Plant fibers are rich in cellulose and they are a cheap, easily renewable source of fibers with the potential for polymer reinforcement. The presence of surface impurities and the large amount of hydroxyl groups make plant fibers less attractive for reinforcement of polymeric materials. Hemp, sisal, jute, and kapok fibers were subjected to alkalization by using sodium hydroxide. The thermal characteristics, crystallinity index, reactivity, and surface morphology of untreated and chemically modified fibers have been studied using differential scanning calorimetry (DSC), X-ray diffraction (WAXRD), Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM), respectively. Following alkalization the DSC showed a rapid degradation of the cellulose between 0.8 and 8% NaOH, beyond which degradation was found to be marginal. There was a marginal drop in the crystallinity index of hemp fiber while sisal, jute, and kapok fibers showed a slight increase in crystallinity at caustic soda concentration of 0.8 -30%. FTIR showed that kapok fiber was found to be the most reactive followed by jute, sisal, and then hemp fiber. SEM showed a relatively smooth surface for all the untreated fibers; however, after alkalization, all the fibers showed uneven surfaces. These results show that alkalization modifies plant fibers promoting the development of fiber-resin adhesion, which then will result in increased interfacial energy and, hence, improvement in the mechanical and thermal stability of the composites.

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SUMMARY: Two chemical treatments were applied to hemp, sisal, jute and kapok natural fibres to create better fibre to resin bonding in natural composite materials. The natural fibres have been treated with varying concentrations of caustic soda with the objective of removing surface impurities and developing fine structure modifications in the process of alkalisation. The same fibres were also acetylated with and without an acid catalyst to graft acetyl groups onto the cellulose structure, in order to reduce the hydrophilic tendency of the fibres and enhance weather resistance.Four characterisation techniques, namely XRD, DSC, FT-IR and SEM, were used to elucidate the effect of the chemical treatment on the fibres. After treatment the surface topography of hemp, sisal and jute fibres is clean and rough. The surface of kapok fibres is apparently not affected by the chemical treatments. X-ray diffraction shows a slight initial improvement in the crystallinity index of the fibres at low sodium hydroxide concentration. However, high caustic soda concentrations lower the fibre crystallinity index. Thermal analysis of the fibres also indicates reductions in crystallinity index with increased caustic soda concentrations and that grafting of the acetyl groups is optimised at elevated temperatures. Alkalisation and acetylation have successfully modified the structure of natural fibres and these modifications will most likely improved the performance of natural fibre composites by promoting better fibre to resin bonding.ZUSAMMENFASSUNG: Naturfasern aus Hanf, Sisal, Jute und Kapok wurden mit zwei chemischen Methoden behandelt, um eine bessere Faser-Matrix-Anbindung in naturfaserverstärkten Verbundmaterialien zu erreichen. Die Fasern wurden mit Natronlauge verschiedener Konzentration behandelt, um Oberflächenver-unreinigungen zu entfernen und die Faserfeinstruktur zu modifizieren. Diese Fasern wurden dann mit und ohne sauren Katalysator acetyliert, um die Hydrophilie der Fasern zu reduzieren und deren Witterungsbestän-digkeit zu verbessern.Die Auswirkung der chemischen Modifizierung wurde mit XRD, DSC, FT-IR und SEM untersucht. Nach der Behandlung ist die Oberflächentopographie der Hanf-, Sisal-und Jutefasern sauber und rauh. Die Oberfläche der Kapokfasern blieb offensichtlich unverändert. Röntgenbeugungsmessungen zeigen einen leicht erhöhten Kristallinitätsindex der mit niedrigen NaOH-Konzentrationen behandelten Fasern. Hohe NaOHKonzentrationen erniedrigen jedoch den Kristallinitätsindex. Die thermische Analyse der Fasern weist ebenfalls auf eine Erniedrigung des Kristallinitätsindexes mit zunehmender NaOH-Konzentrationen hin und daß die Acetylierung bei höheren Temperaturen leichter abläuft. Mit der Alkalisierung und Acetylierung konnte die Struktur der Naturfasern erfolgreich modifiziert werden, was deren Leistungsvermögen durch verbesserte Faser-Harz-Bindung erhöhen dürfte.

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