Preparation And Toughening Properties Of Polypropylene Based Blends

Polypropylene(PP)is light,inexpensive,chemically stable,non-toxic and easy to process,and widely used in vehicles,aerospace,medical and packaging applications.However,the low temperature and impact resistance of PP is relatively poor.The PP can be modified to expand the application fields.Therefore,it is important to develop PP products with excellent toughness and study the toughening mechanism.In this thesis,a series of PP-based blends were prepared by melt blending method using physical toughening modification,and the effects of different toughening agents on the toughness of PP were investigated and the toughening mechanism was revealed.(1)PP/HDPE blends were successfully prepared using high density polyethylene(HDPE)as a toughening agent by melt blending method.The notched impact strength of PP/HDPE blends increased with increasing HDPE content.PP/HDPE blends formed“sea-island” structure particles with HDPE as the dispersed phase and PP as the continuous phase.The toughening mechanism was that when the PP/HDPE blends were subjected to external forces,the PP matrix and HDPE would debond and form a cavity,which would absorb energy.HDPE would produce a “drawing” phenomenon in PP,and the long chain linear structure of HDPE made the particles act as stress concentration points when the PP/HDPE blends subjected to external forces,and the edges of the particles would selectively extend in the direction perpendicular to the force.The stress was transferred from the PP to the HDPE,absorbing the energy generated by the external force.(2)PP/SEBS/HDPE blends were successfully prepared by melt blending method using hydrogenated styrene-butadiene block copolymer(SEBS)and HDPE as toughening agents.All the dispersed particles in the PP/SEBS/HDPE blends exhibited a distinctive core-shell particle structure with HDPE as the core and SEBS as the shell.The formation of core-shell structure particles in the PP/SEBS/HDPE blends made a synergistic toughening effect between SEBS and HDPE and improved the toughness without loss of strength and modulus,exhibiting a rigidity-toughness balance.The brittle-tough transition behaviour was dominated by the SEBS phase.The high chain entanglement degree between the dispersed phase and the matrix was a prerequisite for the toughening of the core-shell particles.The mechanism of core-shell particle toughening of PP was that when the blends was subjected to an external force,the core-shell particles acted as a whole as a stress concentration point and the SEBS shell acted as a bridge between PP and HDPE,transferring the stress from PP to SBES and from SBES to HDPE.The core-shell structure of the particles deformed and the PP matrix underwent shear yielding,requiring the absorption of a large amount of energy in the process led to the PP/SEBS/HDPE blends sample a good toughness.(3)PP/SEBS/β-NAs blends were successfully prepared by melt blending method using SEBS and β nucleating agents(β-NAs)as toughening agents.The notched impact strength of the PP/SEBS/β-NAs blends increased with the increase of SEBS content.The synergistic effect of SEBS and β-NAs was not only able to toughen PP,but also to promote the processing properties.The addition of β-NAs induced an earlier onset of brittle-tough transition behaviour at low SEBS content.The toughening mechanism was based on the increase of β crystals content in the PP/SEBS/β-NAs blends,resulting in finer grain refinement of the PP and finer dispersion of the SEBS particles,which not only SEBS phase cavitated but also induced substantial shear yielding of the PP matrix when the SEBS phase acted as a stress concentration point,giving the PP/SEBS/β-NAs blends good toughness.