Flame retardant lignin-based silicone composites

Abstract

The use of lignin as a filler for polymers to give composites provides both economic advantages and, in some cases, improved mechanical performance. The presence of lignin can also introduce certain advanced properties, including biodegradability, antimicrobial and antioxidant activity. Here we demonstrate that improved thermal insulation, thermal stability, and flame retardancy result when lignin is compounded with hydride-functionalized silicones to give elastomers and foams. In the absence of any additional inorganic flame retardant agent, the V-1 rating (UL-94) could be reached after chemical post-treatment of the materials with NH₃ vapor to remove excess SiH groups (which were identified as a culprit for excessive flammability). The fire resistance was further improved to the V-0 rating (UL-94) by applying additional thermal post-treatment at 220 °C in air. These foams also demonstrated low thermal conductivity, which were comparable with pure silicone foams of similar density. The improved thermal stability is attributed to flame retardant silica char, lignin repolymerization and char and the ability of lignin to scavenge radicals.