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%

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

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

Improvement of mechanical performance of industrial hemp fibre reinforced polylactide biocomposites

In this work, mechanical properties of chemically treated random short fibre and aligned long hemp fibre reinforced PLA composites were investigated over a range of fibre content (0–40 wt.%). It was found that tensile strength, Young’s modulus and impact strength of short hemp fibre reinforced PLA composites increased with increased fibre content. Alkali and silane fibre treatments were found to improve tensile and impact properties which appears to be due to good fibre/matrix adhesion and increased matrix crystallinity. A 30 wt.% alkali treated fibre reinforced PLA composite (PLA/ALK) with a tensile strength of 75.5 MPa, Young’s modulus of 8.18 GPa and impact strength of 2.64 kJ/m2 was found to be the best. However, plane-strain fracture toughness and strain energy release rate decreased with increased fibre content. The mechanical properties of the PLA/ALK composites were increased further due to alignment of long fibres

Characterisation of hemp fibre reinforced Poly(Lactic Acid) composites

In this work, the mechanical properties including tensile strength, flexural strength, impact strength and fracture toughness of the PLA/hemp composites were investigated over a range of fibre content (0-30 wt.%). It was observed that, the tensile strength and Young's modulus of the composites increased with increased fibre content. The flexural strength did not increase with fibre reinforcement, however, the flexural modulus of the composites increased significantly. Impact strength of the composites increased up to 20 wt. % fibre loading, however, further increased fibre loading caused a reduction of impact strength. KIc and GIc values decreased with increased fibre content

Flexural properties of hemp fibre reinforced polylactide and unsaturated polyester composites

In this work, flexural strength and flexural modulus of chemically treated random short and aligned long hemp fibre reinforced polylactide and unsaturated polyester composites were investigated over a range of fibre content (0–50 wt%). Flexural strength of the composites was found to decrease with increased fibre content; however, flexural modulus increased with increased fibre content. The reason for this decrease in flexural strength was found to be due to fibre defects (i.e. kinks) which could induce stress concentration points in the composites during flexural test, accordingly flexural strength decreased. Alkali and silane fibre treatments were found to improve flexural strength and flexural modulus which could be due to enhanced fibre/matrix adhesion

Analysis of mechanical properties of hemp fibre reinforced unsaturated polyester composites

Different chemically treated hemp fibre reinforced unsaturated polyester composites were investigated over a range of fibre content (0–60 wt%). Although Young’s modulus of all the short fibre reinforced unsaturated polyester composites was found to be higher than that of unreinforced unsaturated polyester; however, tensile strength of the composites exceeded that of the unsaturated polyester matrix only for the combined alkali- and silane-treated fibre composites at 40 wt% fibre content. The decrease in tensile strength of the composites could be attributed to stress concentrations caused by the fibres in conjunction with the brittle matrix. Impact strength of all the treated fibre composites was higher than that of the untreated fibre composites at all fibre contents. KIc and GIc of the composites decreased initially and thenincreased as the fibre content increased because more and more fibres being available to pull-out. The mechanical properties of the composites were increased further due to the alignment of long fibres