•Review of targeted fibre pre-treatments and factors affecting mechanical properties of natural fibre reinforced composites.•Physical, chemical, and biological natural fibre pre-treatment ...methods.•Novel illustrations of hemp fibre anatomy, composition, and mechanical properties.•Structure-function relations of hemp fibres for mechanical properties in biocomposites.•Methods affecting hydrophilicity/hydrophobicity and surface properties of natural cellulosic fibres including hemp fibres.
Global interest in the use of plant fibres in natural fibre reinforced composites (NFCs) is growing rapidly. The increased interest is primarily due to the advantageous properties of natural fibres including biodegradability, low cost, low density and high stiffness and strength to weight ratio. In order to achieve strong NFCs, well separated and cellulose-rich fibres are required. Hemp is taking a center stage in this regard as a source of suitable natural plant cellulose fibres because natural hemp bast fibres are long and inherently possess high strength. Classical field and water retting methods have been used for centuries for removal of non-cellulosic components from fibrous plant stems including from hemp, but carries a risk of reducing the mechanical properties of the fibres via damaging the cellulose. For NFCs new targeted fibre pre-treatment methods are needed to selectively and effectively remove non-cellulosic components from the plant fibres to produce cellulose rich fibres without introducing any damage to the fibres. A key feature for successful use of natural fibres such as hemp fibres in composite materials is optimal interfacial contact between the fibres and the hydrophobic composite matrix material. Targeted modification of natural fibres for NFCs must also be targeted to optimize the fibre surface properties. Consequently, improved interfacial bonding between fibres and hydrophobic polymers, reduced moisture uptake, increased microbial degradation resistance, and prolonged durability of NFCs can be achieved. This review, using hemp bast fibres as an example, critically and comprehensively assesses the targeted pretreatment technologies and data available for producing well separated cellulose bast fibres having optimal chemical and physical properties for maximizing the mechanical performance and durability of NFCs.
Due to the increasing awareness around the environmental crisis and the depletion of petroleum resources, together with the inherent issues connected with the use of composite systems such as their ...disposal and recycling and the health problems associated with the use of some raw materials, new legislative rules on global scale have been promulgated to fight environmental issues and will be hoping adopted by several countries.
Based on these considerations, the new generation of composite systems will have to minimize the environmental impact with a more efficient use of energy resources and materials and where possible, replace synthetic or petroleum materials with more sustainable components, like natural ones.
Therefore, the aim of this paper is to investigate the sound transmission loss of a new sandwich structure in bio-composite material based on hemp fibers and bio-epoxy resin in order to further extend the application fields of natural fibers composite (NFC) materials.
Measurements in the impedance tube were first used to assess the sound transmission loss at normal incidence. By using measured mechanical properties, a Finite Element Method was used to predict sound transmission loss in the same boundary condition. Finally, the so determined mechanical model has been used to predict the sound transmission loss in a diffuse field.
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•Diagonal compression tests on unreinforced and strengthened adobe masonry wallets.•Two textile reinforced matrices (TRMs) based on hemp (H) and glass-fibre (G) grids.•Significant ...improvement of shear response provided by both TRM systems.•H-TRM (most effective TRM) with 30% strength increase and 260% ductility increase.•High efficiency of H-TRM also for clay and tuff wallets from comparative analysis.
Many people live in adobe masonry (AM) constructions in earthquake areas even if most of the existing constructions are not engineered, thus prone to seismic damage. Therefore, seismic retrofitting of existing AM structures is a critical issue. In this study, experimental results of diagonal compression tests on 21 adobe masonry wallets, before and after the installation of textile reinforced mortar (TRM) with either hemp or glass fibres, are presented. Hemp-TRM is found to be more effective than glass-TRM in improving the shear capacity of AM, showing a mean increase in shear strength and ductility of 25% and 260% in hemp-TRM-strengthened wallets and 12% and 100% in glass-TRM-strengthened wallets. The hemp textile allowed compatibility of the TRM to the masonry substrate, preventing premature debonding and local failure. Experimental results are compared to existing data on TRM systems made of vegetable fibres, evidencing a significant capacity enhancement through flax- and hemp-TRM systems.
•Changes induced by wet preservation, improve the mechanical properties of hempcrete.•Using wet-preserved hemp means that less water must be added to the mix of concrete.•Wet preservation of the hemp ...is equivalent to other pre-treatments of the hemp fibres.•The hempcretes studied are a good alternative material for sustainable construction.•Wet preservation of hemp fibres reduces cost of the final building materials.
This paper evaluates two types of concrete with hemp fibres as natural aggregates prepared with inorganic binders, based on reactions (Si-Na) and (Si-Ca). It also was tested two states of conservation of the hemp: 1) fresh and 2) preserved in moist conditions for six months. The results indicate that the changes induced by wet preservation, above all the increase in the percentage of cellulose, improve the mechanical properties of the concretes and is equivalent to other pretreatments conducted to improve the conditions of the hemp fibres confirmed by performing compositional, mineralogical and physical studies of the raw materials and the concretes. The dosages used in this study provided high quality concretes in comparison with other studies with similar dosages.
•The extraction of cellulose nanocrystals (CNC) has been carried out from hemp fibres.•Carded and not carded fibres were chemically and enzymatically pre-treated.•Carding allowed removal of most ...impurities from the fibres.•CNC were extracted by acid hydrolysis from pre-treated carded hemp.•The enzymatic treatment resulted in major changes in X-ray diffraction of carded hemp.
The extraction of cellulose nanocrystals (CNC) from Carmagnola hemp fibres has been carried out. Before CNC extraction, the effectiveness of two pre-treatment methods, an alkaline chemical and a pectinase enzymatic treatment, applied on the pristine and carded hemp fibres, were compared. Carding allowed removal of most impurities from the fibres, while it had only a modest effect on their structure. After chemical treatment, hemicellulose was removed, more efficiently in carded hemp, and X-ray diffraction suggests an increase in the size of cellulose crystallites. Carded hemp fibres, after interaction with pectinase, show the total decomposition of pectin and hemicellulose. On the basis of these results, carded hemp was selected as start material for CNC extraction and the acid hydrolysis for CNC synthesis was applied on carded hemp after both chemical and enzymatic procedures. The yield of the different hydrolysis methods remains approximately at the same level (around 19%): on the other side, a carding procedure, combined with an alkaline treatment to hemp fibres, represents an optimized process for CNC extraction by acid hydrolysis.
Natural fibre reinforced composites are lightweight materials widely applied to automotive interior parts. However, there are some problems such as volatile organic compound (VOC) emissions exceeding ...standard levels and weak interfacial bonding, which limits their use in industrial applications. In order to overcome the existing problems, a new degumming method (cryogenic and mechanical treatment) is proposed in this paper. Firstly, hemp fibre bundles became loose after cryogenic treatment, and there was some micropore creation or microcracking. Secondly, mechanical treatment separated hemp fibres from the fibre bundles. Finally, hemp fibres were cleaned with alkaline liquid. The influences of cryogenic and mechanical treatment on the fibres were investigated by using scanning electron microscope (SEM), Fourier transform infrared (FTIR) spectroscopy and thermogravimetric analysis (TGA). The results revealed that cellulose content was increased from 66.25% to 78.93%, and hemicellulose and lignin were reduced to 7.16% and 2.82%, respectively. Decreases in the diameter and tensile properties of the treated hemp fibres were also observed. In addition, in differential thermogravimetric analysis (DTG), the untreated hemp fibres had a significant peak at about 336 °C, while the significant peaks of treated hemp fibres were approximately at 360 °C. The significant peaks at 336 °C and 360 °C represent the maximum oxidative decomposition rate, and indicate that the treated fibres had a higher thermal stability than the untreated fibres. It was found that cryogenic and mechanical treatment is feasible for the degumming of hemp fibres.
•VOC and weak bonding limit the application of natural fibre reinforced composites.•Cryogenic and mechanical treatment is proposed.•There is a significant increase in the cellulose content by the treatment.•Thermal stability of the treated fibres is improved.
•An innovative strengthening system made of hemp-fibre composite grids was developed.•The in-plane behaviour of hemp-fibre strengthened masonry panels was assessed.•The shear strength enhancements ...were comparable with those obtained with FRP meshes.
Sustainability goals are essential driving principles for the development of innovative materials in the construction industry. Natural fibres represent an attractive alternative as reinforcing material due to both good mechanical properties and sustainability prerequisites. The present work investigates the shear behaviour of masonry panels strengthened with a mortar-based system reinforced with an innovative hemp fibre composite grid. The objective of the study is to assess the feasibility of using the proposed strengthening system for external retrofit of existing masonry walls and to compare its performances with typical retrofitting solutions.
•Hemp fibres were extracted using a modified decorticator semi-industrial equipment.•The hemp fibre yield is 28 % of the initial mass of the hemp stem.•The length of the extracted technical fibres is ...sufficient for carded route spinning.•The tensile properties of the individual fibres are higher than the ones processed using hammer mills.•The tensile properties of the fibres are sufficient for the reinforcement of structural or semi-structural composites.
In western Europe, hemp is mainly cultivated for the seeds and the fibre rich straws, randomly aligned following the harvesting with a combine harvester. The straws are mainly valorised for low added value products such as pulp for paper and for insulation. They are also valorised in low property short fibre composites. With the view to use hemp fibres extracted from randomly aligned straws for higher added values, this work proposes to study a different process (fibre opener) for extracting the fibres than the one which is classically used in the industry (hammer mill) and to investigate the extraction performances and the impact of the process on the hemp morphological and mechanical properties.
The morphological and mechanical properties measured at different moments of the extraction indicate that, even in the less favourable case, the length of the fibres (∼ 5 cm) is high enough for textile processing via the carded route. The tensile strength (∼ 660 MPa) and modulus of elasticity of (38 GPa) of the individual hemp fibres are situated above most of the ones of fibres extracted mechanically using a hammer mill (630 MPa and 25 GPa for strength and modulus respectively) despite the fact that large quantities of kink band defects are observed on each fibre. This therefore shows that the “all fibre” opener is suitable for the production of fibres that can be considered for the manufacturing of technical textiles such as load-bearing woven geotextiles or mid-range load-bearing composite reinforcements.
The aim of this study is to achieve a better understanding of the nonlinear tensile behaviour of the elementary hemp fibre. This is of great importance in view of the need to develop an efficient ...predictive tool for the design of natural fibre reinforced composites. This first paper investigates the possible mechanisms responsible for the nonlinear behaviour, using repeated progressive tensile loading with in situ polarised light microscopy.
The persistence of residual strain has been confirmed during testing when the tensile load was released. Only a certain fraction of this residual strain is reversible, and the reversibility is time-dependent. Beyond the yield level, the fibre’s rigidity is not deteriorated, but significantly increased as a function of the number of loading cycles and the level of strain. A new scenario involving a stick–slip mechanism, extension and re-orientation of the microfibrils and shear strain-induced crystallisation of the amorphous cellulose is proposed.
•We studied the effect of hemp fibres on the mechanical performance of an earthen material.•The samples were subjected to static and impact bending.•Hemp fibres greatly increase the fracture ...resistance and the energy absorption capacity.•Both unreinforced and reinforced materials show a strong sensitivity to the rate of loading.•The post-cracking performance is improved by increasing the fibre fraction and length.
The study investigates the enhancements in the load carrying capacity, crack resistance and energy absorption properties provided by the addition of hemp fibres in an earthen material. Notched earthen samples reinforced with two fibre contents (2% and 3% in weight) and three fibre lengths (10, 20, and 30mm) were manufactured and tested under static and impact bending to investigate and compare the influence of the reinforcement on the fracture resistance of the soil material at low and high strain rates. The results of the experimental analyses show that the incorporation of fibres greatly improves the peak load, the post-crack strength, the ductility and the energy dissipation of soil under both static and impact bending. The mechanical response of both unreinforced and reinforced samples is significantly affected by the rate of loading, with samples exhibiting higher values of strength and absorbed energy under impact than under static bending. For both static and impact loading, the post-crack response of the material at large deformations is clearly improved by increasing the fibre content and, at the same fibre content, by increasing the fibre length.
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