Effect of fibre treatments on interfacial shear strength of hemp fibre reinforced polylactide and unsaturated polyester composites

Surface treatment of hemp fibres was investigated as a means of improving interfacial shear strength (IFSS) of hemp fibre reinforced polylactide (PLA) and unsaturated polyester (UPE) composites. Fibres were treated with sodium hydroxide, acetic anhydride, maleic anhydride and silane. A combined treatment using sodium hydroxide and silane was also carried out. IFSS of PLA/hemp fibre samples increased after treatment, except in the case of maleic anhydride treatment. Increased IFSS could be explained by better bonding of PLA with treated fibres and increased PLA transcrystallinity. The highest IFSS was 11.4 MPa which was obtained for the PLA/alkali treated fibre samples. IFSS of UPE/hemp fibre samples increased for all treated fibres. This is believed to be due to the improvement of chemical bonding between the treated fibres and the UPE as supported by FT-IR results. The highest IFSS (20.3 MPa) was found for the combined sodium hydroxide and silane treatment fibre/UPE samples

Mechanical Performance of Industrial Hemp Fibre Reinforced Polylactide and Unsaturated Polyester Composites

This study investigated the effect of fibre content, fibre treatment and fibre/matrix interfacial strength on the mechanical properties of industrial hemp fibre reinforced polylactide (PLA) and unsaturated polyester (UPE) composites. Surface treatment of hemp fibres was investigated as a means of improving the fibre/matrix interfacial strength and mechanical properties of hemp fibre reinforced PLA and UPE composites. The fibres were treated with sodium hydroxide, acetic anhydride, maleic anhydride and silane. A combined treatment, sodium hydroxide and silane, was also carried out. The average tensile strength of sodium hydroxide treated fibres (ALK) slightly increased compared with that of untreated fibres, which was believed to be as a result of increased cellulose crystallinity. In contrast, the average tensile strength of acetic anhydride, maleic anhydride and silane treated fibres slightly decreased compared with that of untreated fibres, which was believed to be as a result of decreased cellulose crystallinity. In the case of combined treatment with sodium hydroxide and silane, the average tensile strength of the fibres (ALKSIL) slightly decreased compared with that of alkali treated fibres (ALK), which was also thought to be as a result of decreased cellulose crystallinity. The average Young's modulus and thermal stability of all treated fibres increased compared with untreated fibres. This was considered to be as a result of densification of fibre cell walls due to the removal of non-cellulosic components during treatment. It was also thought that the grafted molecules in cellulose chains of the acetic anhydride, maleic anhydride and silane treated fibres enhanced resistance to thermal degradation. The interfacial shear strength (IFSS) of PLA/hemp fibre samples increased after treatment, except in the case of maleic anhydride treatment. The increase in IFSS could be due to better bonding of PLA with cellulose of treated fibres (except for the maleic anhydride treatment) as a result of removal of non-cellulosic components evidenced by increased PLA transcrystallinity. The highest IFSS was 11.41 MPa which was obtained for PLA/ALK samples. IFSS of UPE/hemp fibre samples increased for all treated fibres. This could be due to the improvement of chemical bonding between the treated fibres and the UPE as supported by FT-IR results. The highest IFSS (20.3 MPa) was found for the UPE/ALKSIL samples. Short hemp fibre reinforced PLA composites were fabricated using injection moulding. Alkali and silane fibre treatments were found to improve mechanical (tensile, flexural and impact) and dynamic mechanical (storage modulus) properties which appears to be due to the increase in IFSS and matrix crystallinity. Tensile strength, Young's modulus, flexural modulus, impact strength and storage modulus of the PLA/hemp fibre composites increased with increased fibre content. A 30 wt% short fibre reinforced PLA composite (PLA/ALK) with a tensile strength of 75.5 MPa, Young's modulus of 8.18 GPa, flexural modulus of 6.33 GPa, impact strength of 2.64 kJ/m² (notched) and 28.1 kJ/m² (un-notched) and storage modulus of 4.28 GPa was found to be the best, and better than any in the available literature. However, flexural strength, plane strain fracture toughness (Kic) and strain energy release rate (Gic) decreased with increased fibre content. This behaviour could be due to the increase in stress concentration points (number of fibre ends) with increased fibre content. The influence of loading rate and fibre content on the Kic of random short fibre reinforced PLA composites, lacking from the available literature, was studied. Kq (trial Kic) of composites decreased as loading rate increased, until stabilising at a loading rate of 10 mm/min and higher. UPE based short hemp fibre composites were produced by compression moulding. At 20 wt% fibre content, the tensile strength was not increased above that recorded for unreinforced UPE, but thereafter, the tensile strength increased approximately proportionally to the fibre content, except at the highest fibre content (60 wt%) where a decrease in the tensile strength occurred. Kic and Gic reached a minimum value at 30 wt% fibre content and afterwards increased with increased fibre content. The flexural strength was found to decrease with increased fibre content; however, the impact strength and storage modulus increased with increased fibre content. It was also observed that the mechanical and dynamic mechanical properties improved after treatment of fibres. A 50 wt% ALKSIL fibre reinforced UPE composite with a tensile strength of 62.1 MPa, Young's modulus of 13.35 GPa, flexural modulus of 6.11 GPa, impact strength (notched) of 7.12 kJ/m² and storage modulus of 3.5 GPa was found to be the best. To improve the mechanical and dynamic mechanical properties further, aligned long hemp fibres were used to fabricate PLA/ALK and UPE/ALKSIL composites using compression moulding. The mechanical and dynamic mechanical properties of aligned long fibre reinforced PLA/ALK and UPE/ALKSIL composites were found to be superior to those of short fibre composites. The highest tensile strength of 85.4 MPa, Young's modulus of 12.6 GPa, flexural modulus of 6.59 GPa, impact strength of 7.4 kJ/m² (notched) and 32.8 kJ/m² (un-notched), and storage modulus of 5.59 GPa were found for PLA/ALK composites at a fibre content of 35 wt%. In the case of UPE/ALKSIL composites, the highest tensile strength of 83 MPa, Young's modulus of 14.4 GPa, flexural modulus of 6.7 GPa, impact strength (notched) of 15.85 kJ/m² and storage modulus of 3.74 GPa were found for composites with a fibre content of 50 wt%

Influence of loading rate, alkali fibre treatment and crystallinity on fracture toughness of random short hemp fibre reinforced polylactide bio-composites

Plane-strain fracture toughness (KIc) of random short hemp fibre reinforced polylactide (PLA) bio-composites was investigated along with the effect of loading rate, fibre treatment and PLA crystallinity. Fracture toughness testing was carried out at loading rates varying from 0.5 to 20 mm/min using single-edge-notched bending specimens with 0 to 30 wt% fibre. KQ (trial KIc) of composites decreased as loading rate increased, until stabilising to give KIc values at a loading rate of 10 mm/min and higher. The reduction of crazing and stress whitening, as well as a more direct crack path observed in PLA samples combined with reduced plastic deformation observed in composites provided explanation for this reduction. KIc of composites was found to decrease with increased fibre content and fibre treatment with sodium hydroxide. Studies controlling the degree of PLA crystallinity by heat treatment or “annealing” showed that reduction of KIc can be attributed to increased crystallinity

Hemp fibre reinforced poly(lactic acid) composites

The potential of hemp fibre as a reinforcing material for Poly(lactic acid) (PLA) was investigated. Good interaction between hemp fibre and PLA resulted in increases of 100% for Young’s modulus and 30% for tensile strength of composites containing 30 wt% fibre. Different predictive ‘rule of mixtures’ models (e.g. Parallel, Series and Hirsch) were assessed regarding the dependence of tensile properties on fibre loading. Limited agreement with models was observed. Differential scanning calorimetry (DSC) and x-ray diffraction (XRD) studies showed that hemp fibre increased the degree of crystallinity in PLA composites

Effect of various chemical treatments on the fibre structure and tensile properties of industrial hemp fibres

Industrial hemp fibres were treated with sodium hydroxide, acetic anhydride, maleic anhydride and silane to investigate the influence of treatment on the fibre structure and tensile properties. It was observed that the average tensile strength of sodium hydroxide treated fibres slightly increased compared with that of untreated fibres, which was believed to be as a result of increased cellulose crystallinity. The average tensile strength of acetic anhydride, maleic anhydride, silane and combined sodium hydroxide and silane treated fibres slightly decreased compared with that of untreated fibres, which was believed to be as a result of decreased cellulose crystallinity. However, the average Young’s modulus of all treated fibres increased compared with untreated fibres. This was considered to be as a result of densification of fibre cell walls due to the removal of non-cellulosic components during treatment

Poly(lactic acid) composites reinforced with leaf fibers from ornamental variety of hybrid pineapple (Potyra).

While there have been many studies of fibers extracted from pineapple leaves as reinforcement in polymer composites, to date, only commercial varieties have been examined. This work aims to investigate the fibers from the leaves of a hybrid pineapple called Potyra as a mechanical reinforcement in a poly(lactic acid) (PLA) matrix. The fibers were pre-treated in a NaOH solution (1 wt%) and were incorporated into the PLA by a torque rheometer mixer followed by twin-screw extrusion. Samples of each composition were injected. The molded composites showed increases of tensile strength from 58.8 to 69.6 MPa, of Young?s modulus from 1.9 to 3.5 GPa, and of impact resistance from 28 to 44 J/m, and showed an increase of 58C in the heat deflection temperature (Abstract Figure). The measured tensile strength and Young?s modulus values are lower than the theoretical values obtained by micromechanics theory due to the pull-out of the matrix fiber and due to the orientation of the fibers in the composites. It was concluded that the pineapple hybrid fibers have potential for use as mechanical reinforcement in green composites. POLYM. COMPOS., 00:000?000, 2017. VC 2017 Society of Plastics Engineer

Moisture uptake characteristics of a pultruded fibre reinforced polymer flat sheet subjected to hot/wet aging

This paper studies the moisture uptake characteristics of a pultruded E-glass fibre reinforced (isophthalic polyester) polymer after long-term exposure to hot/wet conditions. Both fully exposed samples of varying aspect ratios and selectively exposed samples were immersed in distilled water at 25 °C, 40 °C, 60 °C and 80 °C for a period of 224 days. For the fully exposed condition, bulk and directional diffusion coefficient values were determined. A three-dimensional approach using Fickian theory was applied to approximate the principal direction diffusions at 60 °C by using mass changes from samples having different aspect ratios. This revealed that the diffusion coefficient in the longitudinal (pultrusion) direction to be an order of magnitude higher than in the transverse and through-thickness principal directions. Diffusion coefficients in the three principal directions have also been determined for the selectively exposed condition at 60 °C through the application of one-dimensional Fickian theory. It was found that the size and shape of the samples influences moisture uptake characteristics, and thereby the values determined for bulk and directional diffusion coefficients. Furthermore, the influence of exposure temperature on moisture uptake and mass loss with time was examined. Investigation of the water medium by means of electrical measurements suggested that decomposition of the polymeric composite initiates very early, even after the very first day of immersion. Comparison between the infrared signatures from the pultruded material and the water's residual substances revealed significant decomposition, and this behaviour is verified by Fourier Transform Infrared Spectroscopy (FTIR), Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopic (EDS) analysis as well as the recorded mass loss after 224 days of aging

Consolidation process boundaries of the degradation of mechanical properties in compression moulding of natural-fibre bio-polymer composites

In spite of the volume of literature on natural fibres, bio-matrix materials and their composites, the choices of optimum process parameters such as moulding temperature, pressure and compression time are still largely based on experience, rules of thumb and general knowledge of the chemical and physical processes occurring in the melt during consolidation. The moulding process itself is a complex balance between processes that must occur for the composite to successfully consolidate and the onset of thermal degradation of the natural fibre and/or matrix materials. This paper brings together models of thermal penetration, melt infusion, thermal degradation and chemical degradation of natural polymers to construct an ideal processing window for a bio-composite. All processes are mapped in terms of normalized consolidation progress parameters making it easier to identify critical processes and process boundaries. Validation of the concept is achieved by measuring changes in the mechanical properties of a flax/PLA bio-composite formed over a range of processing conditions within and outside of the optimized window

Up-cycling of agave tequilana bagasse-fibres: A study on the effect of fibre-surface treatments on interfacial bonding and mechanical properties

The aim of this study was to assess the feasibility of upcycling fibre residues from the harvesting and production of tequila to green composites. Specifically, four different surface-modified natural fibres were assessed as raw material for green composite production. Before any surface treatment, the morphology and tensile properties of agave bagasse fibres from the tequila production batches were determined by optical and environmental scanning electron microscopy (ESEM) and single fibre tensile test, respectively. Further to this, agave fibres were exposed by immersion to four surface treatments including alkali, acetylation, enzymatic and silane treatments, in order to improve their morphology and compatibility with polylactic acid (PLA). The effects of these treatments on fibres’ morphology, mechanical properties (i.e. Youngs modulus and ultimate tensile strength), interfacial shear strength (IFSS), and water absorption were assessed. Overall, surface treatments showed improvements in agave bagasse fibre properties with the best results for alkali treated fibres with an ultimate tensile strength of 119.10 ​MPa, Young modulus of 3.05 ​GPa, and an IFSS of up to ~60% higher (5.21 ​MPa) to that performed by untreated samples. These tests allowed to identify alkali treatment as the most suitable for agave bagasse fibres. These results shed light on the interfacial interaction between agave bagasse fibres and PLA and the potential to up-cycle these residue agave fibres to manufacture PLA-based green composites

Cracks, microcracks and fracture in polymer structures: Formation, detection, autonomic repair

The first author would like to acknowledge the financial support from the European Union under the FP7 COFUND Marie Curie Action. N.M.P. is supported by the European Research Council (ERC StG Ideas 2011 n. 279985 BIHSNAM, ERC PoC 2015 n. 693670 SILKENE), and by the EU under the FET Graphene Flagship (WP 14 “Polymer nano-composites” n. 696656)