Surface Modification Mechanism Of Magnesium Hydroxide And Its Application In EVA

With increased environmental awareness and tightening of the related legislations,use of traditional halogen flame retardants is gradually reduced,while inorganic flame retardants are favored.One such chemical is magnesium hydroxide(MDH)which is environmental friendly,non-toxic and helps neutralize acid gases produced from polymer combustion.But MDH is of strong polarity thus easy to agglomerate,with poor affinity for hydrophobic olefinic polymers thus often difficult to disperse evenly in the polymer matrix,resulting in poor application performance.Moreover,due to the low flame retardant efficiency of MDH,high addition levels are typically needed,which can deteriorate mechanical properties of the composite materials.This problem largely hampers the promotion and use of MDH.O ne effective way to solve this problem is to chemically modify MDH surface.Previous studies on MDH surface modification mainly focused on the effect of different types of modifiers on the mechanical properties and flame retardancy of composites.However,the past work lacks completeness in choosing a suitable surface modifier from a variety of chemicals to investigate the mechanism of how these modifiers react with MDH.In this paper,a series of silane coupling agents with different structures were used to modify MDH by dry and wet process for studies of the mechanisms of these processes.The relationships and reaction laws between coupling agent structure,functional groups and their modification properties were shown and summarized,providing reference and better choice for industrial MDH modification and the use of modifiers.The main experimental contents and results are as follows:(1)The Optimization of the Dry and Wet Modification of MDH Using Silane Coupling Agents with Different StructuresEight kinds of silane coupling agents with different structures were selected(O174,O1741,O1744,V151,N308,O1871,S1891 and A110)to conduct dry and wet modification respectively.The modification conditions were optimized with sedimentation curve,activation index and water contact angle.The favorable conditions for dry modification: modifying time being 10-15 min;modifying temperature being 100-120°C,three kinds of methacryloxypropyl silane requiring higher temperature than others;the dosage of modifier being 1.5-2.5 wt%(the modification effect rise with the increasing amount of modifier until it reached the highest value,and then started dropping).The favorable conditions for wet modification: pre-hydrolysis time being 1.5-6.0 h,with huge difference,having effects on the structure of the alkoxy group binding to the inorganic substance(the required time: methoxy < ethoxy < isopropoxy)and the active functional group structure binding to the organic substance(the hydrolysis of aminosilane was significantly faster than that of others);modifying time being 40 ? 80 min;modifying temperature being 40-70°C;the dosage of modifier being 2.0-4.0 wt%,with big difference(the modification effect rise with the increasing amount of modifier without inflection point).(2)The Characterization and Mechanism of Dry and Wet MDH Modification Using Silane Coupling AgentsSedimentation tests,water contact angle tests,SEM,TEM and XRD were conducted to test the MDH modified by the silane coupling agents with different structures in dry and wet process and to get the indirect results of the modification effect.The results showed that all silanes successfully coated the MDH surface in dry and wet conditions,among which the MDH modified by alkyl silane had t he best dispersible,hydrophobic and oleophilic properties,whereas the aminosilane-modified MDH had the worst ones.In terms of mechanism study,the vinylsilane was chosen to explore how the modifiers reacted with the surface of MDH in both dry and wet modification through TGA,FT-IR,NMR and XPS analysis.The results showed that the modifier and MDH in the dry process were chemically adsorbed on monomolecular layer,and the increase of the amount of modifier led to increasing physical adsorption,while in the wet process they were chemically adsorbed on molecular layer and the increase of the amount of modifier led to increasing chemical adsorption.(3)The Effect of Dry and Wet Modification of Silane Coupling Agents with Different Structures on the Properties of CompositesThe MDH modified by silane coupling agent with different structures was added into EVA to prepare composites and tested for their processing,mechanical,flame retardant and smoke suppression properties.In general,after the dry and wet modification,the dispersion of MDH in EVA and its compatibility with the EVA,as well as its processing property,had been improved.The effect of the wet modification,due to its higher amount of modifier,was better than that of the dry one.Among all the silane,Aminosilane was superior in improving the tensile strength and flame retardancy of composites,but worst in increasing the elongation at break.Alkylsilane showed the best performance in enhancing the elongation at break of composites,but worst in enhancing the tensile properties,and it reduced the flame retardant of composites instead of increasing it.The results of this work provided the theoretical and experimental data for the dry and wet modification of MDH using silane coupling agents,verified the mechanism between sliane and MDH and explored the effects on the properties of composites.It provides a reference for selecting and using silane in treating MDH.