Oil palm lignin under subcritical phenol conditions as precursor for carbon fibre production

Abstract

Carbon fibre is as thin as a strand of human hair but is five times stronger than, twice stiffer and lighter than steel; thus, an ideal advanced material in aerospace and automotive industries. However, carbon fibre is 20 times more expensive than steel because of expensive precursors, polyacrylonitrile and pitch, that account for 51% of the total manufacturing cost. Therefore, an alternative precursor, with superior properties, is required. Although successful studies for lignin extraction in subcritical phenol have been reported, there are no studies on evaluation of the lignin fundamental properties related to its suitability as a precursor for carbon fibre. In this study, the extraction of lignin from biomass was conducted in a batch system under subcritical phenol conditions by focusing on its fundamental properties (average molecular weight and glass transition temperature). The effect of temperatures (260–300 °C), reaction times (1–30 min) and solids loading (6 and 10 g) on these properties was determined. The results indicated that lignin from biomass under subcritical phenol conditions fulfilled the requirement as a precursor for carbon fibre. The average molecular weight and glass transition temperature was in the range of 145.5–269.6 g/mol and 40.5–89.3 °C for lignin from empty fruit bunch. On the other hand, the average molecular weight and glass transition temperature of lignin from oil palm frond was 263.8–435.4 g/mol and 71.2–96.3 °C. The increase in temperature reduced the dielectric constant of the subcritical phenol and facilitated lignin depolymerisation. The reactive fragments and the active sites were capped by phenol to suppress the cross-linking reactions and hence the formation of heavier fragments. The minimal decrease in average molecular weight and glass transition temperature of lignin with solids loading supported the conclusion that repolymerisation of lignin likely did not occur under subcritical phenol conditions.