Improved Extraction of Natural Fibers for Sustainable Polymer Composites

Abstract

With increasing amount of global CO2 levels, the demand of carbon negative materials has significantly increased. Natural fibers such as flax which are found on the outer bast region of the plant stem, are gaining importance due to their high specific mechanical properties as a sustainable replacement to glass fiber in polymer composites. But implementing these fibers in industry scale structural composite applications is limited by the high variability in their properties. Besides being inherent, the current process of extraction is a major reason for this variability, where, the stems initially go through a process called dew retting. Here the bonding of the bast fiber layer with the stem’s woody core is degraded by microbial activity in the field, which is inconsistent, limited to conditions of weather and results in fibers with non-uniform diameters. The next mechanical breaking step, where these retted stems are crushed between the rotating gear rollers to separate the bast fiber layer, further induces fiber damage under high bending strains leading to degradation & high variation in mechanical properties of these fibers. The research in this dissertation is aimed at improving natural fiber quality and reducing variability in mechanical properties by elucidating the cause and eliminating the fiber damage occurring through various steps in the natural fiber extraction process. Here, first a lab scale-controlled retting is established using specific enzymes to overcome the limitations of the conventional dew retting for uniform fiber debonding within the plant stem. In this enzymatic retting process, the effect of enzyme concentration & retting duration on the bast-woody core bond strength and resulting fiber diameters is studied by developing mechanical fiber peel tests and microscopy coupled with statistical analysis. It is observed that while the resulting fiber diameters are only affected by retting duration, the bond strength between the bast-woody core interphase is significantly affected by both enzyme concentration and duration of retting. Moreover, from single fiber tensile tests conducted on fibers resulting from different retting conditions, an optimal enzyme retting condition is chosen for later mechanical extraction. To further understand the underlying mechanism of fracturing of these stems during the mechanical breaking step leading to fiber damage, compression and bending tests of retted flax stems is done along with visual observation of the fracture occurring inside the woody core. Finite element modeling (FEM) is implemented for bending analysis of stems under a range of tool geometries and is used as a guide to design an improved roller profile for a lab scale stem breaker. Further, significant improvement in tensile properties of extracted fibers from this improved breaking process is seen when compared to fibers extracted under high bending strains. Finally, long continuous fibers from multiple stems at a time is extracted from the improved extraction process and the fiber yield is evaluated at each step. These fibers are then used to make unidirectional fiber composites. Longitudinal tensile tests are conducted on these composite samples and fiber properties are back calculated using composite’s rule of mixtures. These back calculated fiber properties are found comparable to fibers properties evaluated using single fiber tensile tests. A systematic approach in this dissertation elucidates the effect of retting conditions and fiber property evaluation methods to guide the optimization of extraction process at larger scale for producing high quality natural fibers for sustainable composites.