Preparation,Mechanism And Application Of Thermally Expandable Microspheres

Thermally expandable microspheres(TEMs)have the advantages of controllable foaming volume,foam delicate and uniform cell.Compared with the gas foaming materials,the materials foamed with TEMs possessed better mechanical property,such as material strength.TEMs are useful products.It not only reduces theweightoftheparts significantly,but also improves their dimensional stability.The diameter of TEMs increased from 20 ?m to about 100 ?m when the system was heated from 80 ? to 190 ?,with their shells staying intact.As a result,the density of foamed materials decreased from 1000 kg/m3 to 30?40 kg/m3,and the cell of the as-foamed materials was uniform in size and shape.The core-materials of TEMs are low boiling-point organic solvents such as i-butane,n-hexane,petrol ether,etc.They are liquid at ambient temperature and easily vaporize at relatively low temperatures.TEMs are usually prepared by suspension polymerization in oil-in-water(O/W)suspensions while a monomer phase is suspended in a water phase containing a stabilizing agent.Further polymerization is conducted in the monomer droplets at high pressure.To the best of our knowledge,there are few reports studying the methods of preparation TEMs.To investigate green-environmental core materials,preparation methods and the formation mechanism have profound significance.The main contents in the paper were as following:(1)P(AN-co-MA-co-MMA)@isooctane TEMs with uniform thickness shell were prepared via suspension polymerization under atmospheric pressure.Effects of dispersant type,dispersant content,monomers ratio and core material type on onset foaming temperature and foaming volume were investigated.The onset foaming temperature and foaming volume were controlled by monomers ratio and core material content.(2)TEMs,which using PMMA as shell and NaHCO3 ethanol solutionas core,with core-shell structure were prepared by simple method of thermally induced phase separation(TIPS).Effects of preparation parameters on the morphology,amount of encapsulated NaHCO3 and foaming properties of TEMs were studied;the parameters investigated included:(i)quench rate,(ii)the molecular weight(Mw)and amount of PMMA,and(iii)the amount of NaHCO3.A mechanism for TEMs formation was proposed to comprise the following steps:1)separation of homogeneous polymer solutions into polymer-rich and polymer-poor phases with cooling where decreases in both interfacial tension and interfacial free energy(?G)facilitates the formation of spheres in the polymer-rich phase among polymer-poor phase;2)deposition of PMMA and formation the shell at the interface aided by the acute contact angle(?)and positive spreading coefficient(S);3)the constant deposition of PMMA shell from both the polymer-rich phase and polymer-poor phase with cooling;4)the detachment of TEMs from each other facilitated by shear force from stirring.This mechanism was supported by calculations,the effects of quench rate,and stirring speed.(3)An environmentally friendly protocol to prepare TEMs using water as blowing agent,acrylonitrile(AN),methacrylic acid(MA),and methyl methacrylate(MMA)as monomers was exploited.TEMs encapsulated water was prepared via inverse suspension polymerization under atmospheric pressure.Effects of dispersant type,dispersant content,monomers ratio and core material type on onset foaming temperature and foaming volume were investigated.The onset foaming temperature and foaming volume were controlled by monomers ratio and core material content.(4)Application of TEMs:Melamine resin foam and foamed silicone rubber with P(AN-co-MA-co-MMA)?isooctane,PMMA@NaHCO3,P(AN-co-MA-co-MMA)@H2O and P(AN-co-MMA)@NaHCO3 aqueous solution were prepared,respectively.The properties such as cell structure,thermal stability and compression strength were characterized by SEM,TGA and tensile machine.