Polyurethane Aerogels

In analogy to their resorcinol-formaldehyde homologues, polyurethane wet gels are synthesized by polycondensation. Subsequently, aerogels are commonly obtained by supercritical fluid drying of the organic wet gels [23]. 

Reactions take place in organic solvents (typically in dimethyl sulfoxide, DMSO) and are catalyzed with standard catalysts used for the synthesis of polyurethanes metal salts and tertiary amines [26]. Polymerization mechanisms differ as a function of the catalyst and are briefly discussed below. 

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Figure 10.10. Influence of catalyst ratio (deflned as the ratio between weights of isocyanate and catalyst) and concentration of the sol (% solids) on polyurethane aerogels appearance (P powder, O opaque, T transparent and N no gelation) and density (in g/cm ) [4],...

Figure 10.11. Thermal conductivity of two monolithic polyurethane aerogel plates as a function of partial vacuum of air at room temperature [4].

Figure 10.12. Photograph of a polyurethane aerogel obtained by supercritical CO2 drying of a gel synthesized by cross-linking of pentaerythritol with 4,4-diphenyhnethane diisocyanate under DABCO catalysis.

Figure 10.14. SEM micrographs of polyurethane aerogels coming frcm supercritical CO2 drying of gels synthesized with saccharose in different solutions of DMSO and etac with volume fraction ofDMSO <0.3 (top) and >0.3 (bottom) (microscopy conducted by M. Repoux at MINES ParisTech/CEMEF, Sophia AntipoUs, France) [39].

Figure 26.14. Scanning electron microscopy of a polyurethane aerogel (SEM-FEG microscopy from S. Jacomet, MINES ParisTech/CEMEF, Sophia Antipolis, France), with courtesy of Rigacci A. and Achard P.

Siace the pores ia an aerogel are comparable to, or smaller than, the mean free path of molecules at ambient conditions (about 70 nm), gaseous conduction of heat within them is iaefficient. Coupled with the fact that sohd conduction is suppressed due to the low density, a siUca aerogel has a typical thermal conductivity of 0.015 W/(m-K) without evacuation. This value is at least an order of magnitude lower than that of ordinary glass and considerably lower than that of CFC (chloro uorocarbon)-blown polyurethane foams (54). 

Although the aerogel loses its optical transparency by this measure, its thermal insulation is strongly enhanced. Total conductivities for nonevacuated aerogels of the order of 0.014 W/(m K) are achievable. This is considerably lower than in CFC-blown polyurethane (PU) foams, the best nonevacuated thermal insulators available today [48]. 

Aerogels are particularly well suited for insulation applications because of their exceptionally low density, thermal stability, and high transparency. In fact, they can have a thermal conductivity only one-third that of polyurethane or polystyrene foam, and with recent process improvements that reduce the cost of manufacmre by an order of magnitude their practical use in certain construction applications is now feasible [31]. The insulating properties can be enhanced through the addition of IR opacifiers [32]. The high transparency of aerogels makes them suitable as insulation in windows or translucent panels. 

Rigacci A, Marechal JC, Repoux M, Moreno M, Achard P (2004) lYeparation of polyurethane-based aerogels and xerogels for thermal superinsulation. J Non-Cryst Solids 350 372-378... 

Polyurethane and, to a lesser extent, cellulose-based aerogels, were initially developed for applications in thermal insulation [15, 16]. However, they have been studied for other applications as well [17]. For example, they have been considered as precursors of nanos-tructured carbons [18,19] but, contrary to cellulose-based carbon aerogels (Figure 10.2) that appear promising [20, 21], no clear evidence of superior performance exists relative to resorcinol-formaldehyde-based carbon aerogels, mainly because of foaming during... 

Figure 10.16. Illustration of the densification occurring during evaporative drying of polyurethane gels xerogel (left) versus aerogel (right). Both samples come from gels synthesized with pentaerythritol in solution of DMSO and etac with a volume fraction of DMSO >0.3. Diameters of the xerogel and the aerogel are 3 and 5 cm, respectively.

Biesmans G, Randall D, Francais E, Perrut M (1998) Polyurethane-based organic aerogels thermal performance. J Non-Cryst Solids 225 36-40. 

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