Foamed lignin–silicone bio-composites by extrusion and then compression molding

Abstract

The use of lignin, one of the most abundant natural products, has not gained wide use as a feedstock due to the difficulty of processing it. We have developed a simple route to produce lignin–silicone composite foams via first extrusion and then compression molding. The formulation consists of raw lignin particles, suitable mixtures of hydrosilanes, and a catalyst B(C₆F₅)₃. In order to balance the reaction rates between extrusion and molding, as well as to find other optimized conditions for producing foamed structures, a series of optimizations established that a uniform, closed cell lignin–silicone foam was most effectively made by extrusion at room temperature followed by molding at elevated temperatures under pressure for up to 5 minutes. The morphology and uniformity of the foamed structure depended on many factors, including the quantity of lignin, the catalyst, the crosslinking silicone PHMS, the molecular weight of the spacer silicone H-PDMS-H, and the molding temperature. The content of lignin, acting as both a reinforcing filler and a crosslinker (chemically bonded to the siloxane network), could be varied over a wide range from 25 to 55%. The mechanical performance of the lignin–silicone foam was characterized using DMA and tensile tests (tensile strength up to 0.42 MPa, break-at-elongation up to 249%). The strength of the foam was improved by post-curing at 140 °C. Although the lignin–silicone foam loses some elasticity after post-curing, it maintains reasonable stability even after heating to 300 °C for 12 h. This processing method for lignin-based bio-composites provides new opportunities for better utilization of lignin in silicones and more broadly in organic materials.