DIMENSIONAL STABILIZATION OF MEDIUM-DENSITY FIBERBOARD BY FORMALDEHYDE TREATMENT

Vaporous formaldehyde treatment (formalization) was applied for the improvement of dimensional instability of medium-density fiberboard (MDF). The MDF was treated with cyclic oxymethylenes as well as conventional reagent sources such as paraformaldehyde. The catalyses of sulfur dioxide (SO2) and hydrogen chloride (HCl) were compared from the standpoint of the dimensional stabilizing effects, reaction rates, and mechanical properties. Among the SO2-catalyzed formalizations with various reagents, tetraoxane gave the greatest antiswelling efficiency, more than 60%, which is sufficiently durable for the practical uses of MDF. The catalysis of HCl accelerated the reaction and shortened the reaction time up to within several tens of minutes. The modulus of rupture (MOR) in a dry state decreased 30 to 50% with increasing antiswelling efficiency, whereas the MOR in a water swollen state and the modulus of elasticity (MOE) increased extremely. The decrease of MOR seemed to be somewhat smaller in the HCl-catalyzed reaction than in the SO2-catalyzed one. Other catalyzers or the pre-impregnation of formaldehyde neither did increase significantly the dimensional stabilizing effects nor shorten the reaction times. It was suggested that the dimensional stabilization of MDF is attributable mainly to the interfiber bondings.