Preparation And Properties Of Waterborne Polyurethane/Nano-ZnO Composites

Compared with conventional organic solvent-based polyurethane, waterborne polyurethane (WPU) shows many excellent features, such as the advantages of non-pollution, nontoxicity and no flammability. On the other hand, as a multifunctional inorganic nanomaterial, nano-ZnO has wide applications, especially in the fields of photocatalytic and antibacterial materials, which are closely correlated to the health of human beings. We believe the utilization of nano-ZnO in WPU may result in some new materials with excellent properties.In the present thesis, ZnO nanostructures with different morphologies were successfully prepared by simple hydrothermal processes under different reaction conditions. The morphologies included nanorods, nanoneedles, nanosheets, nanoparticles, nanoplates and flower-like nanowhiskers. The as-prepared nano-ZnO samples were characterized by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The photocatalytic activities of the nano-ZnO samples were evaluated by the degradation of methyl orange under UV irradiation. The results indicated that ZnO nanorods and nanosheets showed efficient degradation capacity, while ZnO flower-like nanowhiskers showed the poorest degradation capacity. Furthermore, the relationship between the morphologies of nano-ZnO and the photocatalytic properties were also discussed.In order to disperse nano-ZnO in the WPU matrix better, nano-ZnO was modified withγ-aminopropyltriethoxysilane (APS). As a result, amino groups were introduced onto the surface of nano-ZnO, which laid the foundation for the further graft of isocyanate groups on the surface of nano-ZnO through the reaction between the amino groups on the surface of nano-ZnO and 3-isocyanatemethyl-3,5,5-trimethyl-cyclohexylisocyanate (IPDI). The IPDI functionalized nano-ZnO samples play a key role in the in-situ polymerization of the nanocomposites in the following steps.Waterborne polyurethane/ZnO nanorods (WPU/ZnO-rods) composites were prepared through in situ polymerization. With low loading levels (≤1.0 wt%), ZnO nanorods were homogeneously dispersed in the WPU matrix. Compared with pure WPU, the tensile strength of WPU/ZnO-rods nanocomposites (1.0 wt% ZnO nanorods) was greatly improved. However, with the increase of the content of ZnO nanorods, the thermal stability of WPU/ZnO-rods composites was decreased significantly but the water swelling character was enhanced. More importantly, the WPU/ZnO-rods composites exhibited strong antibacterial effect against Escherichia coli and Staphylococcus aureus, and the antibacterial rate reached to 99% as the ZnO nanorods content was 2.0 wt%.ZnO flower-like nanowhiskers (f-ZnO) with three-dimensional microstructure can be used as polymer additives for preparing functional composite materials due to their good comprehensive properties such as high mechanical strength, wear resistance, vibration insulation and antibacterial effect. Waterborne polyurethane/flower-like nanowhiskers (WPU/f-ZnO) composites were prepared through in situ polymerization mentioned above. The results showed the properties of WPU/f-ZnO composites, such as mechanical strength, thermal stability as well as water swelling, were strongly influenced by the content of f-ZnO in the composites. With the increase of f-ZnO content, the thermal stability of the composites decreased, however, the water resistance and antibacterial activity of the composites enhanced remarkably. When the content of f-ZnO increased to the optimum value (1.0 wt%), the tensile strength of WPU/f-ZnO composites got to its highest, and then it declined with continued increase of f-ZnO content.In order to investigate whether the morphologies of nano-ZnO affected the properties of WPU/nano-ZnO composites, we also prepared waterborne polyurethane/ZnO nanoneedles (WPU/ZnO-needles), waterborne polyurethane/ZnO nanosheets (WPU/ZnO-sheets) and waterborne polyurethane/ZnO nanoparticles (WPU/ZnO-particles) composites via in situ polymerization mentioned above. The experimental results showed the morphologies of ZnO nanostructures had effect on the properties of WPU/nano-ZnO composites.Compared to pure WPU, the tensile strength of all WPU/nano-ZnO composites with 1.0 wt% nano-ZnO loading was enhanced, of which f-ZnO with 3D structure showed the strongest reinforced effect and ZnO nanoparticles showed the weakest reinforced effect. However, the thermal stability of all WPU/nano-ZnO composites was decreased with the incorporation of ZnO nanostructures. Among the five WPU/nano-ZnO composites, the thermal stability of WPU/ZnO-needles composites was the best but WPU/ZnO-sheets composites showed the worst. Moreover, the water resistance enhancement effect of ZnO nanoparticles was the best, while the f-ZnO was the worst. Furthermore, the antibacterial activity of the WPU/nano-ZnO composites was significantly enhanced with the content of nano-ZnO increasing to 2.0 wt%. WPU/ZnO-sheets composites showed the strongest antibacterial activity, while WPU/f-ZnO composites had the weakest antibacterial activity.In summary, WPU/nano-ZnO composites with excellent mechanical strength, water resistance and antibacterial properties were successfully prepared. The effect of content and dispersion state of ZnO nanostructures in WPU matrix on the overall properties of the WPU/nano-ZnO composites was investigated. Our work provides valuable information for the study of multifunctional nanocomposites in the future.