Лако-красочные материалы — производство

In 1971, Roffael and Rauch [14,15] claimed that the curing time of SSL particleboard could be reduced and, according to Pedersen, the postheating treatment in an autoclave avoided when phenolic resins of the novolak type were added to the SSL [15]. Due to coagulations between calcium-based SSL and PF, the calcium-based SSL has to be trans­ferred into sodium-based SSL. The board properties are strongly dependent on the pH value of the glue: for example, 10% SSL, 4% novolak, and 2.1% hexamethylenetetramine were applied to dry pine wood chips to prepare 9-mm-thick boards at a pressing time of 12 min.

While the highest bending and tensile strengths were obtained between pH 5 and 7, the percentage swelling in water at 20°C, after 24 h, had a minimum at pH 3.5. For this reason, a pH value of 4.7 has been suggested by the authors as a compromise [15]. Both the mechanical strength and the dimensional stability of the particleboard can be improved by higher ratios of novolak in the glue formation or increasing pressing temperatures up to 250°C. Besides conventional contact heating in a flat press, high-frequency heating was applied, raising the temperatures in the core layer during pressing to 220°C for 1 min, which diminished the pressing time. In contrast to their publication in 1971 [15], Roffael and Rauch found in 1973 [16] that phenolic resins of the resol type also improve the binding properties of SSL in particleboard, and the percentage swelling can be improved to meet the German standard specification [17] (6% after 2 h in water at 20°C) by applying a postheating treatment at 200° C for 1 h.

The postheating treatment could be avoided when higher amounts of resol-type resin were used. In conventional PF particleboard the PF resin amounts to about 8%, based on dry wood particles. It has been found [16] that up to 33% of the resol-type adhesive in conventional PF particleboard can be substituted in the surface layers of a three-layer 22-mm board by sodium-based lignosulfonate without major deterioration in the mechanical board properties. In 20-mm one-layer particleboards at pH 9, up to 25% of the PF resin could be replaced by sodium-lignosulfonate under conventional pressing conditions, leading to particleboard meeting the German standard specification [6]. Furthermore, 10% of the PF resin in beech/plywood could be substituted by sodium — based lignosulfonate at a pressing temperature of 165°C, and up to 30% at 190°C [7]. The highest shear strength of the plywood was obtained with an adhesive formulation of pH 12 to 13.