Effects of cellulose nanocrystal-based hybrid modification on flax/polypropylene composites

The composite industry urgently needs to develop effective and efficient surface treatment techniques for high-strength plant fibers. A newly designed hybrid approach which combines surface treatment techniques of three categories is studied in this research, in which sodium bicarbonate treatment is conducted to ‘gently’ clean the flax fiber, silane treatment served to add functional group and silane-grafted cellulose nanocrystals (CNCs) are applied to fiber surface to promote fiber-matrix interaction. Three processes are designed in order to combine silane treatment and CNC treatment. In the first process, CNCs were directly deposited on flax fabric after which flax fabric with CNCs was subjected to silane treatment. In the second process, flax fabric was simultaneously subjected to CNC deposition and silane treatment. In the third process, CNCs were separately subjected to silane treatment and later collected using a centrifuge. Later, silane-grafted CNCs were deposited flax fabric. The combined NaHCO₃-silane treatment improved the tensile strength of flax/polypropylene composites by 26.6%. It was also confirmed that CNCs could enhance fiber/matrix interface via mechanical interlocking. As the concentration of CNC suspension increased, density of CNCs on fiber surface increased. When the concentration is 5 g/L, optimum performance of flax/polypropylene composites is observed. In terms of moisture absorption, it was found that NaHCO₃-silane treatment could reduce moisture absorption (by 49.8%) while CNCs induce higher moisture content (by 31.1% when CNC concentration was 10.0 g/L). More importantly, the results of this study indicated that the moisture absorption and mechanical properties of composites could be largely affected by the nature of the interface. Proper fiber/matrix interface engineering can reduce moisture absorption and enhance mechanical properties.