The value of MOR and MOE reduced as the short length fibres
and resulted in low aspect ratio [26]. According to Ayrilmis and
Buyuksari [27], MOR and MOE values increased due to the fact that
that longer fibre achieve an increased network system by themselves
and result in increased bending properties of fibreboard
[28]. As the length of overlap is shortened, the quality of the bond
between two fibres is reduced [29]. Apparently, higher degree of
hydrolysis had occurred at high pressure refining, causing a reduction
in the fibre strength, thus resulted in the lower MOR value
[23]. Nevertheless, the MOR and MOE value increased with the
increasing of digestion pressure at 7 bar probably due to the short
fibre which consequently make the mat easily compressed thus
gave better compaction. Furthermore, the presence of fine fibre,
i.e. fibrillar fine were relatively owing gap filling ability through fill
fibre-to-fibre gaps. Halvarsson et al. [19] defined fine as particle
shorter than 0.45 mm.
The IB values increased with the increasing of digestion pressure
and time, but decreased with a further increased in digestion
pressure and prolong of heating time as shown in Fig. 5. Sample
manufactured using mild digestion pressure of 3 bar and two heating
times, namely 3 and 5 min resulted in IB values of 0.52 MPa
and 0.46 MPa, respectively. At a digestion pressure of 5 bar for
3 min, the IB value of the sample was 0.75 MPa and 0.68 MPa for
5 min of heating time. On the other hand, sample manufactured
using digestion pressure of 7 bar and two heating times, namely
3 and 5 min resulted in IB values of 0.67 MPa and 0.66 MPa, respectively
due to better adhesive bonding between fibres and binder.
A study conducted by Xu et al. [23] stated that severe digestion
pressure resulted in fibres with low aspect ratio and caused increase
in the bonding area thus improved the bonding. When the
digestion pressure increased, more fine fibres were produced and
resulted in increasing of IB values [29]. Additional explanation
could be that kenaf core inner surface was greater since it had
higher intrinsic porosity, high wettability [30] and thus was more
absorbent [5]. In addition, during refining, the core is subjected to
the steam which might cause thermo decomposition and hydrolization
of the hemicelluloses components. The steam converts
and transforms the hydrolyzed hemicelluloses low molecular
weight water-soluble carbohydrates which can act as resin adhesives
for bonding [31]. Although at more severe digestion pressure,
the IB values of the samples decreased maybe due to the existence
excessively fines. Kangas and Kleen [32] stated that the surface
chemical properties of fibrillar fines rich in extractives and lignin
that influences the absorption, adhesion and strength properties
and finally interrupt the bonding properties of the sample.
4. Conclusion
The refining condition apparently affects the fibre properties
and final board properties. Digestion pressure had more pronounced
effect on the board properties as compared to that of
heating time. Mild refining condition produced long fibre with
higher aspect ratio resulted in high bending strength of the panels,
but they showed lower IB and physical properties of the sample.
On the other hand, severe refining condition produced short fibre
with low aspect ratio with higher IB and physical properties, but
lower bending strength. It was found that the refining condition
was crucial in order to having better properties of MDF. The fibres
refined with 7 bar pressure for 5 min refining resulted in ideal
mechanical and physical properties.
Acknowledgements
The author acknowledges to Universiti Putra Malaysia (UPM)
for funding under EPU scheme grant, Malaysian Palm Oil Board