Isocyanate boards, durability

Performance and durability of wood products bonded with isocyanate and urethane adhesives have been the subject of several investigations. The strength of composition board is generally determined by its modulus of rupture (MOR), which is a flexure to break test its modulus of elasticity (MOE), or stiffness and its internal bond strength (IB), or tensile strength. These tests are run on dry and on water-soaked or wet samples, and are described in ASTM test method D 1037. As compared to phenol-formaldehyde adhesives, composition boards prepared with isocyanate or urethane adhesives are reported to display similar or superior initial strengths. Moisture resistance of isocyanate-bound par-... 

The greatest volume of wood adhesives for structural applications are thermosetting phenol-formaldehyde (PF) or urea-formaldehyde (UF) polymers or their derivatives. PFs are the commodity for exterior applications, while UFs are the commodity for interior applications. Because each composite product and the associated process may require a somewhat different resin, many variations of PF and UF resins are available. Isocyanate crosslinking resins are used to a lesser extent than PFs or UFs in manufactured board. Although these resins impart a durable, water resistant bond suitable for exterior application, the cost per pound of adhesive is greater than for PFs or UFs. 

Applications. Isocyanates are used commercially to a minor extent in manufactured board as a specialty adhesive. They are used in particleboard, OSB, and waferboard for special products where color as well as exterior durability are important. 

The wood chips for oriented strand board (OSB) are milled so that they are dimensionally laiger in the longitudinal direction (i.e., 2-10 cm) and narrow in the radial direction (i.e., 0.1-2 cm). Because wood is anisotropic, the strands are layered with the core strands perpendicular to the surface strands, similar to the layers of veneers in plywood. The resulting board has good dimensional stability. OSB competes with plywood for sheathing and siding applications and is bonded with PFs or isocyanates for exterior durability. 

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