Microcellular Thermoplastic Polyurethane Foam:Structural Regulation and Performance

Author:Ge Cheng Biao

Supervisor:di wen tao


Degree Year:2018





Microcellular polymeric materials have the advantages of low density,large relative modulus,low thermal conductivity,impact resistance and so on,and are widely used in food packaging,tissue engineering,thermal insulation,automobile and other fields.The cell morphology of microcellular foams,such as cell wall thickness,cell density,cell structure(open or closed cell)and cell distribution,has obvious influence on the properties of the corresponding material.Up to now,however,the existing research on microcellular foams mainly focuses on the hard polymer system rather than the thermoplastic elastomers.Additionally,there are many problems that need to be solved in the development of new microcellular foam structure and the relationship between the structure and the properties of microcellular thermoplastic elastomer foam.Consequently,this paper was based on thermoplastic polyurethanes(TPUs),and a series of microcellular TPU materials(film,sheet,block and molded material)had been regulated by using a variety of foaming methods via high-pressure fluid.Meanwhile,the mechanism of formation and the influence of these structures on the performance had been studied,which can be described as follows:(1)For the TPU materials with different geometrical shapes,various innovative physical foaming methods were used to prepare TPU microcellular film,sheet,block and molded bead material,in which the cell morphology was regulated.The first is microcellular TPU film.Restrained escape of physical foaming agent can promote cell nucleation.Hence,the CO2 escape during foaming process was restrained by bonding the gas barrier material on the TPU surface,which ensured the foamed film with 10-40μm thickness was prepared.They had the following characteristics:cell size was about 8μm;cell density in the cross-section was approximated to be 1.1×109 cells/cm3.Meanwhile,the perforated membrane was regulated in the physical foaming process via the induced nucleation mechanism on the material surface and the adjustment of the saturation pressure,the foaming temperature and the membrane thickness.The second is microcellular TPU sheet.The skin-core and the gradient structure were obtained by changing the desorption time and diffusion path in the batch foaming process.In addition,the influence of the hard segment content on the morphology of foamed materials was investigated by selecting the TPU material with same chemical composition while different hard segment content.When the hard segment content was increased to 36.1 wt%,the movement of the molecular chain and the CO2 saturation adsorption were restrained,and the microcrystalline region existed which increased the cell density of foams.Conversely,the foams had high expansion ratio under same foaming condition,when the hard segment content was reduced.Then it is microcellular TPU block.In the process of autoclave foaming,the foamed material with different cell wall thicknesses of 3.4μm to 11.3μm was obtained by adjusting the saturation temperature.Finally,it is the molded TPU bead foam.The steam chest molding process was used to mold TPU bead foams.The water vapor with 0.32 MPa pressure was enough to bond TPU bead foams,which ensured that the corresponding foams had good appearance,strong bonding force and only 0.7%shrinkage.The well interfacial bonding effect benefited from the easily diffused soft segment and the unbroken hard domain during the molding process.(2)The influence of the cell morphology on the performances of the non-chemical cross-linked TPU foams was systematically studied,and the deformation mechanism of the corresponding TPU foams under both tension and compression was discussed.The first is tensile performance.During the deformation process,these cells effectively weakened the damage degree of the hard domain and endowed the corresponding materials with excellent elasticity.For example,the cells of microcellular TPU foam with different hard segment content reduced the residual strain of 20%and the hysteresis of 50%to 10%and 30%,respectively.However,the defect of the large cell structure,the stress concentration phenomenon of the skin-core and the gradient structure,the imperfect interfacial bonding effect of the large TPU bead foam,etc,resulted in poor deformability.The second is compression performance.The compression recovery performance decreased as the density increased.For instance,along with the increased cell wall thickness from 3.4μm to11.3μm,the DSC result showed that the?Hm was gradually reduced and the hard domain was suffered from much greater damage after large deformation.However,the hard domain of low density materials was prevented from being damaged,and they exhibited good recovery performance with the aid of the easy movement of soft segment.Next is the heat insulation performance.The increased expansion ratio and the disappearance of the surface defect endowed the TPU thin film with good thermal insulation performance.The thermal conductivity was as low as 0.162 W/(m·K).Finally,it is the separation performance.For the perforated membrane prepared in this paper,the tensile strength was about 5 MPa and the break elongation was over 150%,which could make such membrane as self-standing medium to accurately separate or enrich PS microspheres.