Tailoring the structure of cellular vitreous carbon foams

Cellular vitreous carbon (CVC) foams have been prepared from tannin-based resin modified by isocyanates of different natures, added in different amounts, and according to different mixing methods. We show that low amounts of isocyanate allow considerable changes in the porous structures of the resultant CVC foams. The latter are now more homogeneous, present thicker cell walls, porous struts, and their cells are less connected with each others. Additionally, average cell size and density can be independently controlled, whereas they were intimately related to each other in the first generation of tannin-based CVC foams. The accessible range of bulk density is also much wider (0.04-0.3 g cm(-3)) than before. CVC foams modified with isocyanate present extremely low thermal conductivity (less than 0.1 W m(-1) K-1), whereas their mechanical properties have been significantly improved (up to 8.4 and 140 MPa for compressive strength and elastic modulus, respectively). Modification by isocyanate thus opened a new route towards CVC foams of controlled porosity and properties. (C) 2012 Elsevier Ltd. All rights reserved.