Modification, Microstructure And Properties Of Polyproplene/Magnesium Hydroxide Composites

Magnesium hydroxide(MH) has attracted considerable interests in reinforcing and halogen-free flame retardation of polypropylene(PP) due to its flame retardation,smoke suppressibility and filling.However,the major challges are the homogeneous dispersion of MH in PP matrix and the enhanced interfacial adhesion between MH and PP in PP/MH composites.In the dissertation,maleic anhydride grafted styrene ethylene/butylene-styrene tri-block copolymer(SEBS-g-MA) as a compatibilizer,and in-situ monomer/MH emulsion copolymerization were used to modify the surface of different MH particles,such as nano-, micro- and whisker-MH.Then,the modified MH particles(MMH) were compounded with PP to prepare PP/MH and PP/MH/red phosphor(P) composites.Their structure and properties were characterized by mechanical testing,dynamic mechanical anlysis(DMA), thermogravimetric analysis(TGA),differential scanning calorimetry(DSC),capillary rheometer and cone calorimetry test(CCT).Based on experiment results,the conclusions were drawn as following:(1) The effects of SEBS-g-MA on dispersion of nano-MHs in PP matrix and interfacial adhesion were studied in order to prepare highly filled PP/MH nanocomposites.The results showed that SEBS-g-MA improved both dispersion of nano-MHs and interfacial adhesion in PP/MH nanocomposites with up to 40 wt%nano-MHs.The elastic moduli of PP/SEBS-g-MA/MH nanocomposites increased marginally and tensile yield strengths were almost invariant with nano-MH loading.Significant impact toughening of these ternary nanocomposites was,however,achieved due to the cavitation of SEBS-g-MA/MH particles and expansion of voids as well as plastic deformation of the PP matrix.When fine MH dispersion and strong interfacial adhesion were achieved with addition of SEBS-g-MA, MH-nucleating was improved and higher relative crystallinity and rates were observed,and. The results of rheological properties show that the PP/MH and PP/SEBS-g-MA/MH nanocomposites are shear-thinning non-Newtonian.With increasing the MH content,the apparent viscosity of PP/MH and PP/SEBS-g-MA/MH nanocomposites increase,but the former increase more.At the same MH content,PP/SEBS-g-MA/MH nanocomposites show lower melt viscosity than that of PP/MH nanocomposites.(2) Firstly,vinylated magnesium hydroxide(MH) nanosheets were prepared with 3-(trimethoxysilyl) propyl methacrylate(γ-MPS) and pristine MH nanosheets,then the MH/polymer hybrid nanoparticles were prepared via in-situ copolymerization of vinylated MH nanosheets and styrene(St) or methyl methacrylate(MMA).The morphology,thermal stability and chemical structure of the final products were investigated in detail by transmission electron microscopy(TEM),scanning electron microscopy(SEM) and Fourier-transform infrared spectra(FTIR).The TEM and FTIR results showed that the uniformly-dispersed core-shell structure of MH/polymer nano-particles with MH-cores and polymer-shell was formed.(3) PP/MH and PP/MMH composites were prepared filled with pristine MH and modified MH(MMH) particles via in-situ copolymerization respectively.The size and shape of particles varied from nano scale,micro scale to whisker MH.The effects of the dispersion size in the matrix and interfacial interaction on the mechanical properties, melting and crystallization properties,and microstructure of the PP/MH composites were systematically investigated.Based on the SEM observation and notched izod impact tests,it were found that impact strength of the composites is significantly dependent on particle dispersion size and the condition of interfacial adhesion.The results showed that the unmodified MH particles are easy to aggregate,and the interfacial adhesion is poor between un-modified particles and composite matrix.After these particles were modified by in situ polymerization,the polymer coated on the MH surface enhances the dispersion of MH in matrixes,and improves the interfacial adhesion.The values of elongations-to-break and impact strength of PP/MMH composites filled with MMA/BA modified MH showed significant improvement over PP/MH composites and PP/MMH composites filled with MMA or St modified MH.It was also observed that the impact strength of PP/micro-MH composites filled with MMA/BA modified MH increased drastically,which contribut to the homogeneous dispersion and debonding around the micro-MH particle/polymer interfaces. Furthermore,after the MH particles are modified by in situ copolymerization,there are more polymer chains grafted on the filler surface,the inorganic particles are more uniformly dispersed in composite matrix due to strong interaction between the matrix and the grafting polymer on the surface of the modified particles,which induces higher crystallization temperature,higher relative crystallinity and crystallization rates.The results of rheological behavior testing show that the PP/MMH and PP/MH composites are shear-thinning non-Newtonian and its apparent viscosity are bigger than those of the PP melt.Moreover,at the same filler content,the apparent viscosities of PP/MMH composites are lower than those of PP/MH composites.This is probably caused by the reinforced interfacial interaction between the matrix and the encapsulating polymer on the surface of the modified MH,which can limited baffling effect of particles aggregate and decrease the apparent viscosity of MMH/PP composites.(4) The PP/MH/P flame retardant composites were prepared by melt blending with PP,P and modified MH(MMH) or unmodified MH particles.The effect of modified MH particles on the mechanical properties,combustion behavior and thermal oxidative degradation mechanism of those composites were systematically investigated by means of LOI,UL-94,TGA and CCT.The results show that PP/MMH/P composites filled with modified MH particles could apparently increase the impact strength,and the largest impact strength was observed in the composites with modified micro-MH particles.It was founded that particles size has little influence on the flammability properties compared to homogeneous dispersion of MH particles.The PP/MMH/P composites filled with modified nano- or micro-MH particles could apparently increase the flammability properties and reach UL-94 test v-0 rating,though decrease the LOI value a little.More over,the data obtained from the TGA curves indicated that residues and therm-oxidative stability of PP/MMH/P composites increased farther,which contribut to the more homogeneous dispersion.The results of CCT show that Pk-HRR,av-HRR and av-MLR were decreased remarkably for PP/MMH/P composites compared to PP,and FPI improved 3-4 times.