Preparation And Control Of Polypropylene/Inorganic-particle Foam

Polypropylene?PP?foam have wide applications in packaging,insulation and construction,as well as in the automotive and aerospace industries due to their light-weight,high impact strength,high fatigue life,high strength-to-weight ratio,superior insulating qualities and low cost.The melt strength of linear PP was so low that the cell wall could not constraint the cell growth resulting in cell coalescence and rupture.In order to improve the cell morphology,the inorganic particles were added to PP matrix.Therefore,heterogeneous nucleation was easier to be obtained during foaming process,and cell density was increased,cell size was dropped.Molecular sieve?MS?and zinc borate?ZB?inorganic particles were chosed as fillers to prepare PP/MS and PP/ZB composites,respectively.The PP/inorganic particle composites foam was prepared by utilizing supercritical CO₂ as a physical foaming agent in a batch foaming process.The dispersion states of inorganic particle in PP and the foam morphology were characterized by Scanning Electronic Microscopy?SEM?.The size and density of spherulites were characterized by Polarized Optical Microscope?POM?.The effect of temperature on the rheological properties of pure PP was studied using ARES-G2 rheometer.The region of foaming temperature was determined by Differential Scanning Calorimeter?DSC?.The compression properties of foam materials were tested by electronic universal testing machine.For PP/MS,pre-existing cavity was formed during foaming process because of the presence of mesoporous structure in the surface of MS,resulting in the reduction of the Gibbs free energy barrier for nucleation.The results indicated that the bimodal structure could be mediated via changing foaming temperature,pressure and concentration of MS.Lower concentration of MS?lower than 3.0%?led to bimodal structure and higher concentration of MS?3.0%⁷.0%?led to the disappearance of bimodal structure.With the addition of MS big pore was nucleated heterogeneously,on the other hand,the size of spherulites was decreased and the density of spherulites was enhanced.Therefore,more heterogeneous nucleation sites for small pore were formed.The size of big/small pore size and the cell density ratio of big/small pore both increased when foaming temperature and pressure improved.Compared to linear PP foaming material,it was demonstrated that foam materials with bimodal structure?0.5%MS?possessed superior compression property.The foam morphology of PP/ZB composites was improved when ZB was added as heterogeneous nucleation sites.The foam morphology could be controlled via changing foaming temperature,pressure and concentration of ZB.On one hand,at high temperature and low pressure,the viscosity of pure PP was low so that the corresponding cellular structure was big pore.The addition of ZB resulted in the reduced cell size,the increased cell density,and the expansion ratio.On the other hand,at low temperature and high pressure,the viscosity of pure PP was high so that the foam structure was small pore,and the addition of ZB hardly affected the foam morphology.