Freeze-dried wheat gluten biofoams; scaling up with water welding

This paper presents a simple and rapid wet welding technique that enables the scaling up of freeze-dried protein (wheat gluten (WG)) biofoams for e.g. thermal insulation applications. The welding occurred by first wetting faces of foam cubes in water and then pressing them together for a limited time period. The water plasticized thin cell-walls of the two foams formed a dense weld when the plasticized cells collapsed under the drying step. The welds were always stronger and stiffer than the surrounding cellular structure. Based on three-point bending, it was shown that welded specimens (four-cube samples) were 7 times stronger than specimens produced directly as one piece with similar total size. This illustrated the problem of freeze-drying larger products; by instead assembling smaller foams into a large object the overall foam structure became more homogeneous. In addition, the dense welds become "walls" that limit gag convection in the mainly open cell structure, beneficial for thermal insulation. This is the first report on combined freeze-drying and water welding. It shows the sustainable potential of the technique for foam production, since only water is used as a foaming/welding agent. (C) 2016 Elsevier B.V. All rights reserved.