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(C 6 F 5 ) 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.

New Control Over Silicone Synthesis using SiH Chemistry: The Piers–Rubinsztajn Reaction

“…An analogous process more efficiently employs the not inexpensive catalyst. The use of low concentrations of B(C 6 F 5 ) 3 and relatively high lignin concentrations allows the preparation of lignin reinforced silicone elastomers and foams . In this case, only the surface of the lignin is modified, such that the particles act as crosslink sites.…”

“…The use of low concentrations of B(C 6 F 5 ) 3 and relativelyh igh lignin concentrationsa llows the preparation of lignin reinforced silicone elastomers [40] and foams. [41] In this case, only the surface of the lignin is modified, such that the particles act as crosslink sites. The resulting elastomeric products containing up to 45 %l ignin by weightw ere surprisingly resistantt oh ydrolytic degradation (in boiling water), in part it is assumed,b ecause ingress of water into ah ydrophobics ilicone elastomer is fundamentally ineffective.…”

Green routes to silicon-based materials and their environmental implications

“…Materials with up to 71 % lignin content were studied, with optimal performance found for samples containing~41 % lignin by having 150 % break elongations, and modulus approaching 5 MPa. They also found in a subsequent paper that these materials could be foamed by first extrusion followed by injection molding to give mechanical properties on par with other silicone-organic copolymers (Zhang et al 2015b). Lastly, they also report that the Piers-Rubinsztijn catalyst system can be combined with short chain "monomers" with silane functionality to positively influence depolymerization of soft lignin polymers (Zhang et al 2014).…”

“…Even though silicones still demand robust efforts to make them more environmentally friendly on their own, many research groups have explored greening silicones by making bio-polymer and siloxane hybrids in order to reduce the amount of siloxane needed in functional materials. One area of bio-based siloxane composites being explored is the incorporation of bio-waste materials such as lignin and gelatin (Zhang et al 2014(Zhang et al , 2015a(Zhang et al , and 2015bMacphail and Brook 2017). Lignin, typically from pulping processes (paper/cardboard) can be a very useful polymer for many structural/composite applications; however, it is often difficult to process and/or depolymerize and mostly burned as a cheap fuel.…”

“…This is especially true of hardwood lignin (Zhang et al 2015a). Brook et al have developed a lignin-silicone-based elastomer and subsequent foam composite based on the reaction of lignin with hydride functionalized silicone using the Piers-Rubinsztijn catalysis method with B(C 6 F 5 ) 3 ( Figure 19); and also studied their degradation mechanisms (Zhang et al 2014(Zhang et al , 2015a(Zhang et al , and 2015b. They report that in this method Si-H groups on the silicone can be reacted with the alcohol groups present on the surface of lignin, making a copolymer/composite that has similar properties to many standard silicone elastomers.…”