Abstract:

Objective: In order to develop a novel approach of preparing lignin-based carbon materials, this paper has exploited a self-bubbling approach of industrial lignin for the preparation of carbon foams based on the physicochemical properties of lignin. Method: A self-bubbling process of lignin was realized using enzymatic hydrolysis lignin as the carbonaceous precursor, zinc chloride as a catalyst and phenolic formaldehyde resin as enhancer in the absence of additional foaming agents. The lignin-based carbons with a high surface area could be prepared from the mixture of lignin, zinc chloride and resoles through the steps of kneading, foaming, curing and carbonization. A thermogravimetric analysis, scanning electron microscope observation and nitrogen adsorption were employed to elucidate the mechanisms of lignin self-bubbling, the foaming processes and the texture of the resultant carbon foams. The effects of the dosage of zinc chloride and phenolic resin on the structure of carbon foams were studied by measuring the density, mechanical properties and porosity of the carbon foams. Result: The thermogravimetric analysis revealed that in the process of lignin self-bubbling, zinc chloride catalyzed the thermal decomposition of lignin leading to an obvious decrease in the temperature of lignin pyrolysis. As a result, the thermal decomposition and softening/plasticizing of lignin could simultaneously take place in the same temperature range, which allows lignin derived volatile pyrolysis products to work as a foaming agent in the softening/plasticizing lignin precursors. Meantime, phenolic resin provided the lignin-based foaming precursors with a higher tenacity and strength by crosslinking with lignin forming a three-dimensional network structure. The results showed that 160-180 ℃ is the suitable temperature range for lignin self-bubbling. The dosage of zinc chloride produced a significant effect on the density and porosity of carbon foams, and the dosage of phenol formaldehyde resin had a main effect on the cell size and open porosity. The self-bubbling process of lignin developed here produced lignin-based carbon foams with a relatively homogeneous cell bubble texture and high surface area, which had a bulk density of 0.26-0.46 g·cm^-3, porosity of 74%-85%, open porosity of 82%-94% and specific surface area of 524-1 055 m²·g^-1. Conclusion: The self-bubbling of lignin might be feasible to prepare lignin-based carbon foams with developed cell structure by regulating the temperature of thermal decomposition and softening/plasticizing of lignin. Therefore, it may be a graat potential to become a novel technology for the preparation of lignin-based carbon material.

Key words: lignin, carbon foam, self-bubbling, phenolic formaldehyde resin, zinc chloride