Improvement of mechanical and thermal properties of epoxy composites with citric acid modified and defibrated cellulose filler

• Cellulose was modified with citric acid via a heating process only. • The modified cellulose has improved affinity with the matrix in the composite. • The defibrated modified cellulose has improved dispersibility in the composite. • The modified cellulose increased the strength of the composites almost twofold. Cellulose is a biomass filler with low environmental impact; however, its application presents challenges such as poor affinity between hydrophobic plastics and hydrophilic cellulose and the aggregation of cellulose. In this study, citric acid-modified cellulose (CAC) was used as an environment-friendly filler for enhancing the mechanical properties of epoxy composites. CAC was prepared via the esterification of cellulose with citric acid. In addition, a defibrated CAC (F-CAC) with a fiber diameter of ∼140 nm was produced using a grinder. The addition of F-CAC doubled Young's modulus increased it from 1.07 GPa to 2.08 GPa, and raised the glass transition temperature by 1.8 °C. The microscopic observations of composites revealed that CAC demonstrated a greater affinity for matrix than that of unmodified cellulose, whereas F-CAC exhibited enhanced dispersibility within the composites. Carboxyl groups introduced during the citric acid-modification may have contributed to the interaction between the filler and epoxy network. The incorporation of CAC or F-CAC as fillers improved the mechanical and thermal properties of epoxy composites. This research expands the range of applications for CAC, an environmentally friendly filler. It is anticipated that CAC will facilitate the development of high-strength, low-environmental-impact epoxy.