Surface Modification Of Magnesium Oxysulfate Whisker And Its Application In Polypropylene

Magnesium oxysulfate whisker(MOSw)is one of the typical inorganic magnesium salt whisker which has been widely used in polymer composite areas,due to its favorable mechanical and flame-retardant properties.However,two main issues have been found: i)the high surface free energy leads to serious aggregation,which is detrimental to the dispersion of whiskers in the polymer matrice;ii)the opposite polarity between hydrophilic MOSw and hydrophobic polymers results in serious incompatibility.Based on these problems,surface modification was employed to alter the surface properties of MOSw,and then the modified MOSw was incorporated into polypropylene(PP)matrix.Also,we investigated the mechanical,thermal,flame-retardant,rheological and crystallization properties of the PP/MOSw composites.The main contents of this paper are shown as follows:(1)The influences of modifier dosage,reaction temperature and reaction time on modification effects were investigated via contact angle and lipophilic degree tests.The purpose of these experiments was to determine the superior conditions using five fatty acids as modifiers.Taking lauric acid(LA)as an example,the mechanism of surface modification of MOSw was further studied.It was found that a chemical reaction similar to the “acid-base neutralization” occurred in the modification process.LA was coated as a thin layer on the surface of MOSw.(2)Analogously,the superior conditions of vinyltriethoxysilane(VTES)modifying MOSw were determined via contact angle and lipophilic degree tests.VTES was hydrolyzed at 25°C for 3 h in a solution composed by V(VTES):V(H2O): V(ethanol)=20:8:72;the dosage of VTES was 5 wt%;the reaction solution was heated to 60°C and held for 60 min.The appearance of Si-O-Si and Si-O-Mg bonds and inexistence of Si-O-C bonds confirmed that VTES was bonded on the surface of MOSw.(3)Analogously,the superior conditions of dodecyl dihydrogen phosphate(DDP)modifying MOSw were determined viacontact angle and lipophilic degree tests.DDP was dissolved in a KOH-H2 O solution after ultrasonic process for 3 min;the dosage of DDP was 3 wt%,the modification temperature was 25°C,and the modification process lasted for 30 min.DDP modified MOSw(DDPMOSw)generated a homogeneously suspension in organic phase.In addition,the appearance of P-O-Mg bonds confirmed that DDPwas bonded on the surface of MOSw.(4)PP/MOSw composites were prepared via melt blending method.The effects of different modifiers on the PP/MOSw composites were investigated by means of mechanical,thermal,crystallization and flame-retardant property tests.The results showed that LA modified MOSw(LAMOSw)performed the best in promoting heterogeneous nucleation of the PP matrix and induced ?-PP forming.As such,LAMOSw showed the best toughening effect for the PP matrix.PP composites containing VTES modified MOSw(VTESMOSw)showed the highest yield strength and Young's modulus,since VTES enhanced the interfacial adhesion between MOSw and the PP matrix.In addition,DDP modifying generated a compact and coherent char based on MgO whiskers backbone,which was helpful to improve the flame-retardant properties of MOSw.(5)PP/MOSw composites with PP-g-MAH or POE-g-MAH as compatibilizer were prepared via melt blending method.The effects of these two compatibilizers on the PP/MOSw composites were investigated by mechanical,thermal and crystallization property tests.PP-g-MAH showed better reinforcing effects for three reasons: i)the MAH content of PP-g-MAH was higher than that of POE-g-MAH,so that more ring-open esterification reaction should be taken place;ii)PP-g-MAH was well compatible with the PP matrix since their similar structures;iii)addition of PP-g-MAH promoted MOSw distribution more evenly in the PP matrix.On the other hand,POE-g-MAH showed better toughening effect for two possible reasons: i)PP-g-MAH induced amount of ?-PP;ii)instrument of the interfacial adhesionbetween MOSw and the PP matrix was not too high with POE-g-MAH.(6)PP composites containing different morphology of magnesium oxysulfate(MOS)were prepared via melt blending method with PP-g-MAH as compatibilizer.The effects of different morphologies MOS on the PP matrix were investigated by means of mechanical,rheological and crystallization property tests.Consequently,the addition of MOS particles(MOSp)greatly improved the toughness of the PP matrix,which was related to the heterogeneous nucleation effect and the formation of ?-PP.Besides,it also found that MOSw network formed in the PP matrix as a mainly reason of MOSw reinforcing the PP matrix.With the increasing of MOSw content,the structure of PP composites were converted from “liquid-like” for PP matrix to “solid-like”.In a word,the results of this thesis provided detailed theoretical supports for the application of the MOSw/polymer composites.Meanwhile,they also had practical reference value to the researches of other whisker/polymer composites.