Particleboard lignin

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). 

One technique being used is to bond solid wood with monomeric materials that are highly reactive with hydroxyl groups such as those in the wood fiber wall. For example, Schoring et al. (65) synthesized particleboards using as a binder hexamethylene diamine and polyvinyl chloride at 140°C. They postulated that the polyfunctional amine covalently linked the wood components, especially lignin, to the polyvinyl chloride. 

Potutkin and associates (14) report changes in the lignin isolated from hot-pressed particleboards compared to that from raw birchwood from which the boards were made. Lignin isolated from boards pressed at 180C showed increased infrared absorption at 800 cirri and 1300-1500 cm"1, which is interpreted to indicate condensation and demethoxylation, respectively. An appearance of a new band at 1370 cm""1 is assigned to aryl-alkyl ether bonds formed by reactions of lignin with other wood breakdown products to yield pseudolignin. 

The filler is usually "furafil," (a ligno-cellulose by-product of furfural production of Quaker Oats Company), Douglas fir or alder bark, wood particleboard sander dust and/or attapulgite clay. A sulfite paper mill lignin is also used as a filler in the Northwest. Between 50,000 and 60,000 tons of filler were used in phenolic glue mixes per year. 

Our houses are built of wood (cellulose and lignin), sheathed with particleboard (wood chips pressed with plastic resin), wrapped with plastic sheeting, clad with siding (vinyl), and decorated with plastic shutters. Buildings with wood siding are covered with paint. Inside, water flows through plastic pipes, the floors are covered with tiles (vinyl) or carpeting (polyester, nylon), the walls are covered with wallpaper (vinyl) or painted (acrylics), and the... 

There have been many attempts to replace these resins with lignin derivatives for wood composite adhesives suitable for plywood, particleboard and waferboard. Most of these studies have been empirical in nature, and few have achieved further consideration for industrial application. As wood binders, technical lignins are variable in quality and poorly reactive in comparison to conventional resin systems such as phenol-formaldehyde (PF) resins. Consequently, they are not utilized on their own. Indeed, if they were, this would adversely affect production quality and times, and necessitate equipment changes. In the wood composite industry, resins having such deleterious effects are not likely to be used even if savings could be made in terms of material costs. 

The use of lignin, essentially as a diluent (extender), has also been investigated (75). In this case, di- or polyisocyanates were reacted with ethylene or propylene carbonates in a solution containing lignin. These mixed ethylene and propylene carbonate-containing organic polyisocyanates were suitable as particleboard adhesives. 

Soda Bagasse Lignin Adhesives for Particleboard Preliminary Results... 

In this chapter, the development of a thermosetting adhesive from soda bagasse lignin is described. The research has concentrated on the development of interior-grade adhesives for particleboard. The local market for exterior boards is smaller than that for the interior panels, and adhesives for exterior boards are already covered by an excellent range of tannin-based adhesives. 

Table IV. Internal Bond Strength of Particleboard, 12-mm-Thick, Prepared from Soda Bagasse Lignin Adhesives (10% on Dry Chips) After Pressing at 170 °C for 15 Minutes...

The moisture dependence of the two PF-extended lignin adhesives was subsequently determined. Single-layer boards were prepared with the resinated chips at different moisture contents. The adhesive content (10%) and press conditions (170 °C, 15 min.) were kept constant. The results presented in Figure 4 indicate the deleterious effect of high moisture contents of the chips on the strength of the boards. The application of the lignin particleboard adhesive thus clearly requires strict control of the moisture content of the chips. 

Figure 5. Strength of three-layer particleboard (20% surface chips) glued with hydroxymethylated bagasse lignin/commercial PF resin (67 33) and pressed at 170 to 180 °C for different times, 12% chip moisture content, final density 680 kg/m3.

Finally, it should be stated that the processes that use lignin alone as thermosetting resin in particleboards 4-6) did not find industrial application, whereas, the processes using lignin in combination with synthetic resins 2,7) are economically feasible though the remaining proportion of synthetic resin is still relatively high (60%). 

Though the contact area between adhesive and wood is much larger in particleboards than in wood laminates, only 75% of the tensile strength was obtained when the lignin-based adhesive was applied as thermosetting system in particleboards (Table II). The following reasons may be applicable ... 

Table II. Versal Tensile Strength of Lignin-Based Adhesive1 Used to Bond Pairs of Wood Laminates and Wood Chips (Particleboard)...

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

The phenoloxidase-lignin-bonded particleboards are free of any emission. 

Forss, K. Fuhrmann, A. Karatex-the lignin-based adhesive for plywood, particleboard and fiberboard. Pa-peri ja Pun. 1976, 11, 817-824. 

Pizzi and colleagues (Chapter 7) have cleverly taken advantage of the special circumstances offered in the kraft lignins from bagasse to develop both cold-setting wood laminating and particleboard adhesives. Lignin oxidations... 

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