Characteristics Of Polyolefin-based Elastomers And Their Application In Toughening Modification Of Polypropylene

Polypropylene has many excellent properties and has been widely used in automobiles,building materials,electrical appliances and other fields.However,the disadvantage which is brittleness at low temperature limits its further application.In order to expand the application range of polypropylene and enhance its market competitiveness,many experts and scholars both at home and abroad have studied the toughening modification of polypropylene.Using elastomers to toughen polypropylene is one of the most important methods to obtain the high impact strength polypropylene.This thesis mainly focuses on the performance and the blending toughening polypropylene of four typical polyolefin-based elastomers.First of all,the performance of four polyolefin-based elastomers:two types of EPDMs which is synthesized by metallocene catalyst(mEPDM)and by conventional Ziegler-Natta catalyst(EPDM),ethylene-a-octene random copolymer(POE)and ethylene-a-octene block copolymer(OBC)were studied.Findings of the research include:(1)The molecular weight distribution of mEPDM is narrower than EPDM,the strength and rigidity are higher,but the processing flowability is poorer.POE and OBC have narrow molecular weight distribution,but higher melt index,partly due to the physical junction structure formed by crystallization of ethylene chain segment in POE and the special "hard segment-soft segment" block structure of OBC's chain segment.(2)The tensile strength and elastic modulus of elastomers are related to the degree of crystallinity of the elastomers.mEPDM has the highest degree of crystallinity,highest elastic modulus and tensile strength.While EPDM which does not crystallize has the lowest stiffness and strength.(3)The difference value between the glass transition temperature and melting point of OBC is the highest,which means the OBC can be used in the widest range of temperature,whereas the mEPDM has minimum difference value,meaning the narrowest applicable temperature range.(4)Elastomers' elongation at break should be related to its glass transition temperature.The glass transition temperature of OBC is the lowest,so its elongation at break is the largest.On the contrary,mEPDM has the highest glass transition temperature,but its toughness is the worst.(5)The extension curve of EPDM shows a typical elastomer's characteristic,mEPDM's shows plastomer's characteristic,OBC's and POE's show thermoplastic elastomer's characteristic.Secondly,we designed the method of orthogonal experiment to study the effects of processing parameters on the alloy toughening effect.Mixing temperature,screw speed,total mixing time and amount of elastomer were selected as influence factors.Three factors and four factors with three levels of orthogonal experiments were designed to determine the optimum blending parameters.For the final,a group of suitable blending parameters were determined:for PP/mEPDM and PP/OBC the blending temperature is 180? the screw rotation speed is 120 rpm,and blending time is 9 mins;for PP/POE and PP/EPDM the blending temperature is 180? the screw rotation speed is 100 rpm,and blending time is 6 mins.In the end,the effect of elastomer content on the properties such as the impact strength,the tensile properties,melt flow index,vicat softening point were investigated.And the results show that:(1)all the four kinds of elastomers have effect on toughening modification of PP,they have almost accomplished "brittle-ductile transition" when their elastomer content of mEPDM&OBC is higher than 20%and POE&EPDM higher than 25%at room temperature;and mEPDM&OBC higher than 35%,POE&EPDM 40%at-20? among which POE&OBC ha-ve better toughening effect on PP.(2)The elongation at break of the PP/POE alloy is significantly better than the other alloys,which is 20 times more than that of pure PP,and twice more than the other alloys.(3)The melting index of PP/OBC alloy is the highest and PP/mEPDM alloy has the lowest melting index.PP/mEPDM's vicat softening point is slightly higher than the other three alloys.(4)The surface morphology of the impact fracture was observed,and the surface morphology of the four alloys was found ductile fracture after brittle-ductile transition.The PP/POE alloy presents co-continuous phase characteristics,especially the presence of"filament" in impact fracture of-20? which also indicates that the PP/POE alloy is more tough.