15 августа, 2015 
Malyar In 1973, K. C. Shen of the Eastern Forest Products Laboratory in Ottawa, Canada, proposed sulfuric acid as a curing agent for SSL waferboard. The pressing conditions were the same as those of conventional PF particleboard, when poplar wafers were first sprayed with 1% of 15 to 20% sulfuric acid and then with 4 to 5% SSL powder, which adheres at the surface of the wet wood wafers. Later [9], concentrated sulfuric acid (9%) was added to the SSL before spray drying, and the powder adhered to the wax-coated wafers.
High pressing temperatures of about 205°C were also necessary, the catalytic effect of the sulfuric acid merely reducing the pressing time to that of industrial conditions for PF particleboard.
The strength properties of the boards, having an average specific gravity of 0.67 g/cm3, were measured by the torsion shear at the center plane of 1 in. x 1 in. specimens, from which the internal bond strength (tensile strength perpendicular to the surface) can be obtained by multiplication with the factor 0.7 [10]. Values obtained for internal bond strength and modulus of rupture (MOR) for dry samples as well as after 2h of boiling met the Canadian standards for particleboard [11]. The torsion shear strength and MOR of dry boards were independent of the pressing time, while the wet strength increased proportionally with the pressing time. This means that for exterior-grade requirements, distinct pressing times are necessary.
The best board properties were obtained with 1% concentrated sulfuric acid, based on dry wood particles. At higher acid concentrations the strength of dry boards decreased, while that of wet boards showed a further increase. However, charring of the wood particles takes place at acid concentrations higher than 0.9%. The thickness expansion of SSL boards obtained with 1% sulfuric acid lay between 26 and 46%, after soaking in water at 20° C for 1 week, and between 51 and 66% after 2h of boiling. These values are considerably worse than those of exterior-grade PF particleboard.
The acidity of the particleboard was found to be pH 3, after disintegration of the boards in 10 times their weight of water [12]. It has been reported that the acidity had no longer-term influence on the mechanical board properties, checked by conventional accelerated aging treatments [12]. In this case, 11-mm boards had been pressed for 6 min and a part of them had been postheated at 149°C for 2h. Of the accelerated aging treatments only 20 days of heating at 149°C showed faster aging of SSL than of PF boards, which is due to the higher acidity of the SSL boards.
On the whole, the SSL particleboard obtained by Shen cannot be compared technically with exterior-grade PF particleboard. The Shen procedure, however, has found no practical application yet, as the results obtained are still far from those obtainable with PF particleboard. In 1977 Shen mentioned that a short production trial run had been carried out at a waferboard plant; although the preliminary results were promising, additional work was still required to modify the binder formulation and production parameters to meet the requirements of plant operation, and to obtain results comparable to those of PF particleboard. The Shen system, however, is used in the manufacture of some types of hardboard (high density fiberboard).
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