The Preparation Of Micro-nano Lignin/inorganic Composites And Their Applications To Waterborne Polyurethane

Lignin is one of the most abundant terrestrial biopolymers,accounting for approximately 30 % of all organic carbon on earth.Unfortunately,less than 2% of industrial lignin resources,mainly produced from pulping and papermaking industry,are recovered and used for chemical and material application,which results in a massive waste of bio-resources.Lignin/inorganic composites has more than three obvious advantages,such as high rigidity or stability,non-toxic and low-cost.Therefore,the preparation of lignin/inorganic composites can provide a new approach toward high value-added application of lignin-based products.However,most of the reported composites were severely aggregated and contained a little amount of lignin,which is often limited their application.In the present study,direct precipitation method,hydrothermal method and sol-gel method are reported for the preparation of well-dispersed lignin/ZnO(< 400 nm)and spherical lignin/TiO2 composites(< 200 nm)based on lignin quaternary ammonium salt(LQAs)that was obtained by quaternization of alkali lignin.The structure and composite of lignin/ZnO,lignin/TiO2 are investigated using SEM,TEM,static contact angle,FT-IR,XPS,BET,XRD,and TG-DSC.An additional goal is to determine the effect of lignin/Zn O and lignin/TiO2 composites on the morphology,mechanical properties,thermal properties,UV-absorbing capacity and UV-aging resistance of waterborne polyurethane(WPU)blended films.Finally,the interaction between composites and WPU will be proposed and the mechanism of enhancement of anti-UV aging properties will also be revealed.The main conclusions in this paper are as follows:(1)The lignin/ZnO composite with different contents of lignin(20~30 wt%)were prepared via direct precipitation method.With the increase of lignin content,the microstructure of composites changed from flake-like to flower-like morphology.These composites were coreshell structures and ZnO inside was typically wurtzite.In addition,It has been examined that the composites were formed through through hydrogen bonding,electrostatic interaction and cation-? interaction.Meanwhile,LQAs/ZnO-4 composite containing 30 wt% lignin had the average diameter of 254.8 nm and BET surface area of 128.0 m2/g.Lower band gap energy(3.20 eV)resulted in excellent UV absorption as well as photo-responsive ability.Furthermore,UV protection factor(UPF)value,the tensile strength,the elongation at break and the rapid thermal decomposition temperature of WPU-based blending film containing 0.6 wt% composite was 38.4,31.76 MPa,414.8 % and 397.1 °C.After 192 h of UV-aging test,the tensile strength and the elongation at break of the blend film was maintained above 25 MPa,360 %.(2)The lignin/ZnO composite with different contents of lignin(10~20 wt%)were prepared via hydrothermal method.These composites were hybrid spherical structures and ZnO inside was typically wurtzite.It has been examined that the composites were formed through Zn-O-C covalent bonds.Meanwhile,LQAs/Zn O-C composite containing 20 wt% lignin had the average diameter of 182.5 nm and BET surface area of 79.1 m2/g.Lower band gap energy(3.11 eV)resulted in excellent UV absorption as well as photo-responsive ability.Furthermore,UV protection factor(UPF)value,the tensile strength,the elongation at break and the rapid thermal decomposition temperature of WPU-based blending film containing 6 wt% composite was 34.7,30.76 MPa,622.1 % and 410.0 °C.After 192 h of UV-aging test,the tensile strength and the elongation at break of the blend film was maintained above 25 MPa,550 %.(3)The lignin/TiO2 composite with different contents of lignin(32~35 wt%)were prepared via sol-gel method.These composites were hybrid spherical structures and ZnO inside was typically wurtzite.In addition,It has been examined that the composites were formed through Ti-O-C covalent bonds.Meanwhile,LQAs-60/TiO2 composite containing 35 wt% lignin had the average diameter of 184.5 nm and BET surface area of 321.4 m2/g.Lower band gap energy(3.13 eV)resulted in excellent UV absorption as well as photo-responsive ability.Furthermore,the tensile strength,the elongation at break and the rapid thermal decomposition temperature of WPU-based blending film containing 6 wt% composite was 14.10 MPa,1066.4 % and 423.1 °C.After 192 h of UV-aging test,the tensile strength and the elongation at break of the blend film was maintained above 8 MPa,800 %.(4)The mechanism studies have shown that there is a hydrogen bond interaction between the-OH groups in the composite particle and the-NH groups in the WPU molecules,which resulted in "Soft segment" in the WPU molecule transforming into an ordered "Hard segment".Therefore,the mechanical properties and thermal stability of the composite films were enhanced.What's more,the composites can serve as anti-UV aging agents to retard the oxidation rate of WPU-based blending films.