Acid-catalyzed Time-efficient Synthesis of Resorcinol-Formaldehyde Aerogels and Crosslinking with Isocyanates
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Aerogels are open-cell foams derived from supercritical fluid (SCF) drying of wet gels. Their large internal void space is responsible for low thermal conductivity, high surface area and high acoustic impedance. Most aerogels are based on inorganic metal or semimetal oxide frameworks. Pekala and co-workers1-4 synthesized resorcinol-formaldehyde (RF) organic aerogels by poly-condensation of resorcinol with formaldehyde in the presence of Na2CO3 as base catalyst, followed by drying with SCF CO2. Low-density RF aerogels prepared by this method exhibit high porosities (>80%), high surface areas (400-900 m2g-1), ultrafine cell-size (<500 >Ã Â ) and densities as low as 0.03 g cm-3. The major drawback though, has been the length of the preparation procedure that typically spans several days. Looking at the mechanism of the process, the RF gel formation has been associated with two major reactions: (1) formation of hydroxymethyl derivatives of resorcinol; and, (2) condensation of those derivatives to methylene (-CH2-) and methylene ether (-CH2-O-CH2-) bridges. The effect of the resorcinol to catalyst (R/C) ratio on the final aerogel structure has been studied extensively. That ratio was typically varied in the range between 50- 300. Formation of particles connected with large necks was reported for low and for very high (~1500) R/C ratios.5-7 the final pore structure and the gelation time depend strongly on the sol pH;8 at low pHs, precipitation rather than gelation was reported. The extensive literature on base-catalyzed RF aerogels has obscured attempts towards acid-catalyzed processing.9-13 Recently, Brandt and Fricke reported an aqueous acetic acid catalyzed route for RF gel synthesis, where they still allowed a two-day period for gelation and aging.10 Reasoning that not only hydroxy methylation of resorcinol, but also subsequent condensation to methylene and methylene ether bridges should be all acid-catalyzed processes, we undertook a systematic look at the reaction of resorcinol with formaldehyde in CH3CN, developing a time-efficient method that yields within a few hours (as opposed to weeks) gels indistinguishable from those reported in the literature. The -OH groups of resorcinol in the resulting gels are reactive with di- and tri-isocyanate crosslinkers in analogy to silica, leading to isocyanate-derived polymer crosslinked RF aerogels, which are more robust, and more resistive to shrinkage than their native (noncrosslinked) counterparts.