Transdisciplinary top-down review of hemp fibre composites: from an advanced product design to crop variety selection

Given the vast amount of available research in the area of natural fibre composites, a significant step forward in the development of next-generation plant fibre-based products would be to devise a framework for rational design. The authors use a top-down approach, starting with an example final product to define the product specifications for high-performance hemp fibre-reinforced composites. Thereafter, all process steps are critically analysed: from textile preform and reinforcement yarn production, to fibre extraction and the agricultural process chain, to the microbiology of field retting, to cultivation and selection of crop variety. The aim of the analysis is to determine how far the current state of knowledge and process technologies are in order to use hemp fibres in high-performance composites. Based on this critical evaluation of the state-of-the-art, it can be stated that hemp will be found in high-performance composites in the short-to-medium term. There is, however, a need for performance optimisation especially through the selection of crop variety, best practices in retting, and effective fibre extraction methods to obtain more consistent fibre qualities suitable for reinforcement spinning and composite preform manufacturing processes

3D-printed polylactide composites reinforced with short lyocell fibres – Enhanced mechanical properties based on bio-inspired fibre fibrillation and post-print annealing

In this study, 3D printable polylactide (PLA) composites reinforced with 10, 20 and 30 mass% of short lyocell fibres were produced by melt compounding PLA modified with maleic anhydride. Based on bio-inspired anchoring systems, fibrillated fibres were also employed in 30 mass% fibre composites. The resulting 3D printed samples displayed outstanding mechanical performance, particularly with high fibre content. Compared to neat PLA, unmodified formulations showed reduced tensile strength and strain at break with the addition of fibres, but they had a moderate improvement in Young's modulus. However, by combining fibre fibrillation, matrix modification, and post-printing annealing, we achieved an excellent balance of tensile strength (85 MPa), Young's modulus (7.2 GPa), and strain at break (3.2%) - the highest reported values for such composites. Incorporating fibres and increasing PLA crystallinity via heat treatment significantly enhanced the thermo-mechanical stability of the composites, raising the storage modulus up to 38 times at 60 °C and 200 times at 80 °C compared to neat PLA. This combined strategy paves the way for the 3D printing of high-performance structures using 100% bio-derived materials

Optimizing Hemp Fiber Production for High Performance Composite Applications

Hemp is a sustainable and environmental friendly crop that can provide valuable raw materials to a large number of industrial applications. Traditionally harvested at full flowering for textile destinations, nowadays hemp is mainly harvested at seed maturity for dual-purpose applications and has a great potential as multipurpose crop. However, the European hemp fiber market is stagnating if compared to the growing market of hemp seeds and phytocannabinoids. To support a sustainable growth of the hemp fiber market, agronomic techniques as well as genotypes and post-harvest processing should be optimized to preserve fiber quality during grain ripening, enabling industrial processing and maintaining, or even increasing, actual fiber applications and improving high-added value applications. In this paper, the effect of genotypes, harvest times, retting methods and processing on the yield and quality of long hemp for wet spun yarns was investigated. Conventional green-stem varieties were compared with yellow-stem ones on two harvesting times: at full flower and seed maturity. Scutching was performed on un-retted stems and dew-retted stems, the un-retted scutched fiber bundles were then bio-degummed before hackling. Both scutching and hackling was performed on flax machines. Quality of hackled hemp, with particular reference to its suitability for high performance composites production, was assessed. The results of fiber extraction indicate that yellow-stem varieties are characterized by higher scutching efficiency than green-stem varieties. Composites strength at breaking point, measured on specimens produced with the Impregnated Fiber Bundle Test, was lower with hemp obtained from stems harvested at seed maturity than at full flowering. On average, back-calculated fiber properties, from hackled hemp-epoxy composites, proved the suitability of long hemp fiber bundles for high performance composites applications, having properties comparable to those of high quality long flax.Highlights:- The trait yellow stem in hemp is an indicator of processability.- Yellow stem varieties have finer hackled fiber bundles.- Controlled dew retting increased yield of hackled fiber compared to bio-degumming.- Retting influenced fiber and composite mechanical properties.- Hemp can achieve properties comparable to high quality long flax for high performance composites

Concrete composites with alternative binder types based on biomimetic structures

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Wool fiber-reinforced thermoplastic polymers for injection molding and 3D-printing

The authors describe the state of the art in research and development in thermoplastic wool fiber-reinforced composites in the present book chapter. In the beginning, wool and horn's structural property relationships as keratinous, natural composites are summarized. This comparison's main objective is to show the potential of keratinous composites and highlight the unique circumstances that need to be taken into account in the development and manufacture of wool fiber-reinforced composites. The published papers in the field of wool fiber-reinforced thermoplastics are critically evaluated and classified according to the thermoplastic material and processing technique based on comprehensive literature research. The achieved properties, depending on the processing methods, are compared and used to describe the micromechanics and the fiber–matrix interaction of wool fiber-reinforced composites. The opportunities and limitations of adhesion promotion to improve mechanical properties are discussed by analyzing literature studies. In this chapter, properties such as combustibility and biodegradation of wool fiber-reinforced composites are critically evaluated. In addition to the literature research analysis, own results for injection-molded and 3D-printed wool/polymer composites are presented. With the help of existing data, suitable products and possible market segments for wool fiber-reinforced thermoplastic polymers are identified. The chapter ends with a final critical evaluation of the state of research. Finally, ideas for necessary future research and development work are presented.35138

Cellulose Fiber-Reinforced PLA versus PP

The present study focuses on a comparison between different cellulose fiber-reinforced thermoplastics. Composites were produced with 30 mass-% lyocell fibers and a PLA or PP matrix with either an injection (IM) or compression molding (CM) process. Significant reinforcement effects were achieved for tensile strength, Young’s modulus, and Shore D hardness by using lyocell as reinforcing fiber. These values are significantly higher for PLA and its composites compared to PP and PP-based composites. Investigations of the fiber/matrix adhesion show a better bonding for lyocell in PLA compared to PP, resulting in a more effective load transfer from the matrix to the fiber. However, PLA is brittle while PP shows a ductile stress-strain behavior. The impact strength of PLA was drastically improved by adding lyocell while the impact strength of PP decreased. CM and IM composites do not show significant differences in fiber orientation. Despite a better compaction of IM composites, higher tensile strength values were achieved for CM samples due to a higher fiber length

Interfacial and Interlaminar Shear Strength of Unidirectional Viscose Fibre-Reinforced Epoxy Composites—an Overview of the Comparability of Results Obtained by Different Test Methods

In this study, the apparent interfacial and interlaminar shear strength (IFSS and ILSS) of single fibres and unidirectional (UD) viscose fibre-reinforced epoxy composites were characterised using different test methods. Microbond and pull-out tests were used to analyse the IFSS of single fibres embedded in epoxy and the transverse tensile test was applied to measure the IFSS of UD fibre-reinforced composites. The short beam shear test, single edge notched bending test (SENB), double-notched tensile test and double-notched compression test were applied to characterise the ILSS. The composites were produced from continuous tows with fibre mass fractions of 20%, 30% and 40% and fibres of different fineness (1.7, 3.3 and 28.0 dtex). The results showed that the different test procedures led to different trends of ILSS depending on the fibre mass fraction and fibre fineness used. The transverse tensile test revealed that the IFSS decreased with increasing fibre mass fraction and fibre diameter. A different trend was found with the short beam shear test and the SENB test for the ILSS. Here, higher values were detected with increasing fibre mass content, and the influence of the fineness was less noticeable. The double-notched shear tests (tensile and compression) showed a different trend: the ILSS increased with increasing fibre mass fraction from 20% to 30%. With a further increase to 40%, the ILSS tend to decrease slightly. An influence of the fibre fineness on the ILSS could not be statistically proven. The different trends of the test methods are attributed to the constitution of the composite and the different load application caused by the test procedures.Article number 709845

Influence of fibre loading, fibre length, fibre orientation and voids on the characteristics of compression and injection moulded cellulose fibre-reinforced polylactide (PLA) composites

The influences of fibre loading, fibre orientation, fibre length and voids on the mechanical characteristics of lyocell fibre-reinforced polylactide (PLA) composites with a fibre loading of 20, 30 and 40 mass-% were analysed. Composites were produced via compression moulding (CM) and injection moulding (IM). An increasing tensile strength was observed with an increasing fibre loading from 20 to 40% for CM composites, while the highest values of IM samples were measured for a fibre loading of 30%. A better fibre/matrix adhesion and compaction was observed for IM composites and the fibre orientation differed not significantly from that of the CM composites. Addtionally the tensile strength of extracted fibres was analysed and has shown only slightly lower values of IM fibres as compared to CM fibres. Despite these findings strength values of IM composites are significantly lower than that of CM composites. It was shown that the fibre length of IM composites drastically decreased during processing with an increasing fibre loading. Additionally, a high proportion of voids was found in the central part of IM samples reinforced with 40% fibres resulting in a notch effect and reduced tensile strength in combination with the low fibre aspect ratio. The lower mechanical characteristics of IM samples with a fibre loading of 20 and 30% compared to CM samples are attributed predominantly to the lower fibre aspect ratio

Influence of sample thickness, curvature and notches on the Charpy impact strength - An approach to standardise the impact strength of curved test specimens and biological structures

The specimen geometry has a significant influence on the Charpy impact strength. This is often a problem when it comes to the analysis of materials that can only be prepared with a curved shape, or which, like some biological structures, are naturally given in curved form like nutshells. The question repeatedly arises to what extent the curvature influences the impact strength. Hence, the present study deals with the dependence of the specimen geometry like specimen thickness, curvature and notches on the impact properties of concrete, gypsum, polyurethane (PUR) and epoxy-based (EP) samples. Increasing sample thickness from 2 to 10 mm resulted in increased toughness for brittle materials, whereas more ductile materials did not show any significant change in toughness. Increasing span length showed an increase of the unnotched impact strength for brittle gypsum, while toughness increased for a more ductile PUR sample. For the curved specimens both, the unnotched and the notched Charpy impact strength could be shown to increase with a reduction of the specimen radius (higher curvature) while the notch-sensitivity was not significantly affected. Material-specific linear relationships between the impact strength and the curvature of the samples were found. These relationships were used to create a model to calculate the influence of the curvature on the impact strength. For all materials, the characteristic values of a normalised flat standard sample could be well predicted from measured values of curved specimens. This calculation approach was used to predict the impact strength of an isotropic biological sample (sweet potato) concerning a flat sample and different curved specimens. The results show a good trend with the measured values