Study On The Low Temperature Impact Toughness Of PP/POE-g-MAH/PA6 Core-shell Toughening System

Polypropylene(PP)is a widely used polymer in many fields of our daily life.It has good comprehensive properties such as small density,good processing performance,non-toxicity,corrosion resistance,and low cost.However,its low temperature performance,such as notched impact strength,is very bad,which has limited its application.Although the impact strength at low temperature can be improved when it is toughened by rubber or elastomer,the tensile strength decrease dramatically.Basing on our previous study,a core-shell toughening system with hard core and soft shell is an effective way to obtain the balance between toughness and strength.In this paper,the impact properties of polypropylene/maleic anhydride grafted ethylene-octene copolymer/nylon 6(PP/POE-g-MAH/PA6)core-shell toughening system at low temperature are detailed investigated.It is well known that the mixing sequences will largely affect the final morphologies in multiphase polymer blends.In our PP/POE-g-MAH/PA6(69/20.7/10.3)system,when PA6 and POE-g-MAH are pre-mixed as a masterbatch before blended with the PP matrix,core-shell structures with PA6 core and POE-g-MAH shell can be effectively formed.However,core shell structures cannot be formed in the systems with pre-mixed PP/POE-g-MAH masterbatch or blended the three phases simultaneously.A series of PP/POE-g-MAH/PA6 core-shell toughening systems with PP as matrix,PA6 as coreand POE-g-MAH as shell are prepared and the volume ratios of PA6 and POE-g-MAH are 1:2,1:1,and 2:1 respectively.The results show that the core-shell particle sizes and inter-particle distances decrease along with the decreasing of PA6 contents at a fixed toughener amount.The brittle-ductile transition temperatures(TBD)of the system decrease when the interparticle distances decrease,which is consistent with Wu's percolation theory.When the volume ratio of PA6 and POE-g-MAH is 1:2,the processing temperatures of the PA6/POE-g-MAH masterbatch and the final PP/POE-g-MAH/PA6(69/20.7/10.3)alloy are all at 235°C(higher than the melting point of PA6),its TBD is around 0°C..,that is,during the alloy formation process,the PA6 core is in a molten state.However,if the processing temperature of the final alloy is lowered to 200°C(lower than the melting point of PA6),at which the PA6 core formed in the masterbatch will no longer melt again,the TBD of the PP/POE-g-MAH/PA6 alloy decreases from 0°C to-10°C.The reason for this phenomenon is not very clear yet,higher core-shell particle formation efficiency of the later process is believed to be an important factor.If the above-mentioned PP/POE-g-MAH/PA6(69/20.7/10.3)alloy prepared at 200 °C is annealed at 120 ? for 3 h,its TBD will increase to 0?.This phenomenon should be ascribed to the decrease of the deformation ability of the rubber layer in the core shell particles.For PP/POE-g-MAH/PA6 core-shell toughened systems,PP and PA6 will shrink to each phase centerduring the injection molding process because they are crystallized.If both phases have sufficient interface strength with POE-g-MAH,this contraction exerts an interface tensile stress on the POE rubber layer.The existence of such additional tensile stress will induce the deformation of the rubber layer,so that it can be cavitated at even lower temperature.As the interfacial strength of the PP and POE-g-MAH phase interface is obtained by molecular entanglements,which will gradually decrease due to rearrangement of the molecular chains during the annealing process,the additional tensile stress between the rubber layer and PP matrix will weaken or even disappear.This will decrease the deformation ability of the rubber,which results in the increase of the TBD and the decrease of the absolute impact strength as well.In order to further prove this viewpoint,we prepare another core-shell toughening system with PA6 as matrix,PP as core and POE-g-MAH as shell,PA6/POE-g-MAH/PP(69/20.7/10.3),and study the impact properties before and after thermal annealing.The results show that the TBD increases from-40 ? to 0 °C after annealing at 150 °C for 3 h.This result fully proves that the presence of tensile stresses at the rubber/matrix and rubber/core interfaces contribute greatly to the deformation of the rubber layer and the improvement of the impact properties of the core shell toughening system with rigid core and soft rubber shell.