Effect Of Condensed Structure Of TPU/EVA Blend On Supercritical CO₂ Extrusion Foaming

Thermoplastic polyurethane(TPU)has excellent characteristics such as good elasticity and cushioning properties,high strength,strong mechanical properties and good processing properties,and is widely used in various fields.However,due to the low melt strength of TPU,there are many difficulties in the physical foaming of TPU.As a non-toxic,non-polluting and non-flammable foaming agent,supercritical CO₂ has received a lot of attention in the field of foaming.However,when supercritical CO₂ is used as a physical foaming agent,the melt strength of TPU is not high,resulting in problems such as large shrinkage and unstable bubble holes in TPU foams.Therefore it is necessary to improve the melt strength of TPU,which can be enhanced by increasing the crystallinity of TPU to improve its melt strength and foaming performance.In this paper,the effects of supercritical CO₂ induced crystallization,TPU/EVA blend phase morphology evolution and crystal morphology evolution on its foaming behaviour were investigated using supercritical CO₂ as the foaming agent.Firstly,a series of TPU foams were prepared using supercritical CO₂ as the foaming agent.the crystallisation and foaming properties of the foamed samples were analysed by differential scanning calorimetry.X-ray diffractometry and scanning electron microscopy.The results showed that the crystallization behaviour of TPU was enhanced with increasing pressure of supercritical CO₂.The supercritical CO₂ induced TPU crystallization,increased the crystallinity of TPU and improved the melt viscosity of TPU.The TPU foamed samples prepared under the process conditions at 170°C and12 MPa showed good pore structure and large foaming multiplicity.Then,in order to improve the crystallinity of the polymer and enhance its melt strength,the TPU and Ethylene vinyl acetate copolymer(EVA)blend foams were prepared by a continuous extrusion foaming method using supercritical CO₂ as the physical foaming agent.The evolution of crystal morphology and changes in crystalline properties of the TPU/EVA blends were investigated.The results show that the addition of EVA improves the crystalline properties of TPU,with TE-10 having the best crystalline properties(crystallinity increased from 1.6%to 5.8%),improves the melt strength of the blends and makes TPU have good foaming properties.At the same time,supercritical CO₂ plays a nucleation role at the interface,forming spherical crystals centred on the vesicles,stabilising the vesicle structure and improving the vesicle morphology.The phase morphology of TPU/EVA blends was investigated by means of various tests in order to further investigate the influence of the phase morphology on the foaming behaviour of the blends.The EVA was added to the TPU matrix to form an incompatible blend system.The TPU/EVA foam material prepared by supercritical CO₂extrusion foaming has good cell size and cell density.Upon the addition of EVA to the TPU matrix,the phase morphology of the blend system changes from a sea-island two-phase structure to a bicontinuous phase structure.The melt viscosity and crystallinity of the blend system first increase and then decrease.Because the interface initiates the heterogeneous nucleation of cells,the cell density increases and the cell size decreases.At the same time,the crystals form spherulites centered on the cells,improving the melt strength of the blend and avoiding cell cracking.By changing the blending ratio,the number of cells and cell size can be controlled.For the EVA content of 10%,a foamed material with a cell size of 105.29?m and a cell density of 3.74×10⁶cells/cm³ is obtained.Due to the TPU/EVA phase morphology structure,the sea-island structure of the blend has better foaming properties than the bicontinuous structure.These research results provide guidance for the preparation of TPU foam materials with high cell density.