Multi-Ligand-Modified Sodium Lignosulfonate-Doped Phenolic Foam as a Flame-Retardant Thermal Insulation Coating

Sodium lignosulfonate (SLS) is an abundant and low-cost by-product generated during the sulfite pulping process, which is typically conducted at 120–180 °C depending on the pulping pH and base cation. Due to its richness in phenylpropane structural units, it holds significant potential as a bio-based resource for the synthesis of phenolic resins. However, its broad molar-mass distribution, limited flame retardance and low reactivity (particularly for SLS obtained from acidic solution process) limit how much of it can be built into phenolic resins. Here, SLS was activated through catalytic oxidation with air over a mixed manganese/cerium/cobalt acetate catalyst under alkaline conditions at 90 °C, then functionalized through an aqueous treatment-blending sequence with sodium hypophosphite, sodium pyroantimonate and triethylenetetramine and recovered as its aluminum salt, referred to as lignosulfonate–aluminum–amine (LAA) salt, to enhance its flame retardance. Gel permeation chromatography showed that scission and coupling run in parallel: the population at Mn = 745 g/mol fell from 98.46% to 95.67% of the peak area, while the high-molar-mass population rose from 1.54% to 4.33% and its Mn from 20,168 to 38,338 g/mol. For SLS oxidized with 0.5 wt% catalyst for 4 h, the onset of the first DSC endotherm shifted from 92.9 to 139.7 °C and the decomposition endotherm from about 230 to about 247 °C. Replacing 30 wt% of the phenol charge with LAA gave a foamed insulation layer of apparent density 43 kg/m3 and thermal conductivity 0.044 W/(m·K). In comparative handling observations, the modified foam appeared tougher than the unmodified foam; in a non-standardized open-flame screening test, it carbonized at the surface and self-extinguished within 10 s after flame removal. These foam coatings are much more suitable for a thick insulation layer foamed and cured in place rather than a thin film. The catalytic process performed at 90 °C using air as the O2 provider avoids the hydrothermal activation normally conducted at 170–260 °C and provides a lower-temperature route to a multi-ligand-modified bio-based resource for phenolic insulation coatings with high bio-based content.