Lignin-polyurethane

F eldman and Lacassel studied the morphology of blended lignin-polyurethane (L-PU) sealants. The objective of the work was to establish the viability of blending lignin (L), a naturally occurring, readily available polymer resource, with polymeric based sealants. The L-PU blends were... 

Macromolecules may be classified according to different criteria. One criterion is whether the material is natural or synthetic in origin. Cellulose, lignin, starch, silk, wool, chitin, natural rubber, polypeptides (proteins), polyesters (polyhydroxybutyrate), and nucleic acids (DNA, RNA) are examples of naturally occurring polymers while polyethylene, polystyrene, polyurethanes, or polyamides are representatives of their synthetic counterparts. When natural polymers are modified by chemical conversions (cellulose —> cellulose acetate, for example), the products are called modified natural polymers. 

The routes give, using well-known condensation and radical reactions, bakelites (I), polyazophenylenes (II), polyimides (III), polyurethanes (IV), nitro compounds and polyamides (V), aromatic polyethers and polyesters (VI), polychalcones (VII), polyphenylene sulfides (IX), ammonia lignin (X), carbon fibers (XI), silicones (XII), and phosphorus esters (XIII). In addition, radiation and chemical grafting can be used to obtain polymers of theoretical interest and practical use. Although the literature on the above subject is very large, there are comprehensive summaries available (1,28,69). 

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 1. TG curves for polyurethanes (PU s) with 20 wt% of lignin and without lignin measured in air. Heating rate 10°C/min. Flow rate 15 ml/min. Solid line PU with lignin and broken line PU without lignin.

Figure 4. Relationship between logarithmic heating rate (log0) and reciprocal absolute temperature (1/T) for the degradation of polyurethane (PU) with 20 wt% of lignin in air. Weight loss, Or 5%, 10%, 15% and...

Heat-Resistant Polyurethanes from Solvofysis Lignin 387... 

Table I. Calculated Activation Energies (E s) for Thermal Degradation of Polyurethanes (PU s) and Lignin...

Elastomeric Polyurethanes from a Kraft Lignin—Polyethylene Glycol—Toluene Diisocyanate... 

Elastomeric, lignin-derived polyurethanes can be produced from a three-component system, in which a polyether diol (like PEG) is used as soft segment, provided that the crosslink density is held at a low level and that the soft segment content is high enough to keep T below room temperature. 

Effect of Soft-Segment Content on the Properties of Lignin-Based Polyurethanes... 

Although the use of lignin as an additive to polyurethanes is not new (15-20), even the most judicious selection of lignin isolation or modification schemes has not allowed researchers to overcome the incorporation limit of 25 to 40 weight percent of lignin as an active component in polyurethanes. Solvent fractionation allows for the isolation of lignin fractions with well defined solubilities and functionalities (21,22). Both of these features are critical for the practical inclusion of lignin into liquid polyol systems. 

Table I. Lignin source and soft segment type for polyurethanes discussed in this study...

KELLEY et al. Soft-Segment Content of Lignin-Based Polyurethanes 407... 

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. 

If toughness is not the limiting consideration, then the simple addition of PEG soft segment appears to offer an attractive route to the production of polyurethanes with 25 to 30 wt.% lignin. It is important to note that both of the studies considered here have employed modified lignins. The relative advantage of hydroxypropylation (16) over fractionation (19) would depend on the relative cost of either procedure. 

---