Co-recovery of cellulose nanofibers and lignin from rice straw for high-performance reusable insulating aerogels

Rice-straw-derived cellulose nanofiber/lignin (CNF/lignin) aerogels were developed as microwave-passive insulating cores for reusable food-container walls. Cellulose and lignin co-recovered from Vietnamese rice straw were reassembled into aerogels containing 0-20 wt% lignin to define a practical formulation window for microwave-reheatable packaging. Moderate lignin addition preserved the cellulose I framework and improved the balance between pore-wall integrity, moisture tolerance, thermal insulation, and dielectric stability. Among the tested samples, CNF/L10 gave the best overall performance, with a density of 0.025 ± 0.001 g cm^-3, porosity of 98.33 ± 0.07%, compressive modulus of 74.6 ± 3.7 kPa, compressive strength of 32.7 ± 1.8 kPa at 70% strain, water contact angle of 79.2 ± 2.8°, and moisture absorption of 49.8% after 48 h at 75% RH. Its thermal conductivity remained low, increasing only from 0.0288 ± 0.0009 W m^-1 K^-1 at 50% RH to 0.0336 ± 0.0011 W m^-1 K^-1 at 75% RH. At 2.45 GHz, CNF/L10 showed ε' = 1.61, ε″ = 0.053, and tanδ = 0.0329 at 50% RH, and ε' = 1.93, ε″ = 0.129, and tanδ = 0.0668 at 75% RH. In a PP/aerogel/PP prototype wall, CNF/L10 reduced the outer sidewall temperature from 62.7 ± 1.2 °C to 45.9 ± 0.9 °C while maintaining food-center heating. After 1000 wash-reheat cycles, it retained 86.8% modulus, 85.0% strength, and 87.8 ± 1.5% microwave transmittance, with overall migration below 10 mg dm^-2. These results support CNF/L10 as a promising bio-based insulating core for lower-plastic microwave food-packaging prototypes.