Morphology and deformation behaviour of natural fibers-thermoplastics composites

Abstract

The present study has been carried out with the objective to develop composite materials based on thermoplastics polymers (isotactic polypropylene, iPP and aliphatic aromatic co-polyester, Ecoflex) using locally available natural fibers (wood fibers of Bambusa nutans, Shorea robusta, Dalbergia sisaoo and Agave sisalana) as fillers and to elucidate their structure-property correlations with special focus on morphology and deformation behaviours. Natural fibers of various particle sizes are chemically modified (such as mercerization, bleaching etc.) and functionalized in different ionic liquid solvents. The functionalized fibers are compared with the commercially available cellulose-Avicel PH101. The differently modified natural fibers are incorporated into the polymer matrix to prepare composites of various compositions (polymer/natural fibers composites: 90/10, 80/20, 60/40, 40/60). The composites are prepared via melt mixing followed by compression molding and characterized by different techniques such as Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), optical microscopy (OM), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA). Mechanical deformation behaviour is investigated via tensile testing and analysis of fracture surface of the composites by scanning electron microscopy. Water absorption behaviour of the composites is also studied by standard methods. In this study, the cellulose content in Avicel has been found to be highest followed by sisal fibers and bamboo fibers. Delignification of bamboo and sisal fibers with caustic soda is confirmed by the removal of characteristic peaks especially at the finger print region, at 820 cm-1, 1250 cm-1, around 1600 cm-1 and also increase in the peaks at 3400 cm-1 and 1050 cm-1.The fibers of bamboo and sisal have been successfully dissolved in ionic liquids. The acetylation and silylation in bamboo and sisal fibers has been confirmed by the decrease in O-H stretching peak about 3400 cm-1, appearance of strong new peaks at 1740 cm-1 for C=O stretching and at 1250 cm-1, 840 cm-1 and 750 cm-1 for the stretching and bending vibrations of Si-OCH3 and Si-CH3. The individual FTIR spectra of the used fibers have been found to be analogous to that of the commercial cellulose. The result shows that there is no significant change in the morphology of the treated and functionalized fillers. However, the cellulose microfiber bundles are relatively finer and clearly visible in chemically modified and functionalized fibers as evident from their SEM micrographs. Mercerization is found to be the proper method for the extraction of cellulose microfibers from natural fibers. The characteristic peaks in FTIR spectra of the natural fiber-polymer composites of variable composition corresponding to wood fiber and the polymer matrix indicate the uniform distribution of fillers in the matrix. This result is also supported by SEM observation of the cryo-fracture surface of the composites and the optical micrographs of the composites. SEM analysis of the alkali-treated filler loaded composites reveals better filler-matrix interfacial interactions, better compatibility and thermal stability than the composites of neat fillers. The treated filler can be loaded up to 40 wt.-% without the loss in its properties. Investigation of deformation behaviour reveals that the weak fiber/matrix interface acts as defects for the failure of the neat fiber loaded composites. In case of bleached fibers, the filler matrix interaction is strong but the bonding between the cellulose microfibrils is weak and hence the failure seems to initiate from cellulose microfibrils in the bundles. However, Ecoflex composites are found to be comparatively more ductile than polypropylene composites. Composites supplemented with 5 wt.-% maleic anhydride grafted polypropylene (MA-g-PP) as compatibilizer have better thermal and mechanical properties. In polypropylene-natural fiber composites, the use of MA-g-PP improved the filler-matrix interfacial adhesion and the effects are reflected in the tensile stress strain properties as well as fracture surface morphology. Furthermore, the effect of compatibilizer is more prominent in alkali-treated filler loaded composites than in the untreated-filler based composites owing to the stronger interfacial interaction through hydrogen bonding with the highly exposed cellulose microfibrills. The thermal characterization of matrix, filler and composites show that the natural fibers have low thermal stability than the polymers and hence the composites were processed below 200 °C above which the natural fibers begin to degrade. In the composites, the change in degradation and melting temperatures than that of the pure matrices and fillers implies the interaction in between the filler and matrix at the interfaces. The treated fillers (particularly in case of bamboo fibers and wood fibers) depict the better compatibility and the thermal stability than the neat fillers in the composites. The compatibilized composites posses improved thermal properties than the uncompatibilized composites and the effect is noticeable in treated fibers loaded composites. The water absorption behaviour of the natural fiber loaded composites shows that their water holding capacity increases with the increase in cellulosic filler content in the composites. In case of polypropylene/natural fiber compatibilized (with MA-g-PP) composites, the water holding capacity of composites decreases (improved) as the treated fibers are used as fillers rather than the neat fibers but in contrast the water absorption property increases in the uncompatibilized composites even in the same compositions. Similar results are also observed in case of Ecoflex/natural fiber composites. The increase in water holding capacity of the natural fiber polymer composites is mainly due to the increase in the hydrophilic cellulose content in the composites but the use of compatibilizer improved the properties significantly. The observation has been discussed in terms of the chemistry at the interfacial region of the composites.