SynopsisA series of polyurethane (PU) films was prepared from chain-extended hydroxypropyl lignins (CEHPL). In appearance, these films ranged from brittle and dark brown to rubbery and bronze.The thermal, mechanical, and network properties of these PUS were investigated by DMTA and DSC analysis. All films exhibited single Tg's which varied between -53" and 101"C, depending on lignin content. From swelling experiments, molecular weight between crosslinks (ac) was determined and found to vary over 2.5 orders of magnitude. The at's were related to the change in Tg that accompanied network formation. Stress-strain experiments showed a variation in Young's modulus between 7 and 1300 MPa. Most of the variation in material properties was related to lignin content and to a lesser extent to diisocyanate type, hexamethylene diisocyanate, or toluene diisocyanate. The source of the CEHPL had no effect on the observed properties. From these results it was concluded that the properties of PUS can be controlled and engineered for a wide variety of practical uses.
INTRODUCTIONHydroxypropyl lignin (HPL) has been used for the preparation of polyurethane f i l m s with glass transition temperatures (T,) between 63 and 198°C and Young's moduli between 1300 and 2400 MPa.' These polyaromatic materials were homogeneous, rigid networks. Their major limitation was poor elongation, usually below 10%. In an effort to improve this behavior and provide for a wider range of thermal and mechanical properties, a series of polyethylene glycol (PEG)2 or polybutadiene glycol (PBD)3 extended ligninbased polyurethanes was prepared by mixing two polyol components prior to croslinking. The addition of a flexible (soft) segment produced lignin-based PUS with a wide range of 2' ' ' s (38-158"C), Young's moduli (380-1670 MPa), and ultimate strain (6-43%).2 Addition of PEG was limited (by miscibility) to 17.8% of the polyol, and phase separation was not observed.2 The molecular weight of the PEG also affected the properties of the PUS. These films contained between 35 and 45% lignin. There has also been research on lignin-PEG mixtures (and solutions) as raw materials for polyurethane f i l m production.* PBD extended films3 were phase-separated even at low levels of PBD addition. The elongation properties of the PBD films were poor until the The current investigation deals with the effects of polyol composition on the thermal, mechanical, and network properties of solution-cast polyurethane films. In addition to the effects of soft segment content, differences in source of the lignin were also investigated.
EXPERIMENTAL
MaterialsLignins. An organosolv (ethanol) Polyols. Preparation and characterization of hydroxypropyl lignin (HPL)7i8 and of chain-extended hydroxypropyl lignin (CEHPL)5 polyols is described elsewhere.Diisocyanates. 1,6-Hexamethylene diisocyanate (HDI) and 2,4-Toluene diisocyanate (TDI) (practical grade) Eastman Kodak Co., Rochester, NY, were used as purchased.
MethadsFilm Preparation. CEHPL (ca. 1 g) was dissolved in 1.5 mL of methylethy...
