Study Of Preparation And Properties Of Polyether-based Thermoplastic Polyurethane Foamed Beads

The preparation and the effect of processing conditions on the properties of polyether-based thermoplastic polyurethane（TPU） foamed beads produced with a bath setup using high-pressure autoclave were studied in this article. Differing with conditional methods,the foamed beads were not cooled with cold water after depressurization in order to increase their expansion ratio. In addition to single-step depressurization, double-step depressurization was innovatively carried out in our methods. Instead of conditional physical foaming agent（i.e. butane）, supercritical carbon dioxide was used as foaming agent because of its non-toxicity, environmental friendly and non-residue. Besides, the mixture of NH₃ and CO₂ was also used as foaming agent to study its effect on foamed beads. The polyether-based TPU was purchased from industry market and used as received without any blending process. The properties of TPU foamed beads investigated in this article included appearance, expansion ratio, cell morphology, cell size, cell number density, cell nuclei and thermal behavior.Firstly, the bead foaming behaviors of polyether-based TPU were studied with the purpose to know its suitable processing parameters and effects of processing parameters on the properties of these foamed beads. It was found that, polyether-based TPU foamed beads with excellent appearance and closed cell could be produced. Every saturation pressure had its corresponding saturation temperature window. The best temperature window for saturation pressure of 10 MPa and 12 MPa is 95-100℃ and 85-95℃, respectively.When the double-step depressurization was applied, it was found that, the foamed beads with excellent appearance could be produced with the temperature window 60-90℃. Compared with single-step depressurization processing, the double-step depressurization could promote the cell nucleation especially under low saturation temperature. When the second pressure drop was kept constant at 10 MPa, the best first pressure drop and saturation temperature was 4MPa and 80℃, respectively. When the saturation pressure was kept constant at 12 MPa, the best first pressure drop and saturation temperature was 2MPa and 80℃, respectively.The existence of NH₃ could improve the shrinkage of foamed beads at some degree and decreased the shrinkage retio of foamed beads. When the mixture of NH₃ and CO₂ was used as foaming agent, the foamed beads had smallest shrinkage ratio at the saturation pressure of 10 MPa.