Preparation And Study On The Structures And Properties Of Polypropylene Foams

Polypropylene(PP)is one kind of material with low density,chemical resistance,high temperature resistance and good mechanical properties.Along with long-term development,it is widely used in the fields of fiber,injection-molding consumer goods,industrial application,automobile,film and so on.Compared with the pure PP and its modified products,PP foaming material only accounts for a very small proportion,which is due to the low melt strength brought by its crystallization characteristics and the narrow range of foaming processing temperature.High melt strength PP by high branched chain structure or wide molecule weight distribution,high requirements for processing technology and not much research resulted in the high cost and limited application.Therefore,it is valuable to further study on the preparation method,clarify the relationship between the structure and property of PP foam material,and try some applications.Based on this,PP foam materials with different expansion ratio were prepared by continuous extrusion foaming with sc CO₂,autoclave foaming and injection molding foaming.The relationship between cell structure and properties was studied,and some promising foam materials were obtained.The main conclusions are as follows:(1)In this paper,PP foams with different expansion ratios were prepared by extrusion with sc CO₂,autoclave foaming and core-back injection molding.PP foam with 30 times,10 to 20 times and less than 50%weight reduction to 7 times can be prepared by extrusion method,autoclave method and core-back injection method,respectively.Among them,it is easy to get foamed materials with open cell structure by extrusion method,closed cell structure by autoclave method,and closed cell structure by injection method which is different with autoclave method.(2)Through the control of melt temperature and the second component,the open cell structure of high melt strength PP HMS1602 prepared by continuous extrusion foaming with sc CO₂ as foaming agent can be designed.The open cell content is high under all the PP formulas and processing parameters.According to the observation of the morphology of the cells by SEM,they are relatively closed,relatively open,flower-like/bamboo-like(cross section/longitudinal section)and hollow structure respectively.These different structures are mainly from whether there is"soft segment"component in the PP matrix and melt temperature and stretching after die.Among them,the flower-like/bamboo-like structure of the bubble structure provides a more appropriate draw resistance,and the adsorption and filtration of cigarette smoke are close to the commercialized samples,which allows us to obtain a more promising PP foam filter through the control of the cell structure.(3)Three carbon black(CB)contents PP foam beads with different densities were prepared by controlling the temperature and pressure of PP micro-pellets impregnated and absorbed by foaming agent CO₂,then discharging the PP micro-pellets together with water.And the PP foam block was formed by steam-chest molding.With the CB content up to 20%,a better reflection loss(RL)can be obtained for the foamed material in a wide frequency range,in which PPCB25 with a density of 45g/L has a good absorption capacity in the range of 3-18GHz,and the ultra-low RL value(-60d B)is obtained.The microwave absorption ability of PP/CB composite foam is mainly due to the loss of incident wave caused by CB.By analyzing the structure of the prepared foamed materials,we found that the surface impedance of the high expansion ratio PP/CB foamed materials is close to that of the air,which makes the reflection of the microwave at the interface small and not obvious.After entering the interior of the materials,the microwave needs to pass through a lot of bubble interfaces,resulting in multiple absorption and the loss of the microwave at the interface,which is a full advantage porous material structure with multiple reflection loss.(4)By using the core-back injection molding foaming process,we have prepared PP/CB foaming materials with different expansion ratio with"skin core"structure,which have better EMI shielding effectiveness.This kind of PP foam is characterized by"skin core"structure which makes the EMI shielding of foamed materials obviously divided into two kinds:reflection and absorption loss.With the increase of expansion ratio,the absorption loss increases obviously,while the reflection decline trend slows down due to the existence of“skin”.Therefore,foaming of PP/CB can effectively adjust the shielding performance of the material.When the CB content is high,foaming can improve the overall shielding effectiveness of the material.On the other hand,through the adjustment of expansion ratio/bubble structure,the absorption and reflection ratio of electromagnetic wave can be effectively adjusted.The existence of cell structure can enhance the absorption loss of electromagnetic wave,so as to reduce the environmental pollution caused by the secondary reflection of electromagnetic wave.(5)Isotactic polypropylene(PP)with?-crystal foam was prepared by core-back injection molding.With the addition of?nucleating agent,the cell structure of the foam becomes more uniform.At high mold temperature,the size of the foam is less than 10?m,and the density of the foam is almost up to 10⁹ cells/cm³.The results show that the elongation at break of?-PP foamed samples increases with the increase of mold temperature,which is related to the morphology and thickness of the foamed materials.The results show that the elongation at break of?-PP foamed samples increases with the increase of mold temperature,the thickness of the foamed materials,the decrease of the cell size and the increase of the cell density.