Research On The Morphology Control And Process Of Thermally Expandable Microspheres

Thermally expandable microspheres are mainly prepared by free-radical suspension polymerization in situ encapsulated blowing agent.In the polymerization process,there are water phase,oil phase(monomer and blowing agent)and polymer phase.Phase separation occurs because the resulting polymer is insoluble in foaming agents.The solubility of monomer in water affects the interfacial tension and viscoelasticity of the reaction solution during polymerization.In addition,the monomer structure and properties affect the degree of movement,swelling,or precipitation of the polymer chain.Therefore,the evolution of the morphology of the microspheres during polymerization and the final morphology of the microspheres are affected.At present,there is a lack of research on the polymerization process and morphology regulation of thermally expandable microspheres.In this paper,the water solubility of monomers,the structure and properties of monomers,initiators,foaming agents,polymerization temperature and other factors were discussed.The effect of phase separation on the morphology and structure of thermally expandable microspheres and the expansion performance of thermally expandable microspheres were studied.The details are as follows:Firstly,P(MMA-ST)thermal expandable microspheres were prepared by suspension polymerization with methyl methacrylate(MMA)and styrene(St)as monomers.The effect of the monomer water-solubility,the initiator,the blowing agent,and the polymerization temperature on the morphology and the structure of thermally expandable microspheres with a core-shell structure were studied from the perspective of the phase separation during polymerization.It showed that when the amount of water-soluble MMA monomer is large,AIBN can initiate the water phase polymerization to form secondary polymer particles adsorbed on the surface of the microspheres,and at the same time accelerated the phase separation process during the polymerization process,which helped to form the core-shell microspheres.LPO could not initiate water phase polymerization and the surface of the microspheres was smooth after polymerization.When the boiling point of the foaming agent is lower than the polymerization temperature,the inside of the microsphere is porous and there are a lot of holes on the surface and with the small amount of blowing agent encapsulated.Furthermore,we also investigate the formation of the core-shell structure when the weight ratio of MMA to St is 75:5 using 2,2'azodiisobutyronitrile(AIBN)and dilauryl peroxide(LPO)as initiators,respectively.AIBN could accelerate the phase separation process,while in the LPO system,the phase separation was slow and there was an intermediate state with a porous surface.However,all of them were core-shell microspheres and had the same amount of blowing agent.Secondly,P(MMA-MAA)thermal expandable microspheres were prepared by polymerization of MMA and methacrylic acid(MAA).The effects of monomer hydrophilicity,initiator,sodi?m nitrite(NaNO2),crosslinking agent and foaming agent on the morphology and expansion property of microspheres were studied.The results showed that AIBN could induce water phase polymerization and form flocculent polymers with irregular morphology.Adding an appropriate amount of NaNO2 to the AIBN initiation system can inhibit the polymerization of the water phase and generate core-shell polymer microspheres.However,LPO could not initiate the water phase polymerization,the particle polymer chains are separated from the blowing agent before they moved and melted to the surface of the microspheres.So there were many small particles inside the formed microspheres.When the amount of foaming agent is increased to 40 g,microspheres with a core-shell structure are formed.In the AIBN or LPO initiation system,compared with the divinyl crosslinker divinylbenzene(DVB)and ethylene glycol dimethacrylate(EGDMA),the swelling of the microspheres prepared by the trivinyl crosslinker trimethylolpropane trimethacrylate(TMPTMA)is better.Finally,the effects of monomer water solubility,monomer structure and properties and vitrification transition temperature on the morphology and structure of microspheres were studied.The results showed that when the amount of St in MMA/St ratio was less than or equal to 1/4 wt%(total monomer mass percentage,the same below),complete phase separation occurred to form core-shell microspheres;Incomplete phase separation occurred when the St dosage was 1/2 wt%,and microspheres with holes were generated.Butyl acrylate(BA)and isopropyl methacrylate(IBOMA)have the same solubility in water.When the amount of MMA in the ratio of MMA/BA and MMA/IBOMA is 15/16 wt%,microspheres with core-shell structure were formed.With the increase of the amount of BA,due to the larger viscosity of the BA and the lower glass transition temperature of the BA polymer,the foaming agent passed through the microsphere shell under the action of agitation and shear force,so there are pores inside and on the surface of the microsphere.With the increase of the amount of IBOMA,the glass transition temperature of the IBOMA polymer is relatively high,and the IBOMA polymer chain has a large rigid side group,so the polymer microsphere is a solid structure.When the MMA/ Methyl acrylate(MA)ratio ranges from 15/1-1 to 1/1,complete phase separation occured and core-shell microspheres were formed.Acrylic acid(AA)is miscible with water.At the initial stage of polymerization,AA diffuses to the aqueous phase,leading to early phase separation.The resulting microsphere is internally coated with many small particles...