Lignin isocyanates

However, lignin may find application as a suitable wood-composite adhesive in adhesive formulations if chemically modified in some manner. In this respect, the recent developments with soda bagasse lignin-resorcinol-formaldehyde and lignin-isocyanate adhesives allow for some cautious optimism for the future. 

A study of the reaction kinetics of the reaction of butyl isocyanate with wood has been performed (West and Banks, 1986 West and Banks, 1987). Reactions were performed without catalyst and using pyridine, triethylamine, 1,4-diazobicyclo [2,2,2-octane] or di-butyl-tin-diacetate as catalyst. The data showed that no catalyst was effective without the presence of a swelling solvent. Kinetic profiles were obtained, which were deconvo-luted to yield two component reaction curves. It was considered that these two curves represented reaction with lignin and the holocellulose component of the cell wall. 

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). 

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). 

Polyurethanes (PU s). SL and 4,4 -diphenylmethane diisocyanate (MDI) were dissolved in tetrahydrofuran (THF), and the solution was stirred for 1 hr at 60°C. A THF solution of polyethylene glycol (PEG 400) and diethyl bis(2-hydroxyethyl)aminomethylphosphonate (polyol containing phosphorous) was added to the reaction mixture, and the reaction time was extended for 1 hr. In all reactions, the molar ratio of the total amount of isocyanate groups to the total amount of hydroxyl groups (NCO/OH) was maintained at 1.2. The lignin content in PU was 20 wt%. Each solution was drawn on a glass plate, and allowed to dry for 48 hr. The residual solvent in a sample was removed under vacuum and curing of each PU film was carried out at 120°C for 3 hr under a pressure of 50 kg/cm2. 

Figure 2. Effect of lignin content on the glass transition temperature (T ) of networks crosslinked with aromatic isocyanate. Formulation type A ( ) type B ( ) and type C ( ).

In most cases the Ta values are related directly to the lignin content of the network. Soft segment-free networks show more scatter than those containing alkylether components, and this may be attributed to the crosslink density of the materials. The method of soft segment incorporation, simple addition or covalently bonded, has a minimal impact on Ts. A comparison of the HDI and aromatic isocyanate-crosslinked networks, at a given lignin content, shows that the HDI-crosslinked polyurethanes generally have a lower Tg than the aromatic isocyanate-crosslinked networks. 

The development of useful adhesive compositions based on the interaction of isocyanate resins with natural polymers such as lignin, proteins, and carbohydrates. 

In real terms, though, the problems that kraft (and other lignins) face are those of poor reactivity, product variability, and discoloration. In order for more reactive kraft lignin adhesives to be made, they must be chemically modified in some manner. This can be done by a variety of means (e.g., introduction of reactive crosslinking sites onto the lignin molecules by means of hydroxymethy-lation, epoxidation, isocyanation, and the like). 

Steam-explosion lignins have received some attention as adhesives mainly because this process offers some potential for utilizing low-quality hardwoods. Like kraft lignin, these preparations also require activation [e.g., by hydroxymethy-lation (70), isocyanation (75,80)] to achieve any type of acceptable wood composite adhesive properties. 

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

Reactions of alkoxylated lignin with diisocyanates produce thermoset materials because the lignin polyol is always polyfunctional with a functionality greater than 2. The isocyanate-alcohol reaction produces a urethane linkage that when repeated creates a crosslinked, nonreformable polyurethane. This is shown in Fig. 6. A broad spectrum of lignin-based urethanes have been made and tested. The data show that these materials match if not exceed the properties of synthetic polyurethanes made without lignin [60]. 

The alkoxlyated lignin requires an isocyanate to hydroxyl group ratio of 1.5 to form effective networks. This is a significantly higher ratio of expensive isocyanate to hydroxyl units than is used in commercial practice. The work... 

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