| Polymethacrylimide(PMI)foam is a kind of foam with a 100%closed cell structure,easy to process and shape,and excellent comprehensive properties.Under the same density,the strength,stiffness and heat resistance of PMI foam are higher than other foams.As a sandwich material,it is widely used in aerospace,transportation,wind power and other fields.In this thesis,PMI composites were prepared by mixing uniformly in a two-step method using acrylonitrile(AN),methacrylic acid(MAA),methyl styrene(AMS)as monomers and crylamide(AM)as crosslinking agent,azobisisobutyronitrile(AIBN))as initiator,formamide(FA)and carbonamide as foaming agents.On this basis,PMI hybrid foams were successfully prepared by adding nano-Si O2,graphene and montmorillonite(MMT)fillers.Optical microscope and scanning electron microsope(SEM)were used to analyze the microstructure of PMI foam and its hybrid foam.The results showed that the cell walls of the PMI hybrid foam with nanoparticles were thickened and the cell pore size was increased.The density of the prepared neat PMI foam was 98.5 kg/m3,and the densities of nano-Si O2/PMI,graphene/PMI,and MMT/PMI foam were 112.6 kg/m3,99.1 kg/m3,108.2 kg/m3,respectively.The hardness of the PMI hybrid foam with nano-particles was reduced,and the hardness of the neat PMI foam,nano-Si O2/PMI,graphene/PMI,and MMT/PMI foam were 40,17,29,and 32,respectively.Compared with other types of PMI foams,nano-Si O2/PMI foam had the best compressibility,with a compressive strength of 3.39 MPa.The compressive strengths of neat PMI,graphene/PMI,and MMT/PMI foams were2.63 MPa,2.75 MPa,and 3.06 MPa,respectively.TGA analysis results showed that at 800?,the residual carbon content of neat PMI foam was 16.07%,and the residual carbon content of nano-Si O2/PMI,graphene/PMI,and MMT/PMI were 23.78%,28.83%,24.64%,respectively.The thermal stability of the PMI foam with nanoparticles can be improved.The friction and wear properties of PMI foam and its hybrid foam were tested under atmospheric environment and dry friction conditions.The wear rate of neat PMI foam had reached 6.35×10-6 cm-3/Nm at a sliding speed of 0.15 m/s.There were more furrows or even fractures on the worn surface.The friction coefficient of nano-Si O2/PMI foam under low load was only 0.069.The surface roughness of the material was greatly improved because of the small size effect and high strength of nano-Si O2.During the friction process,the load applied to the friction pair was borne by the nano-Si O2 particles with higher strength,hardness and better wear resistance,and the resistance to ploughing was enhanced.The maximum wear rate of the graphene/PMI hybrid foam was 6.55×10-7 cm-3/Nm,and the wear rate was greatly reduced.Graphene was lubricated,coupled with the synergistic effect of nano-Si O2 particles,the wear effect was significantly improved,and the wear rate was reduced by an order of magnitude.The minimum friction coefficient of MMT/PMI hybrid foam was 0.07.The self-healing effect of MMT particles and nano-Si O2 particles was synergistic,and the worn surface was filled and repaired during the wear process.The complete friction film on the contact surface was produced,the effect of lubrication was played,and the coefficient of friction was reduced.The PMI hybrid foam with nano-particles had better friction and wear properties compared with neat PMI foam.In order to explore the thermal expandable microspheres of the AN/MAA system,firstly,the thermally expanded microspheres were prepared by Pickering emulsion suspension polymerization using AN and MMA as the main monomers,and the particle size of the microspheres was concentrated in40-50?m.The emulsion was stratified when the stirring speed was too low.The microspheres were ruptured when the stirring speed was too fast,the thermally expanded microspheres with a core-shell structure could not be obtained.The thermally expandable microspheres did not have a complete spherical shape when the reaction pressure was too low,and the shape of the microspheres was irregular,and the expansion ratio was low or even unable to be expanded.When the reaction pressure was too high,the shell walls of the microspheres were thinner and a complete shell-core structure was not formed,the encapsulation of the foaming agent was low,and the microspheres were not expanded. |
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