Study On Structural Adjustment And Properties Of Thermoplastic Polyurethane Foam Fabricated By Supercritical Carbon Dioxide Foaming

Thermoplastic polyurethane(TPU)are linear segmented block copolymers comprised of soft and hard segments.TPU has the advantages of high elasticity,high strength and toughness,and excellent biocompatibility,which is widely used in shoes,constructions,medical instruments,etc..TPU microcellular foam is widely used because of its unique advantages such as good resilience,weight reduction,sound insulation,shock and energy absorption.The supercritical CO2 foaming is a non-toxic and environmentally friendly foaming method.TPU foaming process can be divided into H-foaming and C-foaming according to the temperature control strategy,which have not much research.The type of soft segment affects the strength and toughness of TPU,which have not much research especially in mechanical properties and foaming behavior.In addition,the cyclic compression performance is an feature to characterize the strength and elasticity of foams,and there is still no research on it.Gradient cell foams have many special properties such as sound absorption,electromagnetic shielding,etc.due to their unique cell structure.The preparation process of them is very complicated,because lacking a simple and feasible process to achieve the controllability of gradient cells.To this end,this paper made the following research.First,three kinds of TPU foams using ScCO2 as a blowing agent were prepared under H-foaming and C-foaming,and their cell structure was analyzed by SEM to study the changes of expansion ratio,cell diameter and cell density with diffferent temperature and pressure.Research shows that the expansion ratio is mainly determined by temperature.While the foam cell size and density are mainly determined by the CO2 pressure.The average cell size of the polycaprolactone-based TPU foam(PCLU)is the smallest,and the polyether-based TPU foam(PEU)is the largest.Moreover,the optimal saturation temperature in C-foaming is about 30℃ lower than that in the H-foaming.The foaming temperature required for the polycarbonate-based TPU foam(PCU)is significantly higher than that of the others.Secondly,this paper use single-variable method to study the effects of material type,cell size,foaming method and foaming ratio on the cyclic compression mechanical properties of foam,and the results show that all TPU foams exhibit excellent elasticity.The strength of the foam decreases significantly as the foaming ratio increases.As the foaming ratio increases,the resilience of foam becomes better.Moreover,PCLU has the best resilience and PCU whose elasticity is poor has the best compression mechanical properties.In addition,the TPU foam prepared by the C-foaming have better compression mechanical properties.For the TPU foams,the smaller the cell size,the better the compression mechanical properties of them.The larger the cell size,the better the elasticity of them.Finally,this paper proposes a method for preparing gradient cell foam using gradient pressure.That is,the polymer-gas homogeneous system is reached under a pressure of 5 MPa,and then the pressure is adjusted to 15 MPa at a constant temperature,causing a difference in the concentration of CO2 in polymers.then in this case,the pressure is foamed.At different times of the second pressurization(1-8min),a series of gradient cell foams with different cell sizes were obtained.The results show that the time of the second pressurization is within 1-6 minutes,and the foam shows a clear gradient cell structure.With the increase of the second pressurization time,the area of the middle large hole gradually becomes smaller,and the size of the large hole gradually increases first and then gradually becomes smaller.Through the research,three different types of TPU foam were prepared under different process parameters and foaming methods,so as to realize the adjustability of the TPU foam cell structure.And systematically studied the parameters affecting the foam cyclic compression mechanical properties,which provides experience and direction for further improvement of the TPU foams mechanical properties.A convenient and feasible method for preparing gradient cell foam is also proposed,and the controllability of gradient cell density and gradient range is realized.Finally,it provides valuable theoretical guidance for the expansion of the TPU application field.