Wood adhesives -isocyanates/urethanes

Related articles include Wood adhesives - hot melts, Wood adhesives - isocyanates/ urethanes, Wood adhesives - phenolics and Wood composites - adhesives. 

Wood adhesives - isocyanates/urethanes A PIZZI Bonding to damp wood... 

In a study concerned with the decay resistance provided by isocyanate bonding to wood, the distribution of the methyl isocyanate reaction in southern pine showed that 60% of the lignin hydroxyls and 12% of the holocellulose hydroxyls are substituted at the point where resistance to biological attack occurs. Therefore, it can be surmised that the chemical bonding of wood by isocyanates through the urethane link can contribute significantly to the excellent performance of diphenylmethane-diisocyanate X or polymeric isocyanates XI as adhesive binder in particle boards (24, 25). 

A second type of urethane adhesive is currently being introduced to the U.S. construction industry. The adhesive was originally developed in Japan. The system is two component. The isocyanate component is a modified polymeric MDI. The cross-linkable component contains water with water-soluble or water-emulsifiable resins. The adhesive produces strong, water-resistant bonds and has shown promise as a possible lower cost substitute for resorcinol formaldehyde resins in the adhesion of laminated beams. The product is now being used in Japan to bond wood and various other porous substrates at the rate of about 10 million pounds per year. The first U.S. application of the product has been the bonding of wood doors. The opportunities for products of this type should be excellent in view of the ever-increasing governmental pressures on solvents. 

Isocyanates are used in the forest products industry to adhesively bond wood chips, which are then pressed to form particleboard and oriented strandboard. Urethanes are also used to fill knotholes and surface defects in finished plywood boards ( plywood patch ). These filled systems must cure rapidly and be sanded easily. 

Chem. Descrip. Aliphatic urethane diol resin modifier Uses Modifier for amino or isocyanate crosslinked coatings, elastomers, and adhesives, automotive, aerospace, wood/paper, and general industrial coatings, and inks Features Water-sol. reduces VOCs... 

Urethanes and isocyanates have found widespread use as adhesives, in both industrial and household environments. They can be used to bond a variety of substrates such as rubber, plastics, metals, wood, glass, and ceramics. Polyurethane adhesives display good hydrolytic stability and have... 

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-... 

If wood is treated with an adhesive containing an isocyanate, it is possible that these would react with hydroxyl groups on cellulose or lignin to produce urethane linkages as shown below ... 

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