Urethane adhesives wood bonding

Several large applications for one-component moisture-cure urethane adhesives are available. Polymeric MDI is an exceptional binder for wood products, such as oriented strand board and particleboard. One-component urethane windshield adhesives are used almost exclusively in both the OEM and automotive aftermarket. One-part urethane adhesives are used to assemble the sidewalls for recreational vehicles (RV s), manufactured housing, and mobile homes. In construction applications, one-part urethanes are used to bond metal doors, hardwood flooring, panels, and partitions. 

Dual-cure resins for wood composites. These resins (heat and EB irradiation systems) are based on urethane-acrylate thermoplastic adhesive demonstrate efficacy to bond wood. Eow amounfs of EB radiation were needed to produce a wood bond that survived shipping and that improved the composite modulus. The system demonstrated promise and a new direction for wood adhesives. 

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. 

Further discussion of bonding wood can be found in Wood adhesives - aminoplastics, Wood adhesives - hot melts, Wood adhesives - isocyanates/urethanes and Wood composites - adhesives. 

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

Such adhesives can be applied to adherend surfaces from ketone solution and after drying may be heat- or solvent-activated at 50-70°C. They are especially popular for vinyl and wood bonding. In this, solvent mixtures of a urethane elastomer prepolymer are mixed with a solution of the diisocyanate. This reactive mixture is applied to a substrate where it dries... 

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-... 

Ex. 6 Multranil 176 polyurethane elastomer is dissolved at 20% by weight in a mixture of dry ethyl acetate and acetone, and 8.3 phr of Mondur TM (XII) (for natural and synthetic rubber adhesion) or 5 phr of Mondur CB-75 (XXIX) (for non rubber adhesion) is added. These combinations are versatile adhesives for bonding vulcanized rubber (natural and synthetic) to itself, leather, urethane, PVC, cork, wood, etc. Cure occurs at room temperature, and faster at elevated temperatures. At room temperature bond strengths (peel) are (imme-diate/after 3 days) rubber-to-rubber, 5.5—9.0/ 25 rubber-to-shoe sole leather, 5.5—7.3/24 shoe sole leather-to-shoe sole leather, 7.3—9.0/ 24 rubber-to-shoe sole leather, 9.5/24 Ib/in. width. 

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). 

These adhesives are structural adhesives and have found widespread use in both industrial and household environments. They are designed to bond metals, plastics, rubber, wood, glass and ceramics with thin or wide gaps. They display good water resistance and have good room temperature performance but are poor at elevated temperatures. The following is the simple urethane linkage structure ... 

Similarly, a polyurethane containing polyester segments is an effective adhesive for lamination of PET film to wood. The polyester segment of polyurethane is compatible with the polyester film surface while the urethane linkages provide hydrogen bonding interaction between adhesive and substrate (Diagram 12). 

For bonding molded parts of polycarbonate to other plastics, glass, wood, aluminum, brass, steel, and other materials, a wide variety of adhesives can be used. Generally, the best results are obtained with solventless materials, such as epoxies and urethanes. 

The debate over urethane formation in the wood/pMDI bondline may soon yield to studies of the peculiar morphology of this interphase. Recall that pMDI is a low viscosity, low molecular weight, low surface tension organic liquid. Consequently, pMDI wood binders readily wet and deeply penetrate into wood, as demonstrated by Shi and Gardner [27]. In fact the deep penetration of pMDI into wood contradicts traditional views on wood adhesion. The truly polymeric wood binders sueh as phenol formaldehyde (PF) and urea-formaldehyde (UF) are formulated for only moderate levels of wood penetration overpenetration is undesirable with these resins. By traditional standards, pMDI wood binders overpenetrate and yet they perform as well or better than other wood binding thermosets. One then wonders what becomes of the resin that does not span the gap between bonded wood particles Does it polymerize into a bulk phase within wood cell lumens, providing no benefit Or does the deep penetration provide some performance gain How deep is the penetration ... 

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. 

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... 

NeoPac E-850 is a formaldehyde-free, water-based aliphatic polyester urethane-acrylic dispersion. This system was developed specifically for use in wood furniture adhesives due to its bond strength and excellent water and heat resistance. As a water-based polymer, it can be highly loaded with mineral fillers to reduce the overall water content in the formulated adhesive without loss of adhesion properties. This is a necessary requirement for most wood laminating production methods in controlling viscosity, wood penetration and faster drying. 

The joining of wood by adhesives is widely used in many fields of application. This comprises structural joints, glued-laminated timber, and coatings. In industrial processes, wood is structurally connected by formaldehyde-containing melamine or phenol resorcin resins which cure by polycondensation. Due to the very thin adhesive layers and the minimized formaldehyde content, the expected incorporation into the surrounding air is considerably less than the specified thresholds. In manually operated applications, dispersions of acrylates and polyvinyl acetate as well as urethanes are usually used to bond wood (Richter and Steiger 2005). 

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