Bio-composites materials from engineered natural fibres for structural applications
The large environmental footprint caused by conventional synthetic fibre reinforced petrochemical-derived polymer composites along with potential improvement in mechanical properties for natural fibre reinforced bio-derived polymer composites, have long been a motivation for further research and innovation in natural fibre composites. Harakeke and hemp fibres, amongst other natural fibres, possess respectable mechanical properties along with other advantages such as low cost, low production energy requirements and abundant availability. Polylactic acid (PLA), a sustainable alternative to petrochemical-derived polymers, is produced on a mass scale from 100% renewable resources and can be degraded at end of life by simple hydrolysis under the appropriate conditions. It has high stiffness, strength, thermal and UV stability, but is low in toughness. Limited applications of natural fibre reinforced PLA composites so far is mainly due to low mechanical properties of discontinuous fibre composites made using injection moulding or compression moulding using randomly oriented fibre mats attributed to poor fibre orientation and fibre-matrix incompatibility. Presented in this study are experimental investigations of the properties of PLA reinforced aligned discontinuous harakeke and hemp fibre mats produced using a dynamic sheet former (DSF) to provide improved performance of discontinuous fibre composites. Harakeke and hemp fibres were treated using either a solution of 5 wt% or 10 wt% NaOH or combination of 5 wt% NaOH with 2 wt% Na2SO3 at elevated temperatures in a small pressure vessel. Treated fibres were assessed using single fibre tensile testing, X-ray diffraction (XRD), scanning electron microscopy (SEM) and thermogravimetric analysis (TGA). It was found that 5 wt% NaOH/2 wt% Na2SO3 and 5 wt% NaOH effectively removed non-cellulosic materials from harakeke and hemp fibre surfaces, respectively, giving good fibre separation without greatly reducing the tensile strength of the fibres. It was also found that these treatments lead to a higher crystallinity index and improved thermal stability of fibres. Fibre alignment in fibre mats produced using a DSF was observed using light microscopy. Visual observation supported that the fibre alignment had occurred. Fibre orientation factors (Kɵ) determined for harakeke and hemp composites using the Bowyer-Bader model were found higher compared to Kɵ values obtained for other natural composites prepared using injection and compression moulding. Improved fibre orientation resulted in improved reinforcement giving large increases for tensile strengths up to approximately 90 and 60% for harakeke and hemp composites reinforced with inclusion of 30 and 25 wt% fibre respectively. The effect of silane and peroxide as additional fibre treatments and MA-g-PLA as a coupling agent was evaluated by scanning electron microscopy (SEM), composite swelling analysis, tensile testing, dynamic mechanical analysis (DMA) and thermogravimetric analysis (TGA). Scanning electron micrographs of fractured composite surfaces revealed that the gaps between fibres and matrix for composites with fibres treated using silane and peroxide and composites coupled with MA-g-PLA were smaller compared to the surfaces for composites with fibres treated using just alkali. Improved interfacial strength was also supported by lower swelling indices, lower peaks on tan δ and higher residual composites weights, compared to those values obtained for composites with fibres treated using just alkali, which was found to lead to higher composite tensile strengths. Harakeke and hemp fibre composites were plasticised using hyper-branched polymer (HBP) and assessed using SEM, tensile and impact testing. SEM micrographs for plasticised composites revealed longer pultruding fibres than un-plasticised composites, with gaps between the fibre and matrix appearing to be slightly larger, indicating a weaker interface between fibres and matrix compared to those composites without plasticiser. Improved composite ductility and impact strength were demonstrated for plasticised composites without dramatic reduction of composite tensile strength.
Aligned discontinuous natural fibre composites
Networked Digital Library of Theses and Dissertations