narra branchwood are within the standard value ranges, thus making it suitable for pulp and paper production. The average values of the physical and mechanical properties of narra branchwood also showed closely similar results with the reported values for narra stemwood. Thus, narra branchwood may be used as a substitute for narra stemwood in various uses such as for high-grade furniture and cabinetry, musical instruments, pulp and paper, production of novelty items, and wood parquet.



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ACKNOWLEDGMENTThis study was supported by the Ecosystems Research and Development Bureau of the Department of Environment and Natural Resources through the project entitled “Integrated Research and Development Grant for High Value Forest Crops.”

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Anatomical and Physico-mechanical Characterization of ...philjournalsci.dost.gov.ph/images/pdf/pjs_pdf/vol... · v). Mechanical Properties The mechanical testing was done by using