Preparation Of Core Shell Toughened PA6 Alloys With PVDF As The Core Material And Their Mechanical Properties

Polyamide 6(PA6),a widely used engineering plastics,has excellent comprehensive properties,such as wear-resistance,oil-proofing,good mechanical performance,good self-lubrication,insulation and easy processing.However,PA6 impact strength is sensitive to notch and temperature.It becomes quite brittle with the existence of small cracks or at low temperature,which will limit its application in automobile,electronics and other industries.Core-shell particle with rigid core and thin layer of rubber is an effective way to balance the tensile strength and toughness for rubber toughened system.In our previous studies,maleic anhydride functioned low molecular weight polybutadiene(MLPB,PB-g-MAH)had been melted grafted to the molecular chains of polyethylene(PE)and polypropylene(PP)before they were melt blended with polyamide 6(PA6).Core-shell structures with rigid PE or PP core and soft MLPB shell can be in situ formed via the reaction of MAH in MLPB and end amino group in PA6 molecules during melt blending process.The toughening effect of the core shell toughened system with PE core is much better than that with PP core.However,its tensile yield strength is much lower than that with PP core.SEM images of impact fracture surfaces of the two core-shell toughening system show that PE core has large deformation during the fracture process and the rubber fibrils generated by the cavitation of MLPB rubber shell are stabilized and still can be found in the fracture surface.However,PP inner core can only be slightly elongated during the impact process and the MLPB rubber fibrils are mostly broken.It is indicated that in the core shell toughening system with rigid core and soft shell,the yield strength of the rigid core will determine the yield strength of the core shell toughening system and the stabilization of rubber fibrils that is generated by the cavitation of rubber shell during the whole impact fracture process is the key factor for obtaining high toughness.And the deformation of rigid core during the process of impact(under high strain rate)was essential to the stabilization of the rubber fibrils.By analyzing fracture morphology of the tensile test(low strain rate),large deformation of PP core can also be observed.According to time temperature equivalence principle,it is thought that the glass transition temperature Tg rather than the yield strength of the inner core material should be the key factor to determine the deformation ability under high strain rate(impact test).It is believed that polymers with high yield strength and low glass transition temperature should be the most suitable material to be used as rigid core in the core shell toughening system and get good balance between strength and toughness.Poly(vinylidene fluoride)(PVDF),which has high tensile yield strength(50 MPa)and low glass transition temperature(-35 ?),might be a suitable material to be used as the inner core in core-shell toughening system.In this paper,core shell toughened PA6 systems with PVDF inner core and MLPB shell are prepared.MLPB was first grafted onto PVDF molecular chains via melt blended the pre-irradiated PVDF and then further blended with PA6.The other three kinds of counterpart blends series were also prepared:PVDF/PA6,PVDF-g-MAH/PA6 and PVDF/SEBS-g-MAH/PA6 blends.After comparing the mechanical properties of these four kinds of toughening system,we found that not only the mechanical properties of the particles but also the interfacial adhesion between toughening particles and the matrix resin had great influence on the final performance of the particle-toughened polymer system.When only rigid particles exist(PVDF and PVDF-g-MAH),the mechanical properties of PVDF-g-MAH/PA6 were better than those of PVDF/PA6 blends because of better interfacial bonding between the particles and the matrix.When the particle content is 30 wt%,the tensile yield strength of PVDF-g-MAH/PA6 is 49 MPa and the notch impact strength can be reached to 19 kJ/m;while the tensile strength of the PVDF/PA6 blends is only 44.6 MPa and the impact strength is 4.0 kJ/m2.If part of PVDF is replaced by SEBS-g-MAH rubber,core shell particles with PVDF inner core and SEBS-g-MAH rubber shell can be formed.Although the interfacial adhesion between SEBS-g-MAH rubber layer and PA6 is very good,the interface between PVDF core and SEBS-g-MAH rubber shell is still very weak.When the content of toughener is kept unchanged at 30 wt%,and 30 wt%of PVDF is replaced by SEBS-g-MAH,the impact strength can only be raised to 6.9 kJ/m2,but its tensile strength decrease to 42.5 MPa due to the addition of rubber.While in the PVDF-g-MLPB/PA6 core shell toughened system,the MLPB rubber shell has good interfacial adhesion with both the PVDF core and PA6 matrix.When irradiation dose is 25 kGy,MLPB addition amount is 5 wt%and total toughening agent is also 30 wt%,the tensile yield strength of PVDF-g-MLPB/PA6 system can maintain at 50.7 MPa and impact strength can reach upper than 23 kJ/m,which is much higher than those counterpart toughening systems.SEM images of impact fracture surfaces indicate that in PVDF/PA6 and PVDF/SEBS-g-MAH/PA6 blend system,interface de-bonding happens in the early stage of the fracture process because of the existence of very weak interface either between PVDF and PA6 or between PVDF and SEBS-g-MAH.And almost no deformation of PVDF particles as well as PA6 matrix can be observed.In PVDF-g-MLPB/PA6 toughening system,MLPB shell has good interfacial adhesion with both PVDF core and PA6 matrix.Large deformation of PVDF particles occurs along with the impact fracture process,which is benefit not only to stabilize MLPB rubber fibrils generated by the MLPB shell cavitation but also to induce the yield of matrix and get good toughening effect.At the same time,the existence of rubber fibrils can also be effective bearing stress in material,which is favorable to maintain the tensile yield strength of the toughening system.