Synthesis And Characterization Of Long Chain Branching Polypropylene And Its Properties Study

Most of commodity iPP is produced by Ziegler-Natta or metallocene catalysts, resulting in highly linear chains with a relatively narrow molecular weight distribution. As consequent of this,iPP demonstrates low melt strength which leads to poor processing properties in extensional flow dominated processes,such as thermoforming,extrusion coating,blow molding,and foaming etc.It is necessary to improve the melt strength of iPP in order to further widen its applications.So far grafting of long chain branches(LCB)on iPP backbone is the most effective way to improve its melt strength.In this paper,firstly long chain branching polypropylene homopolymer(LCBPP) was prepared by the means of the reaction between maleic anhydride grafted Polypropylene(PP-g-MAH)with different linking agents such as diamines or dihydroxy in solution.FT-IR test showed that diamines reacted more effective with PP-g-MAH than dihydroxy,the reactive activity of linking agents would decrease with the increase of molecular weight.The introduced long chain branches significantly influenced the viscoelastic properties of the melts:higher value of store module at low frequency,distinct decrease of loss angle,significant increase of zero shear viscosities and relaxation time and profounder shear thinning sensitivities, compared to those of untreated PP-g-MAH.This rheology test confirmed that ethylenediamine(EDA)is the most effective melt strength modificater of PP-g-MAH. Unfortunately higher activation energy of flow was found for modified PP,which mean to high sensitive of rheological behavior to temperature for polymer melt.Secondly,Polypropylene grafting Polyethylene(PP-g-PE)was prepared by the means of the imidization reaction between maleic anhydride grafted Polyethylene (LLDPE-g-MAH or HDPE-g-MAH)and amine grafted polypropylene(pp-g-NH₂)in xylene solution.FT-IR tests showed that imidization coupling reaction was effective. It was found from high temperature GPC test that the product had higher molecule weight,compared with the starting polymers.The branch density of the copolymers was controlled by varying the molar ratio between maleic anhydride and primary amine group.The viscoelastic properties of PP-g-PEs were studied by dynamic rheological measurement,the result showed that PP-g-PEs had most of the characters for long chain branches polymer rheology.And these properties became profounder as the long chain branch density increased.Elevated flow activation energies were observed with measured values lower than pure PP-g-MAH's indicating that the copolymers became less sensitivity of rheological behavior to temperature as PE-g-MAH was brought into PP-g-MAH system.What's more,LCBPP homopolymer had also been prepared by the direct melting reaction between PP-g-MAH and ethylene diamine(EDA)in a two-screw extruder. Compared to the product prepared by solution reaction,reactive extrusion was carried out in melt without solvent which is harmful to environment.The process was continuous and the efficiency was promoted.The obtained products exhibited excellent dynamic rheological characteristics and the tensile strength of LCBPP homopolymer was more than 38MPa and increased over 10%over starting PP-g-MAH.The tensile modulus was close to 2000N/mm²,which was an 25%. increased over linear iPP,and the notched impact strength of LCBPP,which reached to 3.7 kJ/m²,increased 96%compared to that of the linear PP-g-MAH as raw material. The elongation at-break decreased after melt reaction indicating that dimensional stability was improved.Most of mechanical properties except impact strength are higher than commodity HMSPP from Borealis.Blend PP-g-MAH with 20% PE-g-MAH could improve rheological property but promotion was limited;if EDA was added into the blend system to form PP/PE long chain branching copolymer,the rheological property was further promoted and lower sensitivity of rheological behavior to temperature was found in dynamic rheology test.Strain hardening was observed from long chain branching samples,while strain softening from linear samples and its blend in transient extensional rheology test.To improve reaction degree supercritical carbon dioxide(ScCO₂)was injected into two-screw extruder. The presence of ScCO₂ in polymer promoted the mobility of polymer chain segment and diffusibility of small molecules.As a result,the grafting reaction increased significantly. Finally,PP foam was prepared by MuCell technology with ScCO₂ as blow agent, "sandwich" structure of foamed samples with solid skin and foam core was observed by SEM test.Different foam conditions including melt temperature,mold temperature and CO₂ level was compared and a suitable process was determined.And then foam behavior of the linear samples and long chain branching samples was compared. Weight reduce degree,SEM test and modified mechanical properties showed that long chain branching samples were much suitable for foam.The weight of long chain branching samples foam had been reduced over 20%,but the mechanical properties did not compromise badly,this is due to the small size bubble and its uniformly distribution in the "sandwich".