Isocyanate-bonded particleboard

The production of enzyme-lignin-bonded particleboard is much less hazardous than the production of isocyanate-bonded particleboard, for example. 

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. 

Pioneering work in the development of an isocyanate-bound particleboard in the U.S. has been conducted by the Ellingson Lumber Company. This work has resulted in a proprietary process for bonding cellulosic materials with polymeric MDI to produce a multiple-ply structure panel (7.,. 8, 9.) Advantages cited for this process include a tolerance for up to 22% moisture in the wood raw material without predrying and the ability to also include significant quantities of bark and needles. 

Table 10.3.5. Properties of Tannaphen-isocyanate resin bonded particleboards (183)...

Reactive organic chemicals can be bonded to cell wall hydroxyl groups on cellulose, hemicelluloses, and lignin. Much of our research has involved simple epoxides (1 3) and isocyanates (4), but most of our recent effort has focused on acetylation. Acetylation studies have been done using fiberboards (5f6), hardboards (7 11) particleboards (12-20), and flakeboards (21-23), using vapor phase acetylation (8,2 257, liquid phase acetylation (, ), or reaction with ketene (28). 

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. 

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. 

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 ... 

By collecting and measuring the COp evolved from a laboratory particleboard press, Wittman (J3) has calculated that 1/U to 1/3 of the isocyanate groups present, depending upon wood moisture content and binder level, are consumed in the water reaction. This means that a like amount of isocyanate must react with the resultant amine to form substituted ureas. Since at least 50%, and likely more, of the isocyanate is apparently consumed by the water reaction, chemical bonding through urethane linkages appears to be... 

The West German parent of Mobay Chemical Corporation was very active, along with Deutsche Novopan, in the development of isocyanate as a particleboard binder in Europe. The work of Sachs (12, 13, lL) and of Deppe (L, 5.), along with the aforementioned commercial developments, has provided a background for interest in isocyanate binders in North America. This interest is beginning to be reflected in the literature. For example, Hse (15.) has published papers on the development of both plywood and flakeboard adhesives which combine isocyanate and phenolic resins. In the former case, he claims adequate bonding with wetter veneers than can be used with conventional phenolic adhesive. In the case of... 

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. 

Isocyanates by themselves cannot be used to bond plywood because of the characteristic balance of rheology and rate of hardening, which are characteristic of PMDl. However, recently, they have been used as minority additives (10-30%), copolymerized during panel hot-pressing with formaldehyde-based resins such as PF and even UF to yield very strong bondlines. These combinations have been used industrially to yield excellent bonding both in plywood and in particleboard. ... 

Combinations of isocyanates with other resins to be used as particleboard binders have been reported by several research groups. Mixtures with formaldehyde were claimed to give strong, moisture-resistant bonds. Increased and very good moisture resistance were reported... 

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