Study On The Preparation And Properties Of Thermolplastic Polyurethane Microcellular Foams

In this paper,the thermoplastic polyurethane?TPU?microcellular foaming material was successfully prepared by using supercritical fluid as blowing agent and intermittent foaming process of pressure release method.The effects of different ratio of CO₂/N₂ foaming gas combination and different co-blowing agents on the density,microstructure,thermal properties,physical and mechanical properties of TPU foaming materials were studied.The nanocomposites were prepared by TPU with graphene,carbon nanotubes and carbon nanofibers.The effects of filler content on microstructure,foaming property,thermal property,rheological property and electrical property of TPU were studied.TPU foaming materials were prepared by intermittent foaming method with CO₂/N₂ as foaming agent and the ratio of the two as variables.The effects of different ratio of CO₂/N₂ on the foaming performance,solubility and desorption capacity,thermal performance,physical and mechanical properties and cyclic compression properties were analyzed and compared.The results show that with the increase of the proportion of CO₂,the plasticizing effect of TPU increases,which can reduce the temperature needed for foaming;the solubility and desorption rate increase;the cell size of ETPU?expanded thermoplastic polyurethane?becomes smaller and more uniform,the cell shape changes from irregular shape to polygon and fuller,the cell wall is thinner and the cell density increases;the melting point of ETPU decrease,melting range widens and melting enthalpy increase slightly.However,with the increase of the proportion of N₂,the dimensional stability of ETPU is promoted,the shrinkage rate and the compression set are decreased.In addition,components?50%CO₂-50%N₂?and?25%CO₂-75%N₂?can be used to prepare ETPU with lower density,that have better performance in resilience and cyclic compression property.The ETPU beads prepared by component?100%CO₂?have good bonding quality by steam molding,and the tensile strength,tear and split tear strength of the formed ETPU sheet are the largest.In order to improve the efficiency of supercritical N₂ foaming process and perfect the cellular microstructure of ETPU,the synergistic effects of cyclopentane,HCFC-141B,ionic liquid and supercritical N₂ on the properties of TPU microcellular foams were studied.The results showed that the density and cell structure of ETPU could be improved by three kinds of co-blowing agents.Among,the effect of HCFC-141B was the most significant because of its maximum plasticizing effect on TPU,which lead to the final density of ETPU decreased from 0.175g/cm³ to0.139g/cm³,the expanded ratio increased from 6.2 to 8.1,the cell size decreased from117.9um to 19.6um and the cell density increased obviously.DSC analysis shows that cyclopentane has the greatest influence on the thermal properties,which makes the melting point of ETPU decrease visibly and the crystallinity increase.The results of mechanical tests show that the three kinds of co-blowing agents reduce the hardness of ETPU,enhance the resilience,but the compression set is increased.What's more,cyclopentane can plays a positive role in the bonding quality of ETPU beads by steam molding,increasing the tensile strength,tear strength and spilt tear strength,while HCFC-141B reduces the strength of ETPU sheet,but its samples have better cyclic compression performance.The novel TPU nanocomposites and foams were prepared by means of melt blending and supercritical foaming.The effects of G-CNF-CNT?GFT?content on foaming property,thermal property,rheological property and electrical property were studied.The results show that the best foaming temperature of nanocomposites is about 162.5?.A small amount of GFT?0.5%,1%?can promote cell nucleation,improve cellular structure and obtain TPU foam with lower density.The rheological analysis shows that the storage modulus and complex viscosity of nanocomposites increase with the increase of GFT content,and the loss factor decreases.From the thermal properties,with the increase of GFT content,the melting point of the sample decreases,the melting range broadens,the crystallization temperature increases,the crystallinity decreases,and the thermal stability is improved.At the same time,the addition of GFT fillers can improve the electrical conductivity of TPU obviously.The volume resistivity of nanocomposites with content of 4%GFT can be as low as8.934×10⁴?cm,which is 8 orders of magnitude lower than that of pure TPU foam.However,the volume of sample after foaming increases,resulting in disconnection of partial conduction paths or conductive network,which has a negative effect on electrical conductivity.