Comparison of dehydration methods for untreated lignin resole by hot air oven and vacuum rotary evaporator to synthesize lignin-based phenolic foam.

We investigate two different dehydration methods to determine their suitability for preparing resoles for foam synthesis. A simplified process for synthesizing lignin foam (LF) from lignin resole (LR) dehydrated in a hot air oven (HAO) is compared with that dehydrated using a vacuum rotary evaporator (VRE). First, the LR formulation is prepared by mixing phenol with untreated lignin (0%-15% by weight), and subsequently, the prepared LRs are dehydrated using an HAO and a VRE. We find that for the same dehydration time, both techniques yield LRs with the same chemical compositions; however, the HAO technique affords a moisture removal of 13-17% by weight, whereas the VRE technique removes 9-12% moisture by weight. The LR obtained by the HAO is more viscous and maintains a circular shape after being dropped on a plate. In our experimental synthesis of LF containing VRE resole, biofoam is not formed owing to insufficient viscosity, whereas biofoam is obtained with the HAO resole. The synthesized LF exhibits a density range of 44.96-85.68 kg/m and a compressive strength of 103.28-152.27 kPa. Scanning electron microscopy investigations show that the morphology of the foam is a closed-cell structure. The simplified synthesis of LF from the HAO-treated resole offers significant advantages over the complexity of the conventional VRE approach in terms of equipment cost and energy consumption. The resulting foam exhibits a thermal stability and thermal performance comparable with the counterpart properties of phenolic foam.