Adhesive lignin

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... 

The main natural resins used as wood panel binders are vegetal tannin adhesives, lignin adhesives and more recently also soy protein adhesives [1]. Of these, tannin-based adhesives have been used commercially the longest, since 1971. They offer the advantage over the other two types of not needing any reinforcement with an oil-derived synthetic resin [1]. Lignin [2-5] and soy binders [1, 6-8], however, still require between 20% and 40% of the total resin to be either phenol-formaldehyde or most often PMDI (polymeric isocyanate) to satisfy the requirements of relevant board standards. 

Applications. These materials are stiU in developmental infancy. Current production is limited to one commercial process in Europe and a demonstration-scale process in North America. The lignins produced in these processes have potential appHcation in wood adhesives, as flame retardants (qv), as slow-release agents for agricultural and pharmaceutical products, as surfactants (qv), as antioxidants (qv), as asphalt extenders, and as a raw material source for lignin-derived chemicals. 

Wood (qv) is arguably the oldest building material used by humans to constmct their dweUings. It is a natural product obtained from trees, used in both stmctural and decorative appHcations. The chemical composition of wood is largely cellulose (qv) and lignin (qv). Today there are a variety of composite or reconstituted wood products, such as plywood, particle board, wood fiber boards, and laminated stmctural beams, where small pieces of wood or wood fiber are combined with adhesives to make larger sheets or boards (see Laminates). 

Nat. adhesives natural adhesives (tannin, lignin, carbohydrates). 

Use of lignins as adhesive without adding other synthetic resins... 

The application of lignin as an adhesive is possible in principle. The first attempt needed very long press times due to the low reactivity (Pedersen process) [161]. This process was based on lignin polycondensation under strong acidic conditions, which led to considerable corrosion problems in the plant [161]. The particles had been sprayed with spent sulfite liquor (pH = 3-4) and pressed at 180°C. After this step, the boards were tempered in an autoclave under pressure at 170-200°C, whereby the sulfite liquor became insoluble after splitting off water and SO2. 

Nimz, H.H., Lignin-based wood adhesives. In Pizzi, A. (Eds.), Wood Adhesives Chemistry and Technology, Ch. 5. Marcel Dekker Inc., New York, pp. 247-288, 1983. 

Lignins from waste hquors are also used as extenders for phenoplasts and in adhesives for laminated paper and board, hnoleum pastes, animal feedstuff pellets, etc. 

Chen (29) found that the amount of sulfuric acid directly determines the hardening time in the acid condensation of spent sulfite liquors used in plywood and veneers. However, in general the adhesives based purely on acid condensed lignins have often been found to be an uneconomic and qualitatively inferior alternative to adhesives based on synthetic polymers and phenol or lignin-formaldehyde resins. 

A considerable amount of data has accumulated regarding the modification of lignins to engineering plastics. Unfortunately, the incorporation of various monomers and polymers, such as di- and polyvalent epoxyphenols, esters and isocyanates, in the lignin structure in most cases resulted in brittle or tarry materials whose properties designated them as potential adhesives, lacquers, dispersants and films, but not as structural materials (36-40). 

When a new method is reported for synthesis of polymers, in a short time it is also tried on lignins. New lignin-based raw materials are also constantly appearing the use of steam exploded hardwood lignin for making plywood adhesives has recently been explored by Gardner and Sellers (64) and found promising in this intensively competitive area. 

The efficiency of ethylene glycol-water as a delignifying solvent has been demonstrated by Gast and Puls (10). Results showed that sufficiently delignified pulps could be obtained. Also, the lignins produced showed promising results as extenders in phenolic resin adhesives. 

Recent work has concentrated on the use of ALCELL lignin as a substitute for phenol-formaldehyde resins in wood adhesives, particularly wafer-board. Some of the results obtained when a PF resin (Bakelite 9111) was replaced with different levels of hardwood ALCELL lignin in waferboard manufacture will be briefly discussed below. Table III shows the conditions used for waferboard manufacture. 

Some of the most recent work has focused on evaluations of ALCELL lignins outside the pH range initially shown in Figure 6. When ALCELL lignin replaced 30% of the PF resin and its alkalinity or the acidity were increased, dry MOR values above 90% of the MOR of the control (100% PF resin) were obtained. This shows the importance of pH on performance. Future work will include a more detailed look at formulations, pressing conditions, and derivatization as a way of obtaining a better adhesive. 

The objective of this work was to demonstrate the utility of organosolv red oak lignin (a projected cheaper polyphenol than phenol) in phenolic adhesives for wood composites. This work involved three stages ... 

Wood Adhesives from Phenolysis Lignin A Way To Use Lignin from Steam-Explosion Process Iliro-Kuni Ono and Kenichi Sudo... 

The steam explosion process is a recent development in wood processing (1,2). Much attention has been paid to this process from the viewpoint of total wood utilization. Cellulose and hemicellulose from this process can be converted into sugars of commercial value by enzymatic methods (3). However, the conversion of lignin from this process (steam explosion lignin) into useful materials continues to present difficulties. Preparation of adhesives from it is considered to be a feasible way to solve this problem. 

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