Research progress in preparation methods for gradient polymer foams
The properties of polymer foams vary regularly with density change,and density control of the foams is an essential tool for the preparation of gradient functional foams.Due to the different properties of the polymer matrix,the polymer foaming process needs to be regulated according to the matrix properties and processing conditions to control the foam morphology so as to obtain gradient foams with specific structures.The possibility of preparing gradient polymer foams by using the methods of induced nucleation,external force field pretreatment and physical dispersion of prepolymers in the stage of bubble nucleation was firstly introduced.Then the research progress of gradient regulation of polymer foam density by using the methods of temperature differ-ence,pressure difference and concentration difference of the foaming gas in the stage of bubble growth was categorically reviewed.The density gradient regulation means for thermoplastic foams were mostly targeted at the bubble growth stage,and the effect of heterogeneous induced nucleation at the nucleation stage was also obvious,while the density gradient regulation means for thermo-setting foams were more effective at the bubble nucleation stage.Finally,the conditions of use and effectiveness of the above meth-ods were summarized.
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