Preparation And Properties Of Thermoplastic Polyurethane Foamed Materials With Bimodal Cell Structure

Polymeric foams have been demonstrated to be one kind of the most important polymer products for packaging,insulation,and sound absorption,due to their light weight and porous structure.The properties of polymeric foams,such as mechanical strength,heat insulation capacity and sound absorption performance,have strong dependence on cell morphology.Therefore,cell morphology evolution of polymeric foams,including cell size from micrometer to sub-micrometer and nanometer with cell distribution from uniform to gradient and bimodal,becomes an important part of the technical development in this field.Among polymeric foams,the bimodal foams which consist of two kinds of cells with significant difference in cell size attract more and more attention compared with uniform cell structure;bimodal cell structure endows superior sound absorption capacities,heat insulation and mechanical properties to conventional foams.However,fabrication of such bimodal foams is not easy.Normally,significant improvement and/or modification,in foaming process,blowing agent,or polymer composition,have to be made to obtain the bimodal structure.In this work,two kinds of special structures were designed?TPU/PDMS sea-island structure and TPU/G segregated structure?and then foamed by using supercritical carbon dioxide as foaming agent to prepare polyurethane foamed materials with bimodal cell structure.For TPU/PDMS system,the sea-island structure were formed by melt blending and then hot-pressed into the composites.Upon batch foaming using scCO₂ as blowing agent,bimodal structure with large cells of¹1?m and small cells of³?m was generated due to the different cell nucleation in two phases of TPU/PDMS blends.The effects of PDMS content and foaming condition on foaming properties,such as cell morphology,cell size,cell density and bulk density of foamed materials were investigated systemically.Futhermore,the compression behavior and sound insulation performance of the foam materials were studied.It was found that large cells were mostly formed in PDMS phase,while the small cells were mainly formed in TPU phase.In addition,compared with uniform cell structure,bimodal structure shows superior properties in compression strength and sound insulation.In TPU/graphene system,we proposed a new and simple method for fabricating the polymers foams with bimodal structure.That is,the TPU powder and graphene were dry-mixed by a ball-milling and then hot-pressed into the composites in which nanoparticles were selectively distributed at the interfaces between polymer multi-facets.Upon a batch foaming using scCO₂ as blowing agent,bimodal structure was generated due to the selective distribution and heterogeneous nucleation effect of the graphene.The particle size,hot pressing temperature,graphene content and foaming conditions of the polymer matrix were systemically studied.The results showed that the foam morphologies could be easily tuned by changing nanoparticle content,particle size of polymer powder,hot-pressing temperature and pressure,as well as foaming conditions?e.g.foaming temperature and pressure?,indicating the great simplicity and tune-ability of this method.In addition,the electrical properties and strain sensing behaviors of the foamed materials with bimodal structure were investigated.Compared with the materials with uniform cell structure,the foamed materials with bimodal structure showed higher conductivity under same graphene content.In terms of strain sensing behaviors,it showed a good stability and responsiveness under different strains,such as 1%,3%,5%,10%and 20%,which was expected to be applied to the field of pressure sensors.