Preparation And Characterization Of Lignin-Based Polyurethane Materials

As the second most abundant natural biopolymer, and the main source of aromatic structures on earth, lignin has drawn the attention of many scientists for several years. Due to its complexity, nonuniformity, and conjunctive bonding to other substances, lignin has been difficult to isolate without modification and difficult to convert into useful consumer products, and its structure has been difficult to determine. The lignin macromolecule carries multiple functional groups: hydroxyl groups, either phenolic or aliphatic, so it can then be used as a source of OH groups, able to react with diisocyanates to yield polyurethanes. The challenges presented in studying lignin have resulted in a vast amount of published literature about lignin-based materials such as polyurethane foam. The developments of other high-value lignin-based material have drawn much attention in the last decade.In this study, lignin-based polyurethane materials such as polyureas adhesive, polyurethane films and polyurethane nanofibers were successfully prepared use the acetic acid lignin as the basic raw material. The micromorphology, thermal properties, mechanical tensile properties and formation of lignin-based polyurethane material were characterized by scanning electron microscopy(SEM), thermogravimetric analyzer(TGA), differential scanning calorimetry(DSC), universal testing machines and fourier transform infrared spectroscopy(FT-IR).In this paper, we successfully prepared the lignin-based polyurea adhesive via the prepolymer process. High reaction rate and without a catalyst make polyurea possible as adhesive; The tensile strength of lignin-based polyureas adhesive were improved compared to pure polyureas; Control the molar ratio of NCO: NH2 to be 1.5, chose D230 as polyether amines, the polyureas adhesive reach the maximum tensile strength of 6.05 Mpa when the lignin addition was 8%.We prepared the lignin-based polyurethane films and isocyanate terminated polyurethane prepolymers via one step process. Thermogravimetric analyzer shows that both the glass transition temperature and char yield at 700℃increases with increasing the contents of lignin; The polyurethane films reach the maximum tensile strength of 9.30 Mpa when the lignin content was 20%; The tensile strength of films decreases with increasing molecular weight of the polyethylene glycol, while the elongation at break with a sharp increasing to 1570% when chose the polyglycol whose molecular weight was 4000.We chose two kinds of isocyanate components, one is blocked HDI trimers, another is polyurethane prepolymer prepared in chapter III. Lignin separately mixed with the above isocyanate to prepared lignin-based polyurethane nanofibers via electrospinning. The thermodynamic stability, mechanical tensile properties and micromorphology characteristics of the two fibers were discussed. Analysis show that blocked HDI trimers need high temperature at 180℃ to deblocked then to release the free NCO groups, while fibers were molten and the structural were damaged at such high temperture; The mechanical strength of nanofibers improved as add polyurethane prepolymer into lignin; The fibers show orientation as adding 40% contents of polyurethane prepolymer into lignin; Finally, the tensile strength lignin-based nanofibers reach 6.85 MPa and a high elongation at break of 1240% when the polyurethane prepolymer content was 50%, showing a typical high tensile elastic deformation changes. Finally, the application of lignin-based polyurethane nanofiber materials needs further investigation to give the material a broader prospect.The lignin macromolecule carries multiple functional groups: hydroxyl groups, either phenolic or aliphatic, so it can then be used as a source of OH groups, able to react with diisocyanates to yield polyurethanes. Besides, the existence of rigid benzene ring structure in lignin improve the mechanical properties and thermal stability of lignin-based polyurethane materials, to a certain extent. Meanwhile, polyurethane material shows good physical and mechanical properties and biocompatibility, which in turn improves mechanical strength and biocompatibility of the lignin-based materials.This research not only laid a theoretical foundation for the preparation and characterization of lignin-based polyurethane materials, but also shows great practerial significance and theoretical value for the active exploration of the preparation of lignin-based polyurethane nanofiber material by electrospun.