Studies of polyurethane foams prepared with hybrid silicon and hydroxymethylated lignin

Today's petrochemical resources are increasingly strained, and the search for renewable, green, and environmentally friendly clean biomass energy has become a necessity. In order to increase the total hydroxyl content of lignin, enhance its reactivity, and replace some polyols in the preparation of rigid polyurethane foam, alkali lignin was extracted and purified from papermaking black liquid, and the lignin was modified by a hydroxymethylation. Rigid polyurethane foam is extremely flammable, and it is very important for rigid polyurethane foam to reduce its flammability using low smoke, nontoxic, nonhalogenated flame retardants. The waste polyurethane foam was treated by a high‐temperature calcination process to create a halogen‐free flame‐retardant hybrid silicon. The flame‐retardant modified rigid polyurethane foam was synthesized by replacing some of polyols with hydroxymethylated lignin and using hybrid silicon as flame retardant. The process conditions used to prepare the hydroxymethylated lignin were optimized based on the response surface method. The optimal conditions used 33 g of formaldehyde, a reaction temperature of 86°C, a reaction time 2.2 h, and a total hydroxyl content of 14.862 mg KOH/g. Analysis of infrared (IR) spectra confirmed that lignin was successfully modified by the hydroxymethylation reaction. Raman spectra (RS) and IR measurements showed that the hybrid silicon foams contained Si3N4, SiC, and SiO2. The apparent densities of the modified polyurethane foams ranged from 39.5–49.6 kg/m3; the compressive strengths were between 0.184 and 0.241 MPa, and the thermal conductivities ranged from 0.019–0.025 W/(m‧K). The flame‐retardant polyurethane foams prepared from the hybrid silicon foams reached a maximum limiting oxygen index (LOI) of 28.5%. Thermogravimetric (TG) and derivative thermogravimetric (DTG) results showed that the hybrid silicon improved the thermal stability of rigid polyurethane foam and increased the amount of carbon residue remaining after decomposition of the foam. Scanning electron microscopy (SEM) results showed that the flame retardants were uniformly distributed in the hybrid foam and the cell size was uniform.