The negative effect of the RH particles on the flexural modulus
and strength of the sandwich-structured composites was mainly
attributed to their lower length to thickness (slenderness) ratio
in comparison to the wood strands. An increase in slenderness ratio
results in a stiffer and stronger board in flexure [20]. The RH
particles were shorter and wider and their length to thickness ratio
were significantly lower than the wood strands. The flexural properties
of the specimens increased with increasing the strand content
in the face layers. This is because the RH particles with low
slenderness ratios and large specific surface area absorb too much PF resin. Another negative characteristic of the RH particles on the
flexural properties was the fact that the waxy and silica layer encirculating
the RH surface inhibit sufficient direct contact between
the binder and the RH particle. The significant amounts of silica
and waxes, which reduce its interactions through secondary forces
(e.g., hydrogen bonds) with the polar resins. The RH particleboards
(control A) showed considerably lower flexural properties compared
with the strandboard (control B) due to above mentioned
properties of the RH particles (Table 2). This result is consistent
with the findings of previous studies [3,6–8,18].