Cell characteristics of epoxy resin foamed by step temperature-rising process using supercritical carbon dioxide as blowing agent

Abstract

A temperature-rising batch foaming process with supercritical carbon dioxide (ScCO₂) as blowing agent was used to prepare epoxy resin foams consisting of diglycidyl ether of bisphenol A and m-xylylenediamine. The dissolution and diffusion behaviors of CO₂ in pre-cured epoxy resin were investigated, as well as the parameter effect of CO₂ saturation step and foaming step on the cell characteristics. It was proved that 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 epoxy resin samples. With increasing CO₂ concentration from 0.021 g CO₂/g epoxy resin to 0.061 g CO₂/g epoxy resin in the unsaturated samples, the cell size increased from 170.2 µm to 262.6 µm and the cell density decreased from 6.8 × 10⁵/cm³ to 3.1 × 10⁵/cm³. Bubble nucleation and growth occurred simultaneously with curing reaction in temperature-rising step. As the final foaming temperature increased from 60℃ to 120℃, the cell size of samples with dissolved CO₂ concentration of 0.021 g CO₂/g epoxy resin increased from 172.7 µm to 369.0 µm, while the cell density first increased from 6.8 to 7.3 and then decreased to 3.5. The cell size of samples with CO₂ concentration of 0.031 g CO₂/g epoxy resin increased from 145.3 µm to 180.5 µm with foaming time from 5 min to 20 min, but changed slightly when curing reaction almost finished and CO₂ was depleted after 20 min.