Three different hardeners (ammonium chloride, ammonium
sulphate, and aluminium sulphate) were added to the UF resin.
0.85 wt% the hardener (10 wt% water solution) for the surface layers
and 2.5 wt% the hardener (25 wt% water solution) for the core
layer based on the solid content of the UF resin were added in the
resin. The chips were placed in a blender and sprayed with urea
formaldehyde. 1 wt% paraffin (32% water solution) based on the solid
content of the UF resin was used as a water repellent chemical
in the manufacture of particleboards. As a formaldehyde scavenger,
1 wt% of urea (10 wt% water solution) based on the solid
weight of the UF resin was added to resin in the manufacture of
the some particleboard types given in Table 1. The layer composition
of particleboards was designed to consist of 34% chips at the
face layer and 66% at the core layer, and the target density of the
particleboards was 0.63 g/cm3. The mats were then subjected to
hot pressing. The boards were pressed under 2.5 N/mm2 pressure,
at 225 C, for 100 s. At the end of the press cycle, the board was removed
from the press for cooling. The particleboards having
dimensions of 2800 mm 2100 mm 18 mm were produced for
each type of formulation. A total of 24 particleboards, 2 for each
type of formulation, were produced (Table 1). Prior to the tests,
the samples were conditioned in a climatized room at 20 C and
65% relative humidity.