Structure/Anatomy
The coconut palm is a monocotyledon; it has an erect pole-like stem and symmetrical crown; the trunk is 30-40 cm in diameter sometimes reaching a meter at the base. Once formed, it does not increase in diameter due to the absence of a cambium. The slender and branchless trunk reaches a height of 20-25 meters or more. Tall coconut varieties take 3-4 years to develop a stem above the ground. It bears leaf scars showing the insertion of the fallen leaves. The distance between leafscars indicates the rate of growth. Leafscars are closely spaced at the top and at the bottom of the trunk but distantly spaced at the middle portion.
In cross-section, the coconut stem has three distinct zones, namely the dermal, sub-dermal and the central zones with the dermal as the most peripheral portion just below the cortex, the sub-dermal between the dermal and the central zone or core. Its main anatomical elements include the fibrovascular bundles, fibrous bundles and the ground tissue.
The fibrovascular bundles consist of phloem, xylem, axial parenchyma and thick-walled schlyrenchyma fibres. The latter element serves as the palm's major mechanical support. The cell walls of the schlyrenchima fibres become progressively thicker from the centre to the cortex of the stem. The xylem is enveloped by parenchyma cells usually containing two wide vessels, a combination of wide and small vessels or clusters of several small and wide vessels.
The ground tissue is parenchymatous and its cell wall thickness decreases from cortex to the inner zone of the central cylinder.
Physical Properties
The physical properties of cocowood depend largely on its density, moisture content and shrinkage (Table 3).
The oven-dry weight-green volume or basic density of cocowood decreases with increasing height of the stem and, at any given height, increases from the core to the cortex. In addition, the basic density at any particular height increases with the age of the palm. Overall, the basic density ranges from 100kg/m3 at the top core portion to 900 kg/m3 at bottom dermal portion of old coconut palms.
The moisture content is negatively correlated with the basic density, i.e. moisture content decreases with increasing basic density and vice versa. The amount of moisture in coconut stem increases with increasing stem height and decreases from the core to the cortex. The moisture content ranges from 50% at the bottom dermal portion to 400% at the top core portion of the stem.
The dimensional stability of the wood is determined by its shrinkage or swelling which accompanies a decrease or increase in moisture content below fibre saturation point. Shrinkage and swelling cause drying defects such as checks and split. Unlike conventional wood where tangential shrinkage is almost twice the radial shrinkage, the tangential and radial shrinkage of cocowood are not significantly different.
Mechanical Properties
The mechanical properties of coconut which define its end use are positively correlated with the basic density. As a result, cocowood has been classified according to three basic density groups as follows: High density wood (dermal) 600 kg/m3 and above, medium density wood (sub-dermal) 400 kg/m3 to 599 kg/m3, and low density wood (core), below 400 kg/m3.
Table 4 presents the mechanical and related properties of the three density groups of cocowood based on green and dry samples. All values of the strength properties decrease with decreasing basic density. Except for impact bending, the values of the other mechanical properties of cocowood at 12% moisture content are significantly higher than in green condition.
The strength properties of high density cocowood compare favourably with Dipterocarpus grandiflorus, Pentacme concorta, and Shorea polysperma (Table 5) which are commonly used as structural materials for building construction. High density cocowood almost exhibits superior strength properties over the aforementioned conventional wood except modulus of elasticity which shows lower strength values as far as compression parallel to grain is concerned.
The medium density cocowood is comparable to Pentacme concorta in terms of modulus of rupture, stress at proportional limit and maximum crushing strength but it is slightly inferior in the rest of the properties. In contrast, the low density cocowood cannot compare with these wood species hence, it should be used only for non-load bearing structures.
Chemical Properties
Coconut wood is comparable to Philippine hardwood/softwood and bamboo as far as holocellulose, lignin and pentosan content are concerned. However, it contains higher ash than Philippine woods. The proximate chemical composition of coconut wood are the following: holocellulose (66.7%); lignin (25.1%) and pentosans (22.9%).
Sources FAO Report, Assessed on 22 February 2016
The coconut palm is a monocotyledon; it has an erect pole-like stem and symmetrical crown; the trunk is 30-40 cm in diameter sometimes reaching a meter at the base. Once formed, it does not increase in diameter due to the absence of a cambium. The slender and branchless trunk reaches a height of 20-25 meters or more. Tall coconut varieties take 3-4 years to develop a stem above the ground. It bears leaf scars showing the insertion of the fallen leaves. The distance between leafscars indicates the rate of growth. Leafscars are closely spaced at the top and at the bottom of the trunk but distantly spaced at the middle portion.
In cross-section, the coconut stem has three distinct zones, namely the dermal, sub-dermal and the central zones with the dermal as the most peripheral portion just below the cortex, the sub-dermal between the dermal and the central zone or core. Its main anatomical elements include the fibrovascular bundles, fibrous bundles and the ground tissue.
The fibrovascular bundles consist of phloem, xylem, axial parenchyma and thick-walled schlyrenchyma fibres. The latter element serves as the palm's major mechanical support. The cell walls of the schlyrenchima fibres become progressively thicker from the centre to the cortex of the stem. The xylem is enveloped by parenchyma cells usually containing two wide vessels, a combination of wide and small vessels or clusters of several small and wide vessels.
The ground tissue is parenchymatous and its cell wall thickness decreases from cortex to the inner zone of the central cylinder.
Physical Properties
The physical properties of cocowood depend largely on its density, moisture content and shrinkage (Table 3).
The oven-dry weight-green volume or basic density of cocowood decreases with increasing height of the stem and, at any given height, increases from the core to the cortex. In addition, the basic density at any particular height increases with the age of the palm. Overall, the basic density ranges from 100kg/m3 at the top core portion to 900 kg/m3 at bottom dermal portion of old coconut palms.
The moisture content is negatively correlated with the basic density, i.e. moisture content decreases with increasing basic density and vice versa. The amount of moisture in coconut stem increases with increasing stem height and decreases from the core to the cortex. The moisture content ranges from 50% at the bottom dermal portion to 400% at the top core portion of the stem.
The dimensional stability of the wood is determined by its shrinkage or swelling which accompanies a decrease or increase in moisture content below fibre saturation point. Shrinkage and swelling cause drying defects such as checks and split. Unlike conventional wood where tangential shrinkage is almost twice the radial shrinkage, the tangential and radial shrinkage of cocowood are not significantly different.
Mechanical Properties
The mechanical properties of coconut which define its end use are positively correlated with the basic density. As a result, cocowood has been classified according to three basic density groups as follows: High density wood (dermal) 600 kg/m3 and above, medium density wood (sub-dermal) 400 kg/m3 to 599 kg/m3, and low density wood (core), below 400 kg/m3.
Table 4 presents the mechanical and related properties of the three density groups of cocowood based on green and dry samples. All values of the strength properties decrease with decreasing basic density. Except for impact bending, the values of the other mechanical properties of cocowood at 12% moisture content are significantly higher than in green condition.
The strength properties of high density cocowood compare favourably with Dipterocarpus grandiflorus, Pentacme concorta, and Shorea polysperma (Table 5) which are commonly used as structural materials for building construction. High density cocowood almost exhibits superior strength properties over the aforementioned conventional wood except modulus of elasticity which shows lower strength values as far as compression parallel to grain is concerned.
The medium density cocowood is comparable to Pentacme concorta in terms of modulus of rupture, stress at proportional limit and maximum crushing strength but it is slightly inferior in the rest of the properties. In contrast, the low density cocowood cannot compare with these wood species hence, it should be used only for non-load bearing structures.
Chemical Properties
Coconut wood is comparable to Philippine hardwood/softwood and bamboo as far as holocellulose, lignin and pentosan content are concerned. However, it contains higher ash than Philippine woods. The proximate chemical composition of coconut wood are the following: holocellulose (66.7%); lignin (25.1%) and pentosans (22.9%).
Sources FAO Report, Assessed on 22 February 2016
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