Silk cocoons as natural macro-balloon fillers in novel polyurethane-based syntactic foams

With the growing interest in material technologies incorporating bio-based constituents, substitutes to both conventional polymer foams and their synthetic reinforcements are being sought. In this article, we characterise the physical, mechanical, thermal and environmental resistance properties of novel syntactic foams that are based on silkworm cocoons as natural, hollow particulate fillers for a rigid, bio-based polyurethane matrix. Syntactic foams were cast with the natural cocoons replacing 60-90% by weight (40-70% by volume) of the polymer matrix. Although reinforcing the foam with domesticated and wild cocoons increased its density from 45 kg/m3 to ∼60 kg/m3 and ∼120 kg/m3 respectively, a marked increase in both absolute and specific compressive properties was also observed. Comparable thermal and dimensional stability of the proteinaceous cocoon reinforced polymer foams with the unreinforced foam attest to comparable environmental resistance.