ues
of the specimens decreased to 9–35% and 7–29%, respectively.
The TS and WA values of the specimens were lower than those of
the RH particleboards but higher than those of the strandboards.
The RH particleboards made from 100% the RH particles swelled
twice as much as the strandboards made from 100% the wood
strands. Significant differences (p < 0.01) between some group
averages for the TS and WA values are shown in Table 2 as letters.
No delamination was observed between the RH core layer and the
wood strand layer of the specimens after the immersion test.
The low dimensional stability of the RH particleboards without
the wood strands showed that the bonding performance between
the RH particles was poor. The poor bondability of the RH particles
with the PF resin was attributed to the high extractive content such
as silica and wax, the high pH and buffering capacity in comparison
to the wood, and reduced thermal conductivity [15]. The silica covers
almost the entire outer layer of the RH surface which also contains
the waxy water repellent cuticle [16]. The wax and silica
layer encirculating the RH particle inhibit sufficient direct contact
between the binder and the RH particles. The wax and silica coating
of the RH inhibit the penetration of the PF resin to bond with
the active hydroxyl groups of the cellulose, resulting in the poor
adhesion between the PF resin and RH particle, which are polar
and hydrophilic in natüre [15]. For example, Lee et al. [17] reported
that pH and activation energy of water based formaldehyde resin
(urea–formaldehyde) increased with increasing content of the RH
flour in the resin (5–15 wt.%). The pH and acid buffering capacity
of the aqueous extracts from the non-wood lignocellulosic materials
are significantly higher than those of softwood, which increases
the gel time of water based formaldehyde resins and causes bonding
difficulty [16]. The outer thin waxy layer of the RH particles
also lower their wettability, which may influence the bond quality
of the water based formaldehyde resins [16]. The low wettability
decrease the number of the functional hydroxyl (OH) groups which
the lignocellulosic hydroxyl groups could serve as reaction sites
with various functional groups in the PF resin. The deleterious
influence of the silica and wax components on the dimensional
stability of wood-based composites such as particleboard and
medium density fiberboard was mentioned in previous works
[3,6–8,18].