Materials and Methods : Three logs of simul were collected from Potya, Chittagong. The age of the trees was approximately 15 years. The logs were crosscut into 1.25 m length. Preparation of veneers: The logs were peeled into 1.5 mm thick veneers in the Experimental Coe Veneer Rotary Lathe Machine. The Lathe was adjusted to knife angle of 91°-30' where the bar height and the bar opening at 2.50 mm and 0.76mm respectively. The optimum setting was found suitable for veneer of 1.5mm thick. The green veneers so obtained were clipped to a dimension of 65 cm x 65cm. avoiding all types of defects like knots, decay, etc. The surface of the veneers was made smooth, clean and free from dust, resinous and oily materials. The veneers were selected with uniform thickness. Drying and treatment of veneers: Veneers were dried under shed and conditioned in a room maintained at a dry bulb temperature of 21±2oC and 65±5% relative humidity for attaining equilibrium moisture content (EMC) of 12%. The moisture content of the dried veneer sheets was checked with a moisture determination balance. The veneers were treated with hot water (60°C) and cold water (room temp 27-30°C) solution of borax-boric acid (1:1), neem leaves and mahogany seeds of different concentration (1:10, 1:20, and 1:40). The treatment duration were 20, 40 and 60 minutes for hot treatment and 1, 2 and 3 days for cold treatment. The treated veneers were solid stacked for three hours to allow diffusion of the preservative solution into the wood. These were then dried in the shed and finally in oven to a targeted moisture content to about 8%. Spreading of glue: After drying, the veneers were glued into three-ply plywood using urea formaldehyde glue of 50% solid content. The glue was extended with 20% wheat flour and catalyzed with 2% (ammonium chloride) hardener. The adhesive mix was prepared according to manufacturer’s recommendation. Glue was spread on core veneers by the spreader. The weights before and after the gluing were taken to calculate the amount of glue applied. The plywood was assembled immediately after the veneers were spread with adhesives.
Pressing: The three ply-plywood panels were hot pressed in 160 tons Williams and White Laboratory Hydraulic Hot press using the following schedule:
Glue spread : 36 gm glue mixture (double glue line on core)
Press loading : Press was closed within 2 minutes after loading
Closed cold assembly time : 2 minutes
Specific pressure : 1.40 N/mm2
Press temperature : 120oC
Press time : 5 minutes
Test sample preparation Three panels were made using each treatment combination. After cooling, the panels were trimmed to a final size of 50cm x 50 cm. The panels were then subjected to volatilization to remove free formaldehyde and low molecular resin components. The panels were then conditioned in a room maintained at a dry bulb temperature of 21±2oC and 65±5% relative humidity.
a) Water absorption : Water absorption measures the increase in weight due to general absorption of water expressed in percentage of initial weight.
W2- W1
ΔW = -----------------X 100 %
W1
Where,
W1 = Weight of sample before absorption.
W2=Weight of sample after absorption.
b) Thickness swelling (ΔT)
Thickness swelling measures the increase in thickness due to general absorption of water expressed in percentage of initial thickness.
T2- T1
ΔT = ----------X 100 %
T1
Where,
T1= Thickness of sample before immersion in water.
T2 = Thickness of sample after immersion in water.