I made the experiment on the relationship between the spreading of adhesive and drying Velocity of plywood under the constant drying condition (60℃,19% and 40°, 33%). Plywoods used are as follows: I ) Veneer; birch-wood (thickness 0.84 mm). II) construction ; crossbanding plywood (3ply) III) adhesives ; a) urea-formaldehyde resin (U-resin) b) urea-formaldehyde resin containing extender (Wheat flour) (U-･resin). IIII) Spreading : 15, 20, 30, 35, 40, 50 and 60 g/ft^2 V) Pressure : 15kg/<cm>^2 VI) Press temperature; 100℃ VII) Press Period : 10min VIII) assembly time ; 30min VIIII) Specimen of plywood ; 25 × 25(<cm>^2), thickness ; 0.208-0.236cm
Test pieces of about 5×5(<cm>^2) were coated with urea-formaldehyde resin to prevent the drying from four-side faces and then soaked until saturated (initial moisture content ; 90-103%).
Results obtained are as follows
( I )The dying process (moisture content u : drying time Z) of plywood are shown in Fig 1 and the relationship between log u : Z in Fig. 2. From these facts we can obviously recognize three periods of drying ; constant, first and second stage of falling rate of drying.
(II) We could not recognize the relationship between the drying velocity (du/dz) in constant rate of drying and the spreading, the adhesive of plywood (Table. 1)
(III) As the results of Fig 1-3, the drying Velocity (du/dz) in falling rate of drying was given by the following formula ;
du/dz=-1/k (u-u_<eq>)
Coefficient K and its reciprocal 1/k are given in Table 2. The value of 1/k was decreasing corresponding to increasing of spreading of adhesive in each drying condition.
(IIII) The relationship between the Vav/Vo (Vav = um = ueq, Vo = uo-ueq, Um = average moisture content, ueq = equilibrium moisture content, uo = initial morsture content) and t/((a/2)^2)(t = drying time, a = thickness of plywood) are shown in Fig. 4. The moisture diffusivity K was decreasing corresponding to increasing of spreading of urea-formaldehyde resin under the drying condition 60℃, 19%, but this relation was irregular under other conditions.