Isocyanate boards

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. 

Mark Cullen is professor of medicine and public health at Yale University School of Medicine. His research interests are in occupational and environmental medicine, including isocyanate exposure in automobile-shop workers, lung cancer in people exposed to asbestos, and lead toxicity in workers. He has published several textbooks, including Clinical Occupational Medicine and Textbook of Clinical Occupational and Environmental Medicine. Dr. Cullen received his MD from Yale University and did his residency in internal medicine. He is a member of the DuPont Epidemiology Review Board, a member of the MacArthur Foundation Network on Socioeconomic Status and Health, and a corporate medical director for the Aluminum Company of America. Dr. Cullen is a member of the Institute of Medicine and served as a member of its Board on Health Sciences. 

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. 

Figure 25 Modulus of rupture of particleboards combined with chemically modified particles. U, Untreated particles A, acetylated particles B, benzylated particles SG, specific gravity of board. Note isocyanate resin was used as a binder for particles U and A. Benzylated particles were used on the surfaces of the boards. Willow Salix Arakiana koidz.) particles were used to manufacture boards.

Using an isocyanate resin, at 3% solids content, on the same aspen flakes as described above, dry internal bond strength decreased by only 2%, MOR decreased by 23%, and MOE decreased by 15% on boards made from acetylated flakes as compared to boards made from nonacetylated flakes [37]. This is just one more example of how the type and level of the resin used influenced composite board mechanical properties. 

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). 

Isocyanates. The influence of various manufacturing parameters on the properties (IB, MOE, MOR, boiled MOR) of waferboard bonded by polymeric isocyanates was examined. The amount of adhesive used was 1.5-2.25%, the press time was 1-3 min, the temperature was 177-204 °C, and the density of the boards was 0.679 g/cm. The experiments indicated that under the conditions used the National Bureau of Standards (NBS) 2-B-2 standards can be achieved (J93). 

In three-layer flakeboards based on five softwood and hardwood species of 4, 10, and 18% moisture content, with press temperature of 177 °C, 6-min press time, and 3% isocyanate binder, the results showed that moisture content of wood was the most important variable at 18% moisture level, IB and bonding properties were lowest. Species of wood influenced strongly the bonding efficiency. In almost all cases the bending properties were the key characteristic of the panel performance. Southern pine produced the boards with lowest IB (81-116 psi), and red oak gave the highest IB values (98-213 psi) (J95). 

Particleboard and other products made with isocyanates emit only little formaldehyde (IJ, but these adhesives are expensive and require expensive manufacturing procedures. In contrast, phenolic soft wood plywood is a well established product that is predominantly used for exter i or appIi cat i ons. It conta i ns forma Idehyde i n chemically strongly bonded form and also emits little formaldehyde, as shown in a later chapter in this book. In fact, under almost all common use conditions this type of board contributes not much more formaldehyde than is already present in ambient air in many urban areas. The same is true for waferboard, which has recently become popular for replacing plywood. Likewise, phenolic particleboard emits little forma Idehyde, unIess the phenoIi c res in is bIended w i th UFR. Normally, the products with highest potential for formaldehyde emission are those bonded with UFR. During the past year, approximately 300,000 metric tons of UFR have been used for panel manufacturing in the U.S. 

Parti cel board contains between 6-8 wt% UFR (2,3). In 1984 the annual production of UF-bonded particleboard was 5.5 million cubic meters (3.1 billion square feet on a 3/4 inch base) in the U.S. alone. 70% of this board was used in furniture, cabinet construction and fixtures 20% was used in conventional home construction, and 10% in the manufacture of mobile homes. According to industry sources(, in the fall of 1985 90% of the total annual production was capaole of meeting the 0.3 ppm air chamber limit set by HUD for manufactured housing stock (5). The production of UF-bonded particleboard involved 48 plants in the US. Only two plants made phenolic particleboard and only one plant produced isocyanate bonded parti cel board. 

Wood was soaked or impregnated with chemicals. After that they were set in a hot press and pressed at 160 iC/0.5-2MPa for 5-30min. [Heat-Pressed Treatment]. Comparison specimens were set in a hot oven and dried at 160 °C for 2-6hrs. [Heat-Dried Treatment]. Heat pressed material was put on top of each other or untreated material with emulsion polymer-isocyanate adhesives to make a laminated board. 

Renewable materials can be used in the preparation of polyurethanes. Glycosides of polytetrahydrofuran have been used with diisocyanates.164 Castor oil (which contains about 2.7 OH per molecule) has also been used.165 Polyols derived from epoxidized soybean oil have been used to make polyurethanes.166 Lactic acid oligomers can be used.167 Wheat Board is made from wheat straw and isocyanates.168 It is said to be lighter and as strong as and more resistant to moisture than conventional particle board. No formaldehyde is needed for this building material. 

Table VIII. Advantages Claimed for Isocyanate vs. Phenolic Adhesives in Bonding Particle Board...

There are two major disadvantages often cited in the use of isocyanate wood binders. The first is a higher raw material cost as compared to conventional binders and the second is a tendency to adhere to metal transfer plates or press platens. The latter problem has been solved on a commercial scale by producing a multilayer product with solid veneers or phenolic-bound particles as face layers. All isocyanate-bound non-veneered board can be made by treating the metal surfaces with a release material or, more recently, using a self-releasable isocyanate. 

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