Research On High Pressure Foam Injection Molding And Performance Of Large Expansion Polypropylene Foam

Due to light weight,high specific strength,good impact resistence,and superior sound and thermal insulation,as well as other process characteristics,such as simple process,low cost,good machinability,polymer foams have been widely used in many fields,which are aeronautics and astronautics,automobile,tissue engineering,building,transportation,packaging and so on.Comparing with other commom plastics,such as polystyrene and polyethylene,polypropylene foams are drawing more importance due to their advantageous characteristics,which are excellent recyclability,environmental friendliness,heat resisitance and mechincal properties.However,the polypropylene generally exhibiets low melt viscoelasticity,crystallinity and crystallization temperature,leading to poor foams with low expansion ratio and cell density,large cell size.The methods to improve the foaming ability of polypropylene are including crosslingking,branching,blending with polymer and crystal nucleation agents,but each has its advantages and disadvantages.Moreover,batch,extrusion,bead foaming and foam injection molding are the common technologies to manfacture foams.In comparation,foam injection molding has its suriorities,including high efficiency,good quality stability,and high geometrical complexity.Therefore,it is the significance of studying the key technology of foam injection moldin with polyproplyene,to fabricate high-quality polymer foams and further to extend the application of polymer foams,which can efficiently promote the saving of materials and cost,suppression of environmental pollution,and show great potential to achieve global energy and economics sustainability.In this study,the mechanism of high-pressure foam injection molding and performance of polypropylene foam were discussed,and based on the cell nucleation machenism,the in-situ fibrillation and branching methods were applied to improve the melt viscoelasticity and strength,achieving the large expiation-ratio polypropylene foams with high thermal inslation.The main contents of this paper include the following aspects:The major drawback of classical cell nucleation theory?CNT?is that it ignores elastic strain energy's effect on cell nucleation.Based on the CNT,together with the von mises yield criterion,the melt elastic strain energy was introduced in the cell nucleation process,and the change of Gibs free energy during the cell nucleation was calculated.Then,the driving force and resistance of cell nucleation was analyzed,and the effect of elastic strain energy on cell nucleation was discussed.The uniaxial stretching-assisted foaming process and the uniaxial compressing-assisted foaming process were explored and conducted to investigate the effect of the stretching strain energy and the compressive strain energy on cell morphology,respectively,which experimentally verified the mechesim of elastic strain enerfy on cell nucleation.Based on the rheological properties and crystallization behavior of linear polypropylene?LPP?,the low-pressure and high-pressure foam injection molding experiments with LPP were designed and conducted.According to the cell morphology of foams using low pressure foam injection molding,the formation mechanism of solid skin and the evolutionary process were studied,respectively.According to that using high pressure foam injection molding,the effects of the packing time and packing pressure on cell structure were analyzed,and the main factors effecting on the expansion ratio was studied.The in-situ fibrillation process based on a twin-screw compounding was conducted to prepare nano-fibrillar PP/PTFE composite with improved melt strength and foaming ability.To clarify the role PTFE fibrils in the crystallization behavior of PP,the non-isothermal crystallization of PP and PP/PTFE composites were investigated by a differential scanning calorimeter.Furthermore,the dynamic frequency sweep tests,dynamic temperature sweep tests,and extensional viscosity tests were implemented using a rheometer to study the effect of PTFE fibrils on the rheological performance of PP melt.Moreover,thermal conductivity and compressive strength were measured to evaluate the performance of PP/PTFE foams prepared for thermal insulation applications.The mechanisms of cellular structure on thermal conductivity and compressive strength were discussed.The rheological and crystallization behaviors of the LPP,the branched polypropylene?BPP?,and the in-situ fibrillated BPP/PTFE blends were compared,and the effectof PTFE fibers on the melt viscoelasticity and crystallization properties.After conducting the high-pressure foam injection molding with BPP and BPP/PTFE blends,the low-density BPP foam with an expansion ratio of up to 25-fold and fine cellular structures was successfully fabricated,owning a thermal conductivity of as low as 32.4mW·m^-1·K^-1.