Study On Ultra High Molecular Weight Polyethylene By Supercritical Carbon Dioxide Batch Foaming Process

Ultra High Molecular Weight Polyethylene(UHMWPE)is a linear polyethylene with huge molecular weight.It has the same structure as polyethylene(PE),but it has more good properties than PE.In this thesis,the UHMWPE foam was developed by supercritical carbon dioxide batch foaming technology.The effects of saturation temperature,saturation pressure and different foaming routes on the average cell diameter and cell density of UHMWPE foaming samples were research.In addition,the UHMWPE modified materials were also foamed.The cell structure,cell density and cell average diameter of the modified UHMWPE foam samples were research.The effect of fluidity on UHMWPE foaming was research.The effective foaming temperature range of UHMWPE,C1 material(UHMWPE as base material,adding modifier)and C2 material was studied.The surface of the cell was scanned by scanning electron microscopy.The morphology and structure were observed.The variation of cell average diameter,cell density and expansion ratio at different saturation temperatures were analyzed in detail.The differences between different UHMWPE materials were compared.The results show that:when the saturation temperature is lower than 130?,UHMWPE can not be foamed.At 130??150?,with the increase of temperature,Expansion ratio and cell density of UHMWPE foaming samples increase first and then decrease,and both of them reach the maximum around 132.5?.Cell average diameter decreases first and then increases;C1 material foaming temperature is between 128? and 135?.With the increase of the saturation temperature,cell density decreases,but cell average diameter increases.When the temperature reaches 135?,Cell begin to collapse and rupture,expansion ratio increases first and then decreases;C2 material foaming temperature is between 126?and 130?,Temperature range is very narrower.Follow the temperature raising,Expansion ratio increase first and then decrease,cell density reduced,however,the cell size is increased.when the temperature is 127.5?,the cell begins to rupture.The supercritical C2 foaming technology is used to research the variation of cell average diameter,cell density and expansion ratio of UHMWPE,C1 modified and C2 material under different saturation pressure.When saturation pressure increases,the density of UHMWPE cells increases first and then decreases.The average diameter of the cells decreases first and then increases.Under the pressure of 13.79MPa,the cell density reaches the maximum value;the C1 and C2 materials are saturated.With increase of pressure,the cell density increases fastly,the average cell diameter decreases rapidly,and the expansion ratio of both increases first and then decreases,and the cell morphology of C1 material is still good under the pressure of 20.685MPa,but C2 foaming samples showed wrinkles under pressure of 20.685 MPa.By using different foaming processes for UHMWPE foaming,the temperature of UHMWPE foaming samples are first increased to 180?,the crystal in the UHMWPE is completely disappear,the diffusion rate of CO2 is increased,and then decreasing temperature for foaming.Experimental results show that When the saturation temperature is 120?,UHMWPE can be foamed.With the increase of saturation temperature,the cell density of UHMWPE increases first and then decreases.When the temperature is 125?,the cell density reaches the maximum value;The cell density increases with the increase of pressure.Although the melt strength of UHMWPE during foaming is reduced by increasing the temperature at first,cell size become larger,At the same time,the cell density decreased because of the absence of crystals and heterogeneous nucleation.By testing the melt index of UHMWPE,C1 and C2,the fluidity of the three materials at the same saturation temperature and same saturation pressure is obtained,C2>C1>UHMWPE.According to the experimental results,C1 and C2 are suitable for foaming.The mechanical properties of C1 and C2 foaming samples at different saturation temperature and saturation pressure were tested.The results show that the modulus of elasticity and yield stress of foaming samples decrease with the increase of temperature in the online elastic stage,and the yield stress and compressive mechanical properties of foamed materials increase with the increase of saturation pressure.