Preparation And Properties Of Melamine Based Composite Aerogels

Aerogel is a new kind of nanoporous material with many excellent physical and chemical properties,such as low density,high porosity,high specific surface area,adjustable surface chemical properties and so on.Due to these unique properties,the aerogels have been applied widely in many fields,such as adsorption materials,super capacitors,bearing materials,insulation materials and the inertial confinement fusion(ICF)target.Aerogels mainly include inorganic aerogels,organic aerogels and carbon aerogels.Melamine formaldehyde(MF)aerogels belong to organic aerogels.Compared with resorcinol formaldehyde aerogels,there are a few reports on MF aerogels in recent years.MF aerogel has its own advantages:chemical structure is stable;it does not change color because of oxidation reaction;the reaction monomer has high degree of functionality.But at the same time,the MF aerogel also has its disadvantages:the gelation time of MF aerogels prepared by using traditional method is too long;the density of MF aerogels is higher than that of other organic aerogels;the mechanical strength is higher,but the toughness is poor,these will went against enginery processing.In this paper,the natural polymer modified MF aerogels were prepared by introducing starch,chitosan and graphene into MF system,and the preparation process,structure and properties of the modified MF aerogels were studied.Starch/MF composite aerogels were prepared through physical cross-linking between MF and starch by adding starch into the MF system.The effects of the concentration of reactants,the ratio of reactants,the pH value of the system and the reaction temperature on the gelation process,the microstructure and the specific surface area of the aerogel were investigated.It was found that the gelation time was greatly reduced(7 h)when a small amount(5%)of starch was added to the reactants.Meanwhile,the prepared starch/MF composite aerogels with a density of 38 mg/cm~3 had a extremely low volume shrinkage(0.35%)during the solvent exchange and supercritical drying.Compared with the pure MF aerogel,the pore size distribution of the composite aerogel was more uniform,and the mechanical toughness was improved obviously(compressive deformation up to 79%).In order to further shorten the gelation time and strengthen the network structur e of gels,chitosan/MF composite aerogel with the density less than 10 mg/cm~3 was p repared by adding the chitosan containing amino groups into the MF system.The effects of reactant concentration,reactant ratio,pH value and reaction temperature on the sol-gel process and the microstructure and morphology were discussed.It was found that the comprehensive performance of chitosan/MF aerogels was best when the content of chitosan in reactants was 10 wt%.The gelation time was reduced to 10 min,and the prepared aerogel had many excellent properties,such as low shrinkage(3.5%),high surface area(392.2 m2/g)and best mechanical toughness.Compared with the starch/MF aerogel,the chitosan/MF composite aerogel prepared under different reactant ratios all had excellent mechanical toughness.In order to achieve the purpose that the composite aerogel has both higher Young’s modulus and good mechanical toughness,the chitosan/graphene/MF composite aerogel was prepared by adding graphene into the chitosan/MF system.The effects of the addition of graphene on the gelation time,morphology and specific surface area were studied.Graphene contains a large number of active groups,which could further enhance the chemical crosslinking of the composite system.It was found the gelation time was further shortened and the composite aerogels had lower volume shrinkage.Graphene has excellent mechanical properties and its young’s modulus is about 1 TPa.Therefore,the introduction of graphene in chitosan/MF system could effectively improve the young’s modulus of composite aerogels.Compared with chitosan/MF aerogel,the three element composite aerogel had good mechanical toughness and its young’s modulus was further improved.This method provides a new possibility for the preparation of MF composite aerogels with excellent comprehensive properties.The carbon aerogels were prepared by the thermal annealing process,and the structure and properties of the carbon aerogels were studied.The results showed that the carbon aerogels had excellent electrochemical properties and adsorption properties.