Preparation And Foaming Behavior Of Heatproof High-temperature Thermally Expandable Microspheres

Thermally expandable microspheres with a huge volume expansion capacity have important applications in the fields of weight-lightening,patterning,and three-dimensionally shaping of materials.For example,in the field of three-dimensional printing,after adding thermally expandable microspheres,besides of the original pattern,the printed textiles may display a three-dimensional effect.Suspension polymerization is a common method for preparing thermally expandable microspheres.Most of the previous reports have focused on the study of the foaming temperature and expansion performance of thermally expandable microspheres.There are few researches on the high-temperature foam stability of expanded microspheres.Normally,it is difficult to achieve good expansion performance and high temperature foam stability,simultaneously.To reach a balance between these two performances requires fine adjustment of many reaction parameters.Therefore,we thoroughly investigated the synthsis parametes influencing the particle characteristics and foaming properties(foaming temperature,foaming ratio,and high temperature foam stability)of microspheres in this thesis.On the basis of the dependence of the particle characteristics and foaming properties of the microspheres on the synthesis parameters,the synthesis process,the composition of shell copolymers and foaming agents were reasonably designed and optimized to match the expansion ability of the foaming agent with the deformation ability of the shell polymer.Finally,thermally expandable microspheres with controllable size,adjustable foaming temperature,excellent expansion properties,and high foam stability at elevated temperatures were prepared.In this thesis,thermally expandable microspheres were prepared through suspension polymerization by using a high concentration sodium chloride solution as the aqueous phase,magnesium hydroxide as the dispersant,acrylonitrile(AN),methyl methacrylate(MMA),methacrylic acid(MAA),and N,N-dimethylacrylamide as the comonomers,alkanes as the blowing agent.The influence of synthesis parameters onthe particle size,particle size distribution,and particle morphology of the microspheres,the foaming temperature and expansion ratio of the microspheres,and the foam stability of the microspheres at high temperature were systematically studied.Firstly,the evolution of droplet/particle morphology,and particle size and particle size distribution during the polymerization process,as well as the effect of synthesis parameters on the particle morphology,particle size and particle size distribution of the microspheres were investigated.It is found that the shear rate and shear time of high-speed dispersion have minor effect on the morphology,particle size,and particle size distribution of thermally expandable microspheres.The agitation rate has significant influence on the colloidal stability of the reaction system,and the particle properties of microspheres.The colloidal stability of the system becomes worse,the particle size of microspheres becomes larger,and the particle size distribution becomes wider when the agitation speed is too fast or too slow.It is appropriate to use an agitation of 400 rpm.The addition of sodium dodecyl sulfate could improve the colloidal stability of the suspension polymerization system and the controllability in the particle size and size distribution of thermally expansdable microspheres.The high concentration of sodium chloride solution could effectively reduce the solubility of polar monomers in the aqueous phase,and reduce the particle nucleation of the aqueous phase,contributing to a more uniform particle size distribution of thermally expandable microspheres.The amount of cross-linking agent has a significant effect on the particle size and distribution of the microspheres.When the amount of cross-linking agent was 0.03 mol%,the particle size distribution of the microspheres was relatively narrow.Secondly,we studied the infleucne of the synthesis factors on the foaming temperature and expansion properties of microspheres.It is found that the concentration of sodium chloride solution,the amount of initiator,the composition of foaming agent,the type and amount of cross-linking agent,the amount of polar monomer MAA and the dosage ratio of MMA and AN would affect the foamingtemperature and expansion performance of the thermally expandable microspheres.High concentration of sodium chloride solution could improve the foaming temperature of the microspheres,but only has a little effect on the expansion ratio.Increasing the amount of initiator,the foaming temperature of microsphere only changes a little.The higher the boiling point of the foaming agent is,the higher the foaming temperature is.When a mixture of isooctane and isopentane was used as the blowing agent,with the increase of isooctane content,the foaming temperature of microsphere is increased.When the foaming agent was composed of 60 wt% of isooctane and 40 wt% of isopentane,the micorspheres displayed a high foaming temperature and high expansion ratio.The cross-linking agent is the key factor that determines the foaming properties of microspheres.The microspheres prepared by using a polymeric cross-linking agent containing double vinyl groups showed an excellent comprehensive foaming properties.With the increase of the amount of cross-linking agent,the foaming temperature of microspheres increases slightly,but the foaming ratio first increases and then decreases significantly.The monomer composition has a significant effect on the foaming properties of the microspheres.Microspheres had good foaming properties when the dosage ratio of MMA and AN was 18.5: 81.5 and the amount of MAA was 6.85 mol%.Finally,we studied the influence of the concentration of sodium chloride solution,the type and amount of cross-linking agent,the composition of the foaming agent,and the monomer composition on the foam stability at high temperature.The results show that the high concentration of sodium chloride aqueous solution helps to improve the high temperature foam stability of thermally expansdable microspheres.The addition of the polymer cross-linking agent P1 can obviously improve the foam stability of the microspheres.When the dosage of the crosslinker P1 is 0.033 mol%,the microspheres exhibit excellent foam stability.When the foaming agent was composed of isooctane and isopentane,the stabilization time of expanded microspheres increased first and then decreased with the increase of isooctane content.The expanded microphseres show a good high temperature foam stability when the dosage ratio of MMA and ANwas within a certain range.However,a too high amount of MMA will lead to a reduced high temperature foam stability of microspheres.The use of polar monomer MAA can significantly improve the heat resistance of the microspheres.With the increase of the MAA amount,the high temperature foam stability of microspheres would be enhanced.Under optimized process conditions,the thermally expandable microspheres with adjustable foaming temperature,high foaming ratio,and high temperature foam stability could be synthesized.Through the research in this dissertation,the corresponding relationship between the synthesis parameters and the particle characteristics of the thermally expandable microspheres was clarified,and the effective regulation on the particle size and size distribution of thermally expandable microspheres was achieved.Through the rational design of the composition of shell polymer and foaming agent,the expansion ability of the foaming agent and the deformability of the shell polymer could be well matched.The excellent expansion performance and high temperature foam stability of the microsphere could be achieved simultaneously.The research results of this thesis may have high values for the preparation of thermally expandable microspheres with controlled particle size,adjustable foaming temperature,excellent expansion performance,and good high temperature foam stability.