The Resorcinol-Formaldehyde Chemistry

The resorcinol-formaldehyde polymer is classified as a phenolic resin. In fact, phenol-formaldehyde condensates are the first synthetic polymers introduced commercially (Bakelite) in the beginning of the twentieth century. Thus, the phenolic resin chemistry, on which there is a vast amotmt of information, can be used to understand the chemistry of the RF aerogel synthesis. In the first step, resorcinol reacts with formaldehyde to form hydro-xymethylated resorcinol in the second step, the hydroxymethyl groups condense with each other to form nanometer-sized clusters, which then crosslink by the same chemistry to produce a gel. The formation of clusters is influenced by typical sol-gel parameters such as the temperature, pH, and concentration of reactants. Though the first RF aerogel is synthesized by the aqueous polycondensation of resorcinol with formaldehyde with the use of sodium carbonate as catalyst [6, 12, 13], now several literature reports exist for aerogels made from phenol and formaldehyde [14—16]. 

Phenolic resins made from phenol and formaldehyde are classified as resole or novolac based on the presence of methylene ether (-CH2OCH2-) or methylene (-CH2-) 

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