Improving the performance of polypropylene matrix composite materials using engineered hemp fibre mats
Projections of continuing demand for materials across the world is driving the development of more sustainable materials. The low energy consumption requirements, as well as recyclability found within the spectrum of natural fibre composites, has led to increased interest in improving these sustainable materials. Although the use of natural fibre composite materials has been documented in early civilisations, growing environmental concerns coupled with technological advancements have encouraged the expansion of their use in recent times. However, there are still significant issues, including their limited mechanical performance, that limit the ability to compete for future use. Amongst natural fibres, hemp fibres are an attractive alternative reinforcement to synthetic fibres due to their favourable mechanical properties as well as availability. Additionally, compared to other natural fibres, hemp fibres are more valuable for the bio-based economy due to its environmental benefits such as can be grown without pesticide and high yield of technical fibres. The hemp is cultivated in most of the EU countries. However, to improve hemp fibre composites to further replace synthetic fibre composites, research is required. Presented in this thesis are experimental investigations on the properties of polypropylene matrix composites reinforced with hemp fibre mats produced using dynamic sheet forming (DSF) to align the short fibres used in the mats. The overall aim of this research was to improve the mechanical performance of hemp fibre mats produced using DSF and to assess the potential of these mats as reinforcement in polypropylene composites. Two different alkali treatments were carried out on hemp fibres with the goal of producing fibre mats from high strength fibres using DSF, one at ambient and one at a higher temperature. The ambient temperature treatment was carried out using a solution of 5 wt% sodium hydroxide (NaOH), whereas the higher temperature treatment was carried out using a combination of 5 wt% sodium hydroxide and 2 wt% sodium sulphate (Na₂SO₃). The fibres were granulated either before or after the treatment and the effect of granulation was evaluated. Treated and untreated fibres were assessed using single fibre tensile testing, X-ray diffraction, scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR) and thermogravimetric (TGA) analysis. It was found that the high temperature alkali treatment increased the tensile strength of the fibres by about 51 %, whereas the ambient temperature treatment decreased the tensile strength of the fibres to even lower than that of untreated fibres. Although granulation before or after the high temperature treatment had no significant effect on the tensile properties of the fibres, the fibres were found to have better separation when granulated after the high temperature treatment. Additionally, dynamic sheet forming (DSF) was found to be only possible with these better separated fibres for the production of aligned fibre mats. An investigation was conducted into the effect of nozzle geometry in dynamic sheet forming (contraction ratio and exit shape) on the orientation (alignment) of fibres within the mats produced. Nozzles of different geometries were designed, 3D printed, fitted to the dynamic sheer former (the machine) and trialled to produce fibre mats. For the assessment of orientation, control samples, including highly aligned control sample and random mats, were produced. The alignment of fibres within hemp fibre mats was assessed using ImageJ (OrientationJ) and X-ray diffraction. Fibre orientations were quantified, mainly by means of coherency factors from image analysis of microscopic images using ImageJ and Herman’s order parameter from analysis of results using XRD. These techniques were found to be in good agreement showing that the mats produced using DSF possess alignment with respect to the drum rotation direction (machine direction). Based on the literature, it was expected that nozzles with higher contraction ratios and circular exit shape would result in improved alignment of fibres within the mats compared to the nozzles with lower contraction ratio such as the current nozzle fitted to the machine. Although there was a trend of increasing fibre orientation for nozzles with the increase in contraction ratios, however, it was only significant for extreme cases (nozzles with the lowest and highest contraction ratios); the exit shape of a nozzle was found to have less influence on fibre orientation. A coherency factor of 0.23 and Herman’s order parameter (𝘧) of 0.464 was obtained for the mats made using the current nozzle in DSF compared to 0.11 and 0.139 for the random mats, respectively, indicating the potential of DSF to produce aligned fibre mats. Improvement attained for fibre orientation in this work is indicated by the higher values of coherency factor (0.31 compared to 0.23) and Herman’s order parameter (0.511 compared to 0.464) obtained for the fibre mats made using a high contraction ratio nozzle compared to those mats made using the current nozzle in DSF. The improvement was further supported by about 11 % increase in tensile strength for the composites with 30 wt% fibre mats made using the high contraction ratio nozzle compared to the tensile strength for those composites with fibre mats made using the current nozzle in DSF. An investigation was conducted to assess the effect of surface treatments on fibre mats produced using DSF with the goal of improving the interface between the fibre and polypropylene. The fibre mats were treated with the addition of stearic acid (SA) or cellulose nanocrystals (CNCs). Untreated and treated fibre mats and composites made from these fibre mats were assessed using SEM, FTIR, Raman spectroscopy, swelling studies, water retention testing, TGA and tensile testing. The stearic acid treatment involved exposing mats to SA vapour, which reduced the hydrophilic nature of the fibre mats. It was found that improvements in tensile strengths in composites were obtained with the maleic anhydride polypropylene (MAPP) coupling agent and the stearic acid vapour treatment compared to composites with alkali only. The improvement with SA treatment was apparent only in composites without the MAPP coupling agent. The combination of SA and MAPP did not provide additive benefits in tensile strength of the composites. However, scanning electron microscopic images of their tensile fracture surfaces revealed more consistent interaction at the fibre-matrix interface with SA treated composites compared to the composite with only MAPP. This was also supported by the homogeneity in the tensile strength of composite samples indicated by the lower standard deviations with SA treatment. The CNC treatment involved a water-based spray and drying step, which increased the tensile strength of the fibre mats with 2 wt% CNC in water. In composites with 15 wt% fibre mats and with 2 wt% CNC treatment, the tensile strength and Young’s modulus of composites significantly increased, by about 15 and 16 %, respectively. In contrast, composites with higher fibre contents (25 or 30 wt%) exhibited poor consolidation when CNC treated fibre mats were used. This poor consolidation is believed to related to CNC films formed between the fibres in the treated fibre mats; these CNC films are likely to hinder the flow of polymer melt, resulting in insufficient wetting of fibres by the polymer. In order to improve the mechanical performance of composites through high fibre content, different polymer sheet thicknesses and stacking arrangement were investigated. Generally, the strength and stiffness of fibre composites are expected to increase with increased fibre content, provided these composites have reasonable interfacial bonding between matrix and fibres, as the fibres are usually stronger and stiffer than the matrix. However, initial attempts to increase fibre content above 30 wt% resulted in declines of tensile properties of the composites. Scanning electron micrographs of tensile fracture surfaces of these composites revealed insufficient wetting of fibres by the matrix material. It was found that decreasing the overall thickness of fibre mats between two polymer sheets within the stacking arrangements of composites which decreased the travelling distance of polymer improved the fibre wetting through the composite and therefore improved the tensile properties. The strongest composite produced had a fibre content of about 60 wt%. At this fibre content, tensile strength and Young’s modulus of the composites were found to be 3.0 and 6.9 times, respectively, higher than the control samples (PP/MAPP), while figures for flexural strength and flexural modulus were 3.4 and 3.6, respectively. The interface of the PP/hemp composites was assessed using a single fibre pull-out test. The composite was found to have an average interfacial shear strength of 8.7 MPa and a critical fibre length of 0.85 mm. In conclusion, the investigation demonstrated that aligned short fibre mats could be produced from high strength hemp fibres using DSF. The performance of these fibre mats in composites can be improved in the following ways: • Improving fibre orientation using a nozzle of high contraction ratio for dynamic sheet forming. • Through altering surface properties using treatments such as stearic acid vapour treatment and spraying on cellulose nanocrystals, and • Improving stacking arrangement/sheet thickness to allow high fibre content in composites while obtaining effective reinforcement and good fibre wetting and enhanced mechanical performance.
Dynamic sheet forming, Hemp fibres, Alignment of fibres
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