Acid-Catalyzed Time-Efficient Synthesis of Resorcinol-Formaldehyde Aerogels and Crosslinking with Isocyanates
Resorcinol-formaldehyde (RF) aerogels are used as precursors to carbon aerogels, and they are typically prepared by a base-catalyzed condensation reaction followed by supercritical fluid drying. Gelation has been associated with formation of hydroxymethyl derivatives of resorcinol followed by condensation to methylene (-CH2-) and methylene ether (-CH2-O-CH2-) bridges. The major drawback though, has been the length of the process that typically spans several days. Reports on acid catalyzed processing (e.g., with acetic acid) have been sparse. Reasoning that not only hydroxy methylation of resorcinol (an electrophylic aromatic substitution), but also subsequent condensation should be all acid-catalyzed reactions, we undertook a systematic look at the RF gelation in CH3CN, developing a time-efficient HCl-catalyzed process that yields within hours RF gels indistinguishable from those reported in the literature. Reasoning further that the –OH groups of resorcinol in the gels should be reactive towards di- and tri-isocyanates in analogy to recent reports with silica, we demonstrate isocyanate-derived coatings covering the RF skeletal nanoparticles conformally, thus preserving the porosity (~90%) and the surface area (360-460 m2g-1) of the native RF framework. Conformally coated RF aerogels are more robust, and more resistive to shrinkage than their native (untreated) counterparts. Thus, although isocyanate-treated RF aerogels may uptake 100% their weight in new material, their density is usually lower than the density of the corresponding native gels.
S. Mulik et al., "Acid-Catalyzed Time-Efficient Synthesis of Resorcinol-Formaldehyde Aerogels and Crosslinking with Isocyanates," Abstracts of Papers of the American Chemical Society, American Chemical Society (ACS), Sep 2006.
232nd ACS National Meeting (2006: Sep. 10-14, San Francisco, CA)
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