Research On Improving Properties Of TPU And Its Foams By Adding PLA And RGO

This dissertation aimed at improving properties of thermoplastic polyurethane?TPU?and its foams by adding polylactic acid?PLA?and reduced graphene oxide?rGO?.First,TPU/PLA bends were prepared,and supercritical carbon dioxide?Sc-CO₂?foaming was performed on the blends.The effect of the PLA content on phase morphology and mechanical property of the blends and cellular structure of their foams was investigated.Secondly,TPU/rGO nanocomposites were prepared via a combination of solvent-exchange and in situ chemical reduction of graphene oxide?GO?.The effects of the GO reduction and the resultant rGO exfoliation and dispersion on dielectric and mechanical properties of the prepared nanocomposites were investigated;the Sc-CO₂ foaming was performed on the nanocomposites,and the effect of the nanocomposite viscoelasticity on the cellular structure of their foams was investigated by adjusting the rGO content and foaming temperature.Finally,TPU/rGO nanocomposites were continuously prepared via water-assisted mixing extrusion?WAME?with injecting vitamin C aqueous solution.And the effect of water on in situ reduction of the GO was analyzed.With increasing the PLA content from 20 to 40 wt%,phase morphology of the TPU/PLA blends changed from sea-island to co-continuous,where small PLA and larger TPU droplets were dispersed in TPU and PLA continuous phase,respectively.Mechanical property of the TPU was enhanced and dynamic storage modulus of TPU was increased by PLA.Open cell content of TPU foam was improved by the PLA.For the foamed samples prepared at 20 MPa saturation pressure and 160°C foaming temperature,open cell content increased from 2.2%for the TPU to 26.6%for the TPU/PLA 60/40,which was related to the phase morphology of the blend and significant difference in viscoelasticity between the TPU and PLA.The TPU/rGO nanocomposites with successfully reduced GO were prepared via a combination of solvent-exchange and in situ chemical reduction of the GO by adopting vitamin C as reducing agent,and the resultant rGO layers were well exfoliated and dispersed in the TPU matrix.According to rheological testing results,it was speculated that the rGO percolative structure was developed in the TPU nanocomposites with 0.5 and 0.75 vol%rGO.Perhaps because of the percolation structure,dynamic storage moduli and melt strengths of the two samples were obviously higher.Dielectric permittivity????at low f range for the two samples exhibited strong f-dependent behavior,which suggested a possible formation of nano-capacitor network of the rGO layers.The successful reduction of the GO enhanced interfacial polarization ability of the resultant rGO,the combination of which and the rGO percolative structure leaded to much higher??of the two samples???at 1 kHz was 151 for the TPU nanocomposites with 0.75 vol%rGO,21.6 times of that of the TPU?.The higher dynamic storage moduli and melt strengths of the TPU nanocomposites with0.5 and 0.75 vol%rGO effectively restricted excessively cell growing in their foaming processes.Therefore,nanocellular structure was developed in the two foams at the foaming temperature of 180oC.The much higher dynamic storage moduli and melt strengths also suppressed cell nucleation,thus cell density of the two foams was low.Raising the foaming temperature to 200oC,the nanocellular TPU nanocomposite foams with high cell density and expansion ratio?up to 2.21×10¹22 cells/cm³ and 1.81,respectively?were developed.Dielectric loss of the TPU nanocomposites was obviously decreased by foaming,which was ascribed to that the rGO percolation structure was disturbed in the Sc-CO₂ foaming process.According to numerically simulated results of TPU melt flow in flow domains formed by different screw configurations,kneading block with a stagger angle of 90°arranging upstream of reverse-screw element could help to enhance pressure build up ability of the screw,and increased shear stress on and residence time of the melt.Tested viscosity of TPU melt under high pressure water environment showed that water promoted the plasticization of the TPU melt.This improved the crystallization and hydrogen bond formation of TPU in WAME process and therefore improved mechanical property of the prepared TPU sample.In the WAME process of TPU/GO composite,in situ thermal and chemical reductions of GO were enhanced by water,as suggested by the effective removal of carboxyl and hydroxyl groups from the GO.This was ascribed to the plasticization of water and mixing of screw.The plasticization promoted intercalation of the TPU molecular chains into the GO galleries,which increased interaction area between the TPU and GO layers and thus enhanced in situ thermal reduction of the GO.In addition,the mixing of the screw promoted dispersion of the vitamin C aqueous solution into the GO galleries.This increased the reaction area between the vitamin C and the GO and thus enhanced the in situ chemical reduction of the GO.Therefore,the dielectric property of the TPU/rGO nanocomposite was improved.