Flexible natural tannin-based and protein-based biosourced foams

Abstract

Flexible tannin foams as opposed to the rigid tannin foams already prepared, were obtained by the addition of an external (non-reacted) plasticizer, namely glycerol. Glycerol was chosen for its high boiling temperature and the lack of evaporation, coupled to its lack of toxicity. Flexible albumin protein foams were also prepared using glycerol as an external plasticizer. Flexibility and spring-back of these experimental foams when subjected to a cyclic compression force followed by spring-back and compression again was quantified by both thermomechanical analysis at different temperatures as well as by compression/spring back hysteresis cycle tests in a universal testing machine. Tannin foams containing formaldehyde and without glycerol have been shown to reach a stress plateau indicative of structure crushing. Tannin foams without formaldehyde but without glycerol too, becomes very fragile, brittle and rigid just two months after their preparation again presenting structure crushing with ageing. Instead, tannin foams without formaldehyde but with glycerol added do not show any change of flexibility with time and remain truly flexible. Albumin foams to which glycerol was added also do not show any change of flexibility with ageing. The glass transition temperature of the materials constituting the foams was measured by thermomechanical analysis.