Designing cellulose nanofiber surface for high density polyethylene reinforcement

Abstract

Because of their high mechanical performance, high specific surface area, and high aspect ratio, there is a strong interest in cellulose nanofibers (CNFs) as a reinforcing material for plastics. Although three hydroxyl groups per repeating unit exposed on the surface is a unique characteristic of CNFs, an effective chemical treatment to improve the reinforcing efficiency of CNFs for hydrophobic thermoplastic resin has not yet been reported. In this study, six systematically designed aliphatic ester groups with linear, cyclic and branched structures, were incorporated on the surface of CNFs using the hydroxyl groups with a degree of substitution of 0.4. The melt compounding of the esterified CNFs and HDPE was performed at 140 °C using a twin screw extruder followed by injection molding at 160 °C and investigated the CNF dispersibility in HDPE and the CNF reinforcing efficiency in injection-molded HDPE samples. Incorporation of linear long chains results in the best dispersion of CNFs in HDPE compared with cyclic and branched chains, but the latter chains give higher reinforcing efficiencies in Young’s modulus and tensile strength. Especially, the bulky t-butyl group gave the highest reinforcing efficiency. Thus, the structure of ester groups incorporated on the CNFs very much affects on the dispersibility in HDPE, Young’s modulus and tensile strength, and coefficient of thermal expansion (CTE) of the esterified CNFs-reinforced HDPE composites. Addition of pivaloylated CNFs increases the Young’s modulus of HDPE from 1.20 to 3.32 GPa, and the tensile strength from 23.4 to 51.2 MPa. The CTE is 72.5 ppm/K, which is less than one-third that of the HDPE. The high reinforcing efficiency of these composites is partly explained by the formation of the double shish–kebab crystal structure during injection molding identified by the TEM observation.