Microcellular Foaming And Electromagnetic Shielding Performance Of Epoxy Resin/Carbon-nanofiller Composites

Polymeric microcellular materials typically refer to foamed materials having uniformly distributed cellular structure with cell size of 1-10?m and cell density of10⁹-10¹² cells/cm³,exhibiting unique advantages including lightweight,thermal insulation,sound insulation,insulation,low dielectric constant,high impact strength,low cost,etc.Thus they are widely used in used in food packaging,sports equipment,automotive parts,aerospace,electronic packaging,biomedical materials and so on.Introducing carbon nano-materials such as carbon nanofibers?CNF?,carbon nanotubes?CNT?and graphene?Graphene?into polymeric microcellular materials,which can endow them with new functions of electrical conductivity,heat conduction,electromagnetic shielding,has become a new direction in the field of microcellular foaming research in recent years.However,the existing researches always involve thermoplastic polymers,while the thermosetting polymer nanocomposite foams are rarely reported,especially epoxy-based nanocomposite foams.There is a main problem that the structure of the crosslinking network must be well coordinated with gas infiltration and gas expansion.Based on the supercritical CO₂?SC-CO₂?foaming method,the microcellular foaming and electromagnetic shielding properties of epoxy resin and epoxy/carbon-nanofiller composites were systematically studied.The aim is to increase carbon-nanofiller loading amount and make them disperse uniformly by taking advantage of the low viscosity of epoxy resin before curing,then introduce microcellular structure inducing carbon-nanofillers to oriente,aggregate and interconnect to form conducting pathways,ultimately to improve the electromagnetic shielding efficiency of composites by enhancing the reflection and absorbtion of electromagnetic wave in the microcellular structure,and provide theoretical and experimental basis for the structure-effective relationship between microstructure and electromagnetic shielding effectiveness of thermosetting microcellular electromagnetic shielding materials.The related research contents and results are summarized as follows.?1?The bisphenol A epoxy prepolymer?DGEBA?/2-ethyl-4-methylimidazole?2E4MZ?system was taken as the research object for preparing epoxy foams.In this part,three kinds of epoxy monomers including E51,E44 and CYD-014U were selected to prepare epoxy sheets for foaming.The crosslinking morphology of epoxy resin was controlled by the ratio of curing agent and curing conditions.The results showed the crosslinking density of the cured epoxy reduced with the molecular weight of DGEBA increased.The effect of 2E4MZ content on crosslinking density of cured epoxy resins was investigated.In the certain range of 2E4MZ content,it could be found that the crosslinking density of the cured epoxy reduced with the 2E4MZ content increased,leading to the improvement of the foamability of epoxy resin.When 2E4MZ content is controlled at 35-50 mol%,the crosslinking density of epoxy resin is about 80-232 mol·m^-3,CYD-014U/2E4MZ foams with cell size of 10-12?m and cell density of 0.4-1.2×10⁹ cells/cm³ can be prepared by foaming,indicating content of 2E4MZ is moderate to adjust the crosslinking density of epoxy resin.Therefore,the crosslinking density of epoxy resin can be controlled by adjusting DGEBA molecular weight and 2E4MZ content.Then by adjusting the foaming parameters such as saturation pressure,foaming temperature and foaming time,microcellular epoxy foams with different cell size distribution and cell density can be obtained.?2?Carbon nanofibers?CNF?were treated with silicone surfactant,and then EP/CNF composites were prepared and foamed by supercritical CO₂ foaming.The bubble morphology of EP/CNF foams was observed by scanning electron microscope?SEM?,and the heterogeneous nucleation effect of CNF in SC-CO₂ foaming process was investigated.The volume conductivity and electromagnetic shielding effectiveness of EP/CNF samples before and after foaming were tested,and the effects of CNF content and cell structure on electrical conductivity and electromagnetic shielding properties were discussed.The results demonstrated that cellular structure and CNF played positive roles in enhancing the EMI shielding performance of nanocomposites.The introduction of cellular structure would facilitate the formation of effective conducting pathways,resulting in the promotion of electrical conductivity.The conductivity of EP/CNF composites increased from 10^-12to 10^-6 S/cm,and the EMI SE value increased from 4.3 to 16.5 dB,showing good conductivity and electromagnetic shielding properties.In addition,adding CNF into epoxy matrix can enhance the mechanical properties of the materials,and the well dispersed CNF can obviously improve the tensile strength,tensile modulus and impact strength of the microcellular epoxy materials.?3?MWCNT was introduced in epoxy matrix by utilizing a ZrP-assisting dispersion method,and then fabricated EP/F-MWCNT nanocomposite foams using a supercritical CO₂ foaming method.The cell morphology of EP/F-MWCNT foam was observed and analyzed by SEM.The effect of F-MWCNT content and cell structure on the electromagnetic shielding performance of the material was also discussed.The results showed that F-MWCNT with good dispersion in epoxy resin could play a more effective role in heterogeneous nucleation to promote the formation of bubbles and increase the density of bubbles.The improvement of the dispersion of carbon nanotubes in the matrix can promote the formation of conductive percolation network,and further enhance the conductivity and electromagnetic shielding properties of the materials.By conductive percolation analysis,the conductive percolation ranges of EP/F-MWCNT foamed samples and EP/F-MWCNT foamed samples were 0.28-1.12vol%and 0.06-0.32 vol%respectively,indicating that the introduction of microcellular structure decreased the conductive percolation threshold of EP/F-MWCNT composites.?4?Graphene oxide?GO?was obtained by Hummers oxidation method,then the reduced graphene oxide?RGO?was prepared through hydrothermal reduction.EP/RGO composites were prepared using RGO as filler and then foamed by SC-CO₂method.The electrical conductivity and EMI SE of EP/RGO foams were tested,and the effects of RGO content and cell structure on the electromagnetic shielding properties of the materials were investigated.It was found that the introduction of RGO and microcellular structure improved the electromagnetic shielding performance of the materials.Based on the study of EP/RGO electromagnetic shielding materials,MWCNT and RGO were simultaneously introduced into epoxy matrix,the results showed that the EMI SE of EP/RGO/MWCNT foams was superior to EP/RGO foams and EP/F-MWCNT foams under the same filling content,and the EMI SE reached22.6 dB when RGO/MWCNT content was 5 wt%.These results indicated that there is a synergistic effect between one-dimensional material MWCNT and two-dimensional material RGO.The one-dimensional and two-dimensional materials can inhibit the agglomeration of single material to a certain extent,and the conducting pathways are easier to form,resulting in the orientation of CNT and RGO maked the RGO/CNT conductive network further constructed after foaming,and ultimately the conductivity and electromagnetic shielding properties of the material can be improved.