| Thermosetting epoxy resin and its foams,known for their low density,low hydroscopicity,low shrinkage,high strength,high electrical insulativity and heat resistance,are widely applied in resent years.Epoxy resin foams also exhibit higher heat resistance and lower energy cost than other thermosetting foams.High pressure CO2,regarded as a green solvent,plasticizer and blowing agent has avoided the problem of toxic,discharge and residue and has been widely applied in polymer reaction and processing.Especially,the green manufacture technology of polymer foamed by supercritical CO2 is widely concerned,because it can remove the unstable bubbles coming from the large amount of heat during traditional chemical foaming process and produce the uniform microporous materials with high performance.In this work,the curing reaction and two-step temperature rising foaming process of epoxy resin system consisting of diglycidylether of bisphenol A(DGEBA)were studied using high pressure CO2 as reaction solvent and blowing agent.The curing reaction kinetics were established,the available foaming range was obtained,the epoxy resin foams with varied morphologies were prepared and the compressive properties of prepared foams were investigated.The results provide the basic data and theoretical basis for preparation of epoxy resin and foams with high pressure CO2 as green solvent.Firstly,the non-isothermal and isothermal curing kinetics of diglycidyl ether of bisphenol A(DGEBA)with m-xylylenediamine(MXDA)under compressed CO2 was investigated by high-pressure differential scanning calorimetry(DSC)and Fourier transform infrared spectroscopy(FTIR)respectively.During both non-isothermal and isothermal curing processes,the high pressure CO2 can promote curing reaction obviously without affecting the curing mechanism.From the non-isothermal curing kinetics results,the Sestak-Berggren autocatalytic model exhibits good agreement with the experimental data.Moreover,the activation energy determined by isoconversional method decreases with the increasing CO2 pressure.On the other hand,amended Kamal model describes the isothermal reaction rate properly and autocatalysis process dominants the curing reaction.Further,the reaction rate changes slightly between 2 MPa to 6 MPa,and then increases significantly with CO2 pressure from 6 MPa to 16 MPa.When the pressure is higher than 16 MPa,the reaction rate tends to stable.Secondly,the effect of pre-curing step on the foaming results was investigated,and the available curing degree window for foaming was obtained.According to the isothermal curing kinetics,samples with different pre-curing degrees were prepared and then foamed via temperature-rising foaming process.It was found that the pre-curing degree was a crucial index for the foamability of epoxy resin.The results showed that the pre-curing degree from 37.7%to 46.3%,the complex viscosity from 104 to 106 Pa-s and tan? from 0.8 to 10 were the optimum foaming ranges for the chosen epoxy resin.The samples with curing degree lower than 37.7%present coalescing and cracked bubbles due to the low viscosity,while samples with higher pre-curing degree cannot be foamed because of high mass transfer resistance of CO2.With increasing pre-curing degrees,average cell size of epoxy foams decreased from 329.8 ?m to 60.8 ?m while cell density increased from 1.4×105 cells/cm3 to 8.6×105 cells/cm3.Furthermore,the foamed samples with the same pre-curing degree had similar cell morphology regardless of pre-curing conditions.Next,the effect of CO2 saturation and foaming conditions on the foaming results was discussed.The CO2 concnetration was controlled by adjusting saturated time and pressure,and the dissolution and diffusion of CO2 in the epoxy resin are investigated through mass-loss technique and desorption method.The CO2 concentration and foaming temperature were regarded as crucial index for foaming results.From the results,closed-cells could be generated for CO2 unsaturated samples and the cell characteristics with the same dissolved CO2 concentration were similar.The merged and cracked bubble morphologies were usually obtained for CO2 saturated samples.With increasing CO2 concentration from 0.021 g CO2/g EP to 0.061 g CO2/g EP in the unsaturated samples,the cell size increased from 170.2 to 262.6 ?m and the cell density decreased from 6.8×105/cm3 to 3.1×105/cm3.Bimodal-cell structure is obtained when samples saturated completely under 8 MPa,due to the plasticization and reduced strength of epoxy resin.Bubble nucleation and growth occurred simultaneously with curing reaction in temperature-rising step.As the final foaming temperature increased,the cell size of samples increased,while the cell density first increased and then decreased.The cell size of samples increased with foaming time,but changed slightly when curing reaction almost finished and CO2 was depleted after 20 min.Originally,epoxy foams with diameter of 46.8-65.1 ?m were successfully obtained by a two-step saturated method in which dispersion of CO2 gas in epoxy resin was improved.In the first step,the epoxy resin was fully saturated by CO2.In the second step,the degree of supersaturation decreased due to the CO2 diffusion,resulting in less bubble growth as well as little coalescence and burst.Finally,the compressive properties tests of samples with different cell morphologies were performed.The compressive strength increases with curing degree.At a certain curing degree of 100%,the compressive property of closed-cells structure was higher than that of open-cells ones,while the cracked foams was the worst.The compressive properties of closed-cells were affected by foam density and cell size,cell size influenced the mechanical property significantly.Samples which have higher densities,smaller and more cells possess higher compressive strength and modulus.The size of micro-cellulars decreased dramaticly,and resulted in increasing compressive strength and modulus.The smaller cell provides better compressive properties in the bimodal structure,exhibiting better compressive properties than the unimodal foaming materials.Besides,the cell size and distribution affected the compressive properties of epoxy resin foams with bimodal structure.The compressive properties decreases with the broader distribution. |
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