The Regulation Of Foaming Temperature Window And Cell Shape Control Of Polypropylene Supercritical CO₂ Extrusion

Polypropylene?PP?foam has good heat resistance,high specific strength,excellent anti-aging and corrosion resistance,and is widely used in daily catering,transportation,construction,military and other fields and industries.However,PP lacks low melt strength,high crystallinity,poor cell morphology,and narrow foaming temperature range.High melt strength polypropylene?HMSPP?is to insert long chain branches into the PP segment through irradiation or reactive blending to improve its melt strength.Nevertheless,HMSPP foam still has the problems of high crystallinity,poor cell shape and narrow foaming temperature range.In this paper,a series of HMSPP foam samples were prepared by using supercritical CO₂as a foaming agent and continuous series extrusion foaming equipment.Then the sample performance was analyzed.Firstly,the foamed HMSPP sample was extruded and foamed under different head temperature and CO₂flow,with supercritical CO₂as the foaming agent.The crystalline properties and cell characteristics of foamed samples under different CO₂flow rates were analyzed.The rheological properties of unfoamed HMSPP samples were also studied.The results show that the melting temperature of HMSPP moves to high temperature with the increase of supercritical CO₂flow rate.The higher the CO₂flow rate,the earlier the appearance of crystals,which is more conducive to foaming.The higher the CO₂flow rate,the earlier the appearance of crystals,which is more conducive to foaming.In the range of CO₂flow rate of 1-2 ml/min and head temperature of 160-164°C,as the flow rate increases,the average cell size of the foamed material gradually decreases,and the cell density increases accordingly.When the temperature of the machine head increases,the average cell size of the foamed material gradually increases,and the cell density decreases.Under the process conditions of a die temperature of 162?and a CO₂flow rate of 2 ml/min,the cell density of the extruded foam sample is 6.334×10⁷cells/cm3,the average cell diameter is 75.141?m,and the foaming ratio reaches 15.07.In addition,the foam sample can be continuously and stably extruded and foamed under this condition,and the surface of the extrudate is relatively smooth.Using HMSPP and ultra-high molecular weight polypropylene?UHMWPE?as raw materials,supercritical CO₂as the foaming agent,the HMSPP/UHMWPE foam samples were prepared by extrusion.And the influence of UHMWPE content on the melting behavior,rheological behavior,extrusion characteristics and cell morphology of the blend was studied.The results show that the addition of UHMWPE increases the complex viscosity and modulus of HMSPP,reduces the tan?of HMSPP,which helps optimize the cell morphology of HMSPP.And with the addition of UHMWPE,the cell density of the foamed blend samples increases,the cell size decreases,and the foaming ratio is also significantly improved.When the die temperature is 162?and the UHMWPE content is 10%,the average cell size of the extruded foaming sample is 40.737?m,the cell density is 5.594×10⁷cells/cm³,and the foaming ratio is 20.8.Therefore,under the condition of unchanged process conditions,by changing the content of UHMWPE,the foaming ratio and cell morphology of the foamed blend can be adjusted.High-impact polystyrene?HIPS?was added to HMSPP,and HMSPP/HIPS foam samples were prepared by extrusion foaming with supercritical CO₂as the foaming agent.The crystallization properties,dynamic rheology and extrusion rheology properties of HMSPP/HIPS blends were analyzed.The results showed that the crystallization ability of the blend weakened with the addition of HIPS,and the crystallinity and crystallization temperature decreased significantly.In addition,the storage modulus,loss modulus and complex viscosity of HMSPP/HIPS blends are all significantly improved,and the relaxation time becomes longer.The extensional viscosity activation energy and melt strength activation energy of the blend are reduced,and the temperature dependence of the extensional viscosity and melt strength of the blend is improved.After adding HIPS,the foaming temperature range of PP material has been expanded from 164?160?to 166?152?.Finally,the upper and lower foaming temperature limits of PP/HIPS blends with different components are predicted and calculated based on the relationship between Arrhenius equation,melt strength and inlet pressure drop,which is partially consistent with the experimental results.