Abstract
Foaming an epoxy is challenging because the process involves the curing reaction of epoxy and hardener (from monomer to oligomer, to a gel and a final three-dimensional crosslinked network) and the loading of gas phase into the epoxy phase to develop the cellular structure. The latter process needs to be carried out at the optimum curing stage of epoxy to avoid cell coalescence and to allow expansion. The environmental concern regarding the usage of chemical blowing agent also limits the development of epoxy foams. To surmount these challenges, this study proposes a solid-state CO2 foaming of epoxy. Firstly, the resin mixture of diglycidylether of bisphenol-A (DGEBA) epoxy and polyamide hardener is pre-cured to achieve various solid-state sheets (preEs) of specific storage moduli. Secondly, these preEs undergo CO2 absorption using an autoclave. Thirdly, CO2 absorbed preEs are allowed to free-foam/expand in a conventional oven at various temperatures; lastly, the epoxy foams are post-cured. PreE has a distinctive behavior once being heated; the storage modulus is reduced and then increases due to further curing. Epoxy foams in a broad range of densities could be fabricated. PreE with a storage modulus of 4 × 104–1.5 × 105 Pa at 30 °C could be foamed to densities of 0.32–0.45 g/cm3. The cell morphologies were revealed to be star polygon shaped, spherical and irregularly shaped. The research proved that the solid-state CO2-foaming technique can be used to fabricate epoxy foams with controlled density.
Highlights
Thermoset foams have become more and more essential in various industry sectors
A steep increase was observed at an elevated temperature of 80 ◦C, as shown in Figure ??. This behavior was totally different from the rheology of diglycidylether of bisphenol-A (DGEBA) and a lower molecular weight hardener such as N-aminoethylpiperazine, (AEP) or isophorone diamine (IPDA) (Bethke et al, 2019), in which the modulus just increased slightly before the gel point and sharply increased at the gel point
PreEs fabricated from DGEBA and 260A could be controlled to achieve various specific moduli and different pre-crosslinked network structures
Summary
Thermoset foams have become more and more essential in various industry sectors. These foams are used in applications requiring good thermal stability, high flame resistance, burning without drips and excellent solvent resistance, for example, in automotive, electronic, marine, aeronautical and space products [? ? ? ]. The process is called the resin direct foaming process, and is a common practice for polyurethane, phenolic and epoxy. Epoxy and hardener in the resin stage have low molecular weight, so the usage of gas blowing agents (GBAs) such as CO2 is inconvenient for foaming because of its high diffusion rate. Directly via absorption and desorption (similar to CO2 batch foaming) [20,21] To implement this technique, a solid epoxy needs to be prepared in advance. He weight uptakes of acetone of the preE and completely cured epoxy were tested. The sample was tested isothermally and at fixed time to achieve the studied modulus, and allowed to cool down at a rate of 10 ◦C/min to 30 ◦C (to obtain the storage modulus of preE at 30 ◦C as St.M30 ◦C). To avoid charging during SEM observation, samples were sputtered with a 13 nm gold layer by using a Cressington Sputter Coater 108auto (Cressington Coating systems, Dortmund, Germany)
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