Bonding of isocyanates to wood

There are a few other chemical reactions on the wood surface that could make important contributions. One is that of moisture on the surface of wood to form an unstable carbamic acid group that quickly decomposes to form a primary amine with evolution of carbon dioxide. The primary amine formed has active hydrogens reactive to isocyanate. Other successive reactions ensue leading first to disub-stituted ureas and then to biurets. Furthermore, isocyanate reaction with urethane to form allophanates, and trimerization of isocyanates to form isocyanurate are also possible to variable extents, under the conditions of bonding. The different reactions are summarized in Scheme 2. 

Bonding methyl, ethyl, propyl, and butyl isocyanates to wood gives good decay resistance at weight gains above about 20%. Determination of the distribution of bonded chemical with methyl isocyanate in southern pine shows that 60% of the lignin hydroxyls are substituted and 12% of the holocellulose hydroxyls are substituted at the point where resistance to biological attack occurs. 

Most investigators agree that the strength and durability of isocyanate-bound wood panels are due to the chemical reaction of the isocyanate group with wood hydroxyls as illustrated by the above equation. Thus, the multifunctional isocyanate molecule forms a chemically bonded bridge between two or more adjacent wood particles. This reaction is only one of several involving isocyanates that can and probably do occur in a hot press during formation of particleboards when isocyanate binder is used. A very important reaction is that of isocyanate with water to produce a very unstable carbamic acid which immediately decomposes to form a primary amine and COg ... 

It is possible to react an organic moiety to the hydroxyl groups on ceU waU components. This type of treatment also bulks the ceU with a permanently bonded chemical (68). Many compounds modify wood chemically. The best results are obtained by the hydroxyl groups of wood reacting under neutral or mildly alkaline conditions below 120°C. The chemical system used should be simple and must be capable of swelling the wood stmcture to facUitate penetration. The complete molecule must react quickly with wood components to yield stable chemical bonds while the treated wood retains the desirable properties of untreated wood. Anhydrides, epoxides, and isocyanates have ASE values of 60—75% at chemical weight gains of 20—30%. 

Johns [116] could show that isocyanate spreads easily on a wood surface. 4% of isocyanate give panels the results which are comparable to those of boards bonded with 8% of a phenolic resin. The good mobility of MDI is based on several parameters [140] (1) MDI contains no water, and it cannot loose its mobility during adsorption on the wood surface (2) it has a low surface tension (ca. 50 dyn/cm) as compared to water (76 dyn/cm) (3) it has a low viscosity. 

Loblolly pine modified by 1,6-diisocyanatohexane (HDI) was found to be resistant to attack by G. trabeum at a WPG of 26 % (Chen, 1992c). At 26 % WPG, 6 % of bonded chemical was lost during a 12-week soil decay test. When moist wood was used for reaction, the HDI reacted mainly to form ureas and biurets. It was stated that the decay resistance of HDI modified wood was probably due to the inability of the modified cell wall to absorb sufficient amounts of water to support decay. Although wood reacted with chloro-sulphonyl isocyanate lost only 1.3 % mass when exposed to G. trabeum in a decay test, it was reported that 50 % of the bonded chemical was lost in this test. 

Various commercial adhesives have been used to provide bond strength with nylon on the order of 250 to 1000 psi. Priming of nylon adherends with a composition based on resorcinol formaldehyde, isocyanate modified rubber, and cationic surfactants has been reported to provide improved joint strength. Some epoxy, resorcinol formaldehyde, phenol-resorcinol, and rubber-based adhesives have been found to produce satisfactory joints between nylon and metal, wood, glass, and leather. Exposure of nylon 6 to oxygen and helium plasmas for 30 s to 1 min improved the adhesion of two-part epoxy adhesives.66... 

Phenol-formaldehyde type polymers had been the only exterior-durable adhesives for wood bonding, until the recent limited use of isocyanates. Both systems are petrochemical-based. Several researchers substituted carbohydrates for part of phenolic adhesives (1-4) > producing solid, fusible novolak resins. Recently, reaction of carbohydrate acid-degradation products with phenol and formaldehyde has produced liquid resols (5). Gibbons and Wondolowski (6,7) replaced a considerable amount of phenol with carbohydrate and urea to pro-... 

Surface oxidation processes have also been used as pretreatments for improving the bonding strength of adhesives. Brink et al. [9] reported that the wet bonding strength of plywoods or particleboards manufactured using phenol formaldehyde increased after pretreatment of wood with nitric acid. Mari et al. [10] also reported that nitric acid oxidation reduced the amount of isocyanate resin adhesive required to manufacture particleboard and improved the mechanical properties and biological resistance of boards. 

Although there are many reports of reactants to achieve dimensional stabilization by ester bonding with hydroxyl groups of the wood components, an additional increase in biological resistance has been studied only for acetic anhydride [1-12] and several isocyanates [13-15] such as methyl isocyanate and allyl isocyanate. 

By taking apart the cell wall of a modified wood specimen and separating the cell wall components from one another, it is possible to determine the distribution of bonded chemicals in the cell wall polymer. It is more difficult to delignify modified wood than unmodified wood, which means that the lignin has been substituted 122, 127, 128). This is true for wood reacted with both acetic anhydride and methyl isocyanate. Table IX shows that the lignin component is always more substituted than the holocellulose components 128). This would indicate that the lignin is either more accessible for reaction than holocellulose or that it is more reactive than holocellulose. Lignin was found to be more reactive than cellulose toward acetylation 129). 

Although all the reactions discussed in this section can contribute to the cross-linking of the adhesive and the chemical bonding of the wood particles, the initial reaction of isocyanate with adsorbed water molecules on the wood surface results in immediate wetting of wood with the formation of polyurea. Even on impervious surfaces like glass or metal, the reaction of isocyanate with adsorbed water results in intimate contact after chemical reaction, including poly-... 

IR spectroscopic evidence for covalent urethane bond formation in the reaction between isocyanates and wood has been obtained. Isolation of holocellulose by the sodium chlorite method, isolation of lignin by the H2SO4 procedure, and subjecting both to IR spectroscopy indicated that isocyanates reacted with both cellulose and lignin (194),... 

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. 

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