Properties And Research Of Thermoplastic Polyurethane Elastomer-based Bead Foams

Thermoplastic polyurethane(TPU)has the advantages of good flexibility and wear resistance,it has been widely used in sports equipment,shoes,automobiles and other fields.The emergence of expanded TPU(ETPU)has expanded the application range of bead foam materials.However,the bead foam products prepared by steam-chest molding have defects of gaps between beads,which affect the mechanical properties of the products.In addition,the isolated structure of the beads is solid,which increases the weight of the product.In this paper,the TPU bead blend foam prepared by solution impregnation combined with supercritical fluid(SCF)foaming technology solved the above problems and explored methods to reduce its shrinkage rate.The main research contents are as follows:The foaming properties of TPU1170A,TPU1180A and TPU1185A beads under 9groups of foaming process parameters were studied.The results show that with the increase of foaming temperature,the cell diameter of TPU foam beads increases,the cell density decreases,the thickness of the cell wall becomes thinner,and the expansion ratio increases.As the foaming pressure increases,the cell diameter decreases,the cell density increases,the cell wall becomes thinner,and the expansion ratio becomes larger.Comparing the cell structure of different grades of TPU foam beads,it is found that the higher content of hard segment(HS),the smaller cell diameter,the greater cell density,the thicker cell wall,and the lower shrinkage rate of the foam beads.Therefore,through the exploration of the above foaming process,the intermediate foaming process parameters(90°C,15 MPa,2 h)and the TPU1170A beads and TPU1185A beads with the largest difference in foaming performance were selected as the foaming conditions and materials of the TPU bead blend.Four different types of TPUbx/TPUcy bead blend foams(b and c represent bead phase and continuous phase,respectively.x and y represent different grades of TPU,respectively)with microcellular segregated network structure(MSNS)were prepared by solution impregnation combined with SCF foaming.The results showed that the interface was well combined,and a bimodal cell structure was produced;the continuous phase constituted a MSNS.Among them,the TPUb70/TPUc85 bead mixture foam had the most obvious bimodal cell structure.TPUb70/TPUc70 bead blend foam has the highest expansion ratio(13.35 times)and the lowest density(0.059 g/cm~3).All TPU blend foams exhibited good springback performance and thermal insulation,among which TPUb70/TPUc70 bead blend foam had the lowest hysteresis loss(3.8%)and the lowest thermal conductivity(0.037 W/m/K).In order to reduce the shrinkage rate of TPU bead blend foams,biodegradable poly(butylene succinate)(PBS)plastic was selected to be added to the continuous phase skeleton.TPU based bead blend foam was prepared by adjusting the proportion of PBS(10%,30%,50%).Firstly,TPU/PBS mixture was prepared by solution method,and the foaming properties of PBS and TPU/PBS mixture were studied.The experimental results show that the cell structure of PBS was destroyed when the foaming temperature and pressure were high.The cell walls of the TPU/PBS mixture thickened with increasing PBS content.For TPU/PBS bead blend foams,the expansion ratio(12.07,10.31,9.03 times)and shrinkage rate(74.70,73.28,67.51%)gradually decreased with the increase of PBS content,indicating that the shrinkage of TPU/PBS bead blend foams can be effectively reduced and a more dimensionally stable foam can be obtained by adding PBS to the continuous phase.