Preparation and characterization of cellulose nanofiber/melamine–urea–formaldehyde composite aerogels for thermal insulation applications

The inherent flammability and low strength of pure cellulose aerogels limit their use in several applications. In this study, novel cellulose nanofibers/synthetic polymeric composite aerogels with excellent flame retardancy and increased strength were prepared under aqueous conditions with an eco-friendly freeze-drying and post-curing process. This composite combined a rigid melamine–urea–formaldehyde network and pentaerythritol phosphate melamine salt as an inorganic flame retardant. The latter is positively charged; hence, it could react with 2,2,6,6-tetramethylpiperidine-1-oxyl-oxidized cellulose nanofibers, which are negatively charged, by the charge neutralization of pentaerythritol. The resultant composite aerogels showed excellent flame-retardant properties, an increased compressive modulus (up to 73.1 kPa), and low densities (8.75–10.7 mg/cm3). The composite aerogels also exhibited a limiting oxygen index as high as 30.2% and a remarkable decrease in the fire indices. Therefore, we believe that the developed composite aerogel demonstrates potential as a lightweight composite thermal insulation for construction applications.