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pounds. Jara-Palacios et al. [117] found quercetin-3-O-glucoside as the most abundant compound in RGPE extracted by C 2 H 5 OH:H 2 O. Wang et al. [118] modified hemp fibre with a lysine-grafted N-halamine organic as an antibacterial agent in hemp fibre using a mild Schiff base reaction. The materials totally eliminated Staphylococcus aureus and Escherichia coli in 5 min, while the inhibition zone increased to 18.4 mm. 3.2. Physical Modification Techniques Previous studies mainly focused on the effects of coupling agents and compatibilisers to tailor the mechanical properties of natural fibre-reinforced composites. Most chemical treatments were successful and resulted in increased thermal and mechanical properties. However, some major problems associated with chemical treatments are the high cost and pollution from the disposal of the chemicals after treatment [119]. Plasma treatment introduces functional groups onto natural fibres that form strong covalent bonds with the matrix, leading to a strong fibre/matrix interface. Plasma treatment is simple, short- duration, consumes little energy, and low cost. The technique requires no water or any potentially hazardous chemicals. Surface etching improves the surface roughness of natural fibres and results in better interfacial interaction with the matrices through mechanical linking [120–124]. 3.2.1. Cold Plasma Treatments Cold plasma techniques are dry, clean processes with less environmental concerns. Such a modification occurs only on the surface with no interference on the bulk properties. Figure 3 shows a schematic representation of the effects of plasma and cationising processes of cellulose-constituting cotton fibres [125]. Sinha et al. [122] studied the influence of physi- cal treatment on the morphology, wettability and impact of the fine structure of fibres on interfacial adhesion of natural fibre-reinforced composites. They found that plasma treat- ment reduced fibre hydrophilicity due to the decrease in phenolic and secondary alcoholic groups and oxidation of the basic structural lignin and hemicelluloses components. Plasma treatment improved fibre/matrix adhesion, as revealed by scanning electron microscopy (SEM) morphology. Figure 4 demonstrates the etched surfaces and increased numbers of new oxygen functional groups present on the surface of sisal and coconut fibres revealed using SEM analysis [126].
Figure3. The effects of plasma and cationising processes. Reprinted with permission from Ref. [125]. Copyright 2011 Springer Nature Ltd.
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