Preparation And Performance Of Epoxy Vitrimer Foams Via Supercritical CO₂ Foaming

In recent years,the application of thermosetting foams in the industrial field has aroused great interest among researchers.Thermosetting foam materials such as epoxy resin foam,polyurethane foam and phenolic resin foam have excellent mechanical properties,corrosion resistance,heat resistance and dimensional stability due to their cross-linked network,which have been widely used for thermal insulation,shock absorption and lightweight components.However,once the thermosetting resin is cured and cross-linked,it is difficult to melt or dissolve,which makes its recovery and reuse extremely challenging.Epoxy resins,as the most widely used thermosetting polymers,can be recycled and reused by the introduction of dynamic covalent bonds into its cross-linked network,yielding dynamic cross-linked epoxy vitrimers.While most of the research on epoxy vitrimers was focused on the structural design,recyclability and functionalization of solid vitrimers,the feasibility of epoxy vitrimers for supercritical CO₂ foaming and reprocessability of vitrimer foams were rarely studied.The development of high-performance epoxy vitrimer foams with excellent reprocessability is of great significance for the industrialization and wide application of epoxy vitrimers.In view of the above problems,based on the exchange mechanism of dynamic ester bonds and the foaming principle of supercritical carbon dioxide（SC-CO₂）,this paper introduced ester bond into the epoxy anhydride system catalyzed by zinc salt,and prepared dynamic crosslinking epoxy resin vitrimer and its microporous foam material with recycling characteristics.We systematically investigated the influence of raw material components and foaming conditions on foaming behavior,mechanical properties,thermo-mechanical properties and recovery properties of epoxy resin Vitrimer.On this basis,conductive functional particles were added to epoxy resin vitrimer to construct conductive foam composite materials,which gave the materials such functions as conductivity and electromagnetic shielding,providing a new idea for the functional application of recyclable epoxy resin vitrimer foam.The main research results of this paper are as follows:（1）A recyclable epoxy resin vitrimer was constructed with giglycidyl ether of bisphenol A（DGEBA）as resin prepolymer,phthalic anhydride（PA）as curing agent.It can be recycled for more than two times after complete curing.The crosslinking density（80-94 mol/m~3）,glass transition temperature（84℃-92℃）and mechanical properties（tensile strength:19-29 MPa）of epoxy resin vitrimer can be controlled within a certain range by changing the ratio of epoxy resin vitrimer.Under the condition of this distribution ratio,the epoxy resin vitrimer matrix has suitable viscoelasticity,which can meet the infiltration and growth of gas.The saturated adsorption capacity of CO₂ of fully cured epoxy vitrimer can reach 56 mg/g after soaking at 45℃and 20 MPa SC-CO₂for 48 h,and the foaming material with uniform bubbles can be obtained through temperature rise foaming at 90-130℃.The morphology of the cell structure can be controlled by controlling the cross-linking density and foaming temperature of the resin:increasing the foaming temperature can reduce the cell density,increase the cell size and change the cell wall thickness.The foaming materials with cell diameter of 4-14μm,foaming ratio of 8-15 and cell density of 3.7-10¹¹-7.110~9cell/cm~3 can be obtained at the foaming temperature of 90℃-130℃;Adjusting the ratio of DGEBA/PA/ZAA can adjust the cross-linking density of the material and then affect the cell morphology,and a microcellular foaming material with a cell diameter of 4-9μm and a porosity of54%Murray 92%can be obtained.The compression property of epoxy resin vitrimer foaming material decreases with the increase of foaming ratio.When the foaming ratio is 9,the compressive strength can reach 0.74 MPa,the compressive modulus can reach 11MPa,and its thermal conductivity decreases with the increase of foaming ratio,and finally tends to be stable.When the foaming ratio is 11:00,the thermal conductivity can be as low as 32.6 m W/m K.（2）Low-layer graphene（GNP）/vitrimer composites were prepared by solution blending.GNP has good dispersion in vitrimer resin.GNP hinders the movement of molecular chain and increases the glass transition temperature（T_g）of the composites.The T_g of the composites with 10 wt%GNP addition is 102℃.In the range of 2-10 wt%,with the increase of the amount of GNP,the electrical conductivity and electromagnetic shielding effectiveness of GNP/vitrimer composites also increase.The electrical conductivity of vitrimer-10（10 wt%GNP）can reach 37S/m and the electromagnetic shielding effectiveness can reach 20.3 d B.GNP/vitrimer composites can be recycled and reprocessed,but their electrical conductivity decreases due to the destruction of the conductive path after recovery.The microcellular foam of GNP/vitrimer composite was prepared by supercritical CO₂ foaming technology.The heterogeneous nucleation effect produced by the addition of GNP not only promoted the cell formation,but also hindered the further growth of the cell,so the dense small cell with narrow range of cell size distribution was obtained.In the range of 2-10 wt%GNP,the average cell size and foaming ratio of GNP/vitrimer foaming materials decreased with the increase of GNP content,from 3μm to 1μm,and from 5 to 1.In addition,the cell destroys the continuity of the conductive path of GNP/vitrimer composites,resulting in a decrease in electrical conductivity compared with that before foaming.