In order to improve durability of strength properties and dimensional stability perpendicular to wood grain, polyurethane resin-impregnated 3-ply plywood type composites were made of veneers in same thickness by using isocyanate of TDI-TMP adduct and ethylene glycol adipate. Ten to thirty percent amounts of propylene carbonate were added to polyurethane-forming systems in order to promote the penetration of resin into wood. The systems were formulated with an excess NCO-content for post-curing. Samples were impregnated with a solution formulated by the polyurethane-forming systems and pressed at 120°C for 30min after 24 hours cold pressing. For evaluating the combined effect of post cure and deterioration, the cured samples were soaked in cold water, boiling water, or a 10 percent aqueous solution of hydrochloric acid or caustic alkali. The impregnated quantities were varied substantially from 90 to 160 percent, including remaining diluent, depending upon aging or deteriorating treatments.
Soaking in boiling water-and dry treatment, followed by volatilizing and leaching of diluent, increased the moduli of rupture and elasticity (MOR, MOE) perpendicular to or parallel to the surfacial fiber direction. The tensile strength of polyurethane resin film treated with soaking in boiling water showed a considerably higher value than the untreated one. The aging treatment gave much higher elasticity than the unimpregnated and untreated one.
Acid or alkali treatments imparted much resistance to the resin impregnated samples. However, MOR and MOE of the samples subjected to weathering were less than those subjected to aging treatment.
MOE perpendicular to the surfacial fiber direction was apparently related to diluent-excluding but clearly affected by the heat-stabilized and chemical reactivity of NCO-group. Some values of MOE of the samples subjected to weathering were surely higher than the untreated ones.
The tensile-shear adhesive strength of the impregnated samples soaked in boiling water showed a considerably high value when propylene carbonate was added to resin. Other treatments also made an increase in adhesive strength.
Swelling per unit moisture content change of the impregnated samples could be reduced significantly by each treatment except the caustic alkali treatment, accompanying reduction in equilibrium moisture content.
Even if each treatment showed some aging and/or deteriorating effects, the combination of polyurethane resin in excess of isocyanate compound and crossed structure of veneers improved the mechanical properties perpendicular to grain, dimensional stability and bonding properties.