Foaming Behavior Of Polypropylene Structured Melt

Compared with traditional polystyrene（PS）,polyurethane（PU）,polyethylene（PE）foams,microcellular foamed polypropylene（PP）has the advantages of high performance-cost ratio,good heat resistance,high service temperature,good rigidity and recyclability.It has broad applications in aerospace,automotive lightweight,packaging,thermal insulation,sound absorption and the other fields,and has received extensive attention from the researchers.However,solid-state foaming of polypropylene is limited by its high crystallinity,and high-temperature foaming is confined by low melt strength,which make it difficult to be foamed.In order to solve this problem,structured melt was introduced by supercritical CO₂ pretreatment in this paper,which not only acts as a physical crosslinking point to improve the melt strength of polypropylene during foaming,but also behaves as a heterogeneous nucleation sites to improve the cell density.To study the influence of process parameters on its foaming and mechanical properties,different polypropylene foaming processes based on CO₂ pretreatment were used.The main work and conclusions are shown as follows:（1）Supercritical CO₂ pretreatment processStructured melt was introduced into polypropylene by supercritical CO₂pretreatment.DSC results show that pretreatment at the temperature of 162 or 165°C with the pressure of 10 MPa for30 min leads to the formation of double melting peaks,which means that there are rearrangement of crystals and increased thickness of crystal lamellas in polypropylene,forming the partially ordered structure.XRD results showγcrystals are formed after CO₂ pretreatment.The impact strength of samples after CO₂ pretreatment was tested,and it is found that the highest impact strength of the samples is up to 9 k J/m²,it is about 160%higher than that of no pretreatment,which is 3.5 k J/m².（2）Polypropylene batch foaming process based on CO₂ pretreatmentPolypropylenes before and after CO₂ pretreatment were foamed by batch foaming process.Results show that the foaming temperature window of the conventional batch foaming process before pretreatment is only about 2-4°C,and it is increased to 9°C,with an expansion ratio of more than 10 times after pretreatment,and the maximum expansion ratio is 29,which is related to the appearance of structured melt andγ-crystals.（3）Temperature induced foaming process of polypropylene based on CO₂ pretreatmentFoamed materials with nano-cells were prepared by temperature induced foaming method.Results show that the cell density of the foamed samples increases with the decrease of the pretreating temperature,the cell size distribution becomes narrow,and the cell size tends to decrease.The average cell diameter is small to 8 nm,and the cell density can reach 10¹³ cells/cm³.It was found that the cell density and cell size did not be changed significantly with the increasing of foaming temperature.The impact properties of the foamed samples were tested,and it was found that the impact strength of the foamed polypropylene is up to 16.4 k J/m²,which is about 370%higher than that of the un-foamed value of 3.5 k J/m².（4）Lower temperature foaming process based on CO₂ pretreatmentThrough the adjustment of process parameters such as pretreating temperature and time,foaming temperature,pressure,etc.,sub-microcellular polypropylene foamed materials with average cell diameter about 350 nm and cell density of 10¹³ cells/cm³ were prepared.Moreover,it`s an open-cell structure with homogeneous cell morphologies.The mechanical properties of the foamed materials were further studied.The tensile strength of the foamed samples was lower than that of the un-foamed samples.The impact strength of the material increases with the increase of the pretreating temperature.The highest impact strength is 8.2 k J/m²,a 190%improvement compared to the its un-foamed counterpart 2.8 k J/m².This is closely related to the change of material cell structure.