Urethanes and Composite Wood Products: A Review and Brief Research Results

Organic polymers called urethanes, have been used in synthetic fibers, sealants, foam systems, paint, coatings, and adhesive resins. Literature also exists relevant to the production of polyols (materials used to co-cure with certain urethane systems) from wood byproducts such as tall oil or treated pulping wastes. The basic chemistry of isocyanate functional molecules, which are reacted to form the product urethanes, indicate the possibility of formation of covalent bonds with lignocellulosic materials to form an isocyanate-lignocellulose composite. Research reported on the application of urethane or isocyanate reactive molecules to lignocellulose-based materials has included reactions with cellulose, treatment of paper linerboard, dimensional stabilization of wood and others. Reports on plywood bonding show a possibility for cold pressing at low pressure to yield structurally strong bonds. With respect to particleboard, lower density, good physical properties and exterior durability are advantageous. Practical problems in using these systems center on prevention of precure prior to forming the final composite, a marked tendency of urethanes to adhere tenaciously to working surfaces, and an inability to respond to release agents normally used in pressing operations. Experimental results with several isocyanate functional materials used as adhesive showed that bond strengths in excess of 2000 psi could be achieved. Resin or monomer systems, in some cases incorporating catalysts or potential crosslinking agents (wood-based polyols), were applied to maple shear blocks and were tested to failure in accordance with ASTM Standard D-905-49. Results indicate that, with optimization of such formulations, structural wood adhesives based on urethane resins can be developed.

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