Study And Simulation On Microcellular Polyolefin Composites Foaming Process

The development of modern science and technology requires polymer materials to have lighter and better comprehensive properties.In order to meet the needs of people for a better life,microcellular foaming polymer materials can be one of the materials that can meet the above requirements.Microcellular polymer foams is a new material with gas,due to the cells,it not only effectively reduced petrochemical raw material usage and cost,and it also owned many excellent properties,which were widely used in heat insulation,sound insulation,cushion,filtration,medical treatment,car inner decoration,packaging,children’s products,and other fields.In order to make the microcellular polymer foams have a better comprehensive performance,it is required to have a better foaming quality,such as smaller size,more numbers and evenly distributed cells etc,whose depend on the foaming process of polymer foams.Therefore,the control of foaming process is the key to obtain a better foams.Solving this problem is of great significance to the development of high-performance new materials.Presently,the research on the foaming process is mainly based on the models establishment and the analysis of the final morphology,but neither of them can truly and quantitatively reflect the formation and growth of cells in the actual foaming process,and it is difficult to control the foaming process.Therefore,this study used the simple OM-hot stage visualization device,micro-visualization device and molding injection device for polyolefin composites with different conditions,which get the characteristics of cells during foaming process(less-foamed,better-foamed and over-foamed),and combining the related models and SEM to reveal the rules of cells nucleation and growth of polymer foaming process.The mainly conclusions are as follows:(1)The foaming process of LDPE composites was studied with The simple OM-hot stage visualization device,and it was found the foaming characteristics of the three stages were revealed:in the less-foamed stage,the number of cells was small,and the cells size was small and regular.In the better-foamed stage,the cells number increased rapidly and reached stable within a short time.The cells size was large and some of them deformed.In the over-foamed stage,the cells number remained unchanged,and most of them were obviously deformed.Moreover,with the passage of time,the cells broke through or even burst.In addition,the higher the temperature is,the shorter the time of cells appearing,and the more the number of cells is,the more obvious the cells deformation is.(2)In this paper,a simple OM-hot stage visualization device,micro-visualization device and injection molding device are used to observe and characterize the foaming process of polypropylene composites under different conditions,and the results show that,no matter what methods,Polyolefin has experienced three stages with less-foamed,better-foamed and over-foamed stages in the foaming process.In the same way,for PA6 foams under injection molding,through process adjustment and control,it will also appear in three stages.(3)Be based on heterogeneous nucleation theory,the mathematical analysis were used to analyze the cells nucleation of foaming process.The results showed the critical nucleation radius was estimated in order of magnitude 10 nm in this experiment.And the cells parameters(cell numbers,mean cell diameter and size distribution)were investigated.We found cells formation at 160 ~o C enhanced slowly at less-foamed stage,and then increased rapidly at the better-foamed stage,last kept constant at the over-foamed stage,which follows Boltzmann function empirically with a higher correlation coefficient of 0.9988.Subsequently,we adopt the derivatives between cells number vs.time to discuss the cells formation.The first derivation of cell number and time revealed the cells formation rate clearly and emphasized cells formation rate was higher from less-foamed to better-foamed stage and was lower at over-foamed stage.And the second derivation of cell number and time reflected the relationship between driving force and resistance of cells formation directly,where driving force was more than resistance for cells formation at less-foamed stage and better-foamed stage,here cells formation was spontaneous.While resistance was more than driving force for cells formation at over-foamed stage and cells formation power was lower.In addition,the relationship between driving force and resistance was symmetrical with respect to a point,which indicated the influence on cells formation was identical during less-foamed,better-foamed and over-foamed stages.Ultimately,we built a relational expression between the driving force and resistance for cells formation during foaming process.(4)The cell growth process by the foaming process LDPE theoretical analysis and calculation,the results show that the relationship between the cell mean diameter and time follows Logistic function with a high correlation coefficient of 0.9594.Besides,the first derivative of cells mean diameter vs.time illustrated the cells growth rate and emphasized that the cells growth rate was greater in the less-foamed and better-foamed stages than the cells growth rate in the over-foamed stage.Furthermore,the second derivative of cells mean diameter with time reported the relationship between driving force and resistance of cells growth straightly,where driving force was stronger than resistance in the less-foamed and better-foamed stages.On contrary,resistance was stronger than driving force in the over-foamed stage,where obvious cell shape transformation could be found instead of cells expansion.Furthermore,with the relationship between driving force and resistance,which indicated the different influence on cells growth was asymmetrical during less-foamed,better-foamed and over-foamed stages.