Commingled natural fibre/polypropylene wrap spun yarns for structured thermoplastic composites

A major challenge for natural fibre composites is to achieve high mechanical performance at a competitive price. Composites constructed from yarns perform better than composites made from random nonwoven mats. However, the twist structure of conventional ring spun yarns prevents the full utilization of fibre mechanical properties in the final composites. We produced flax/polypropylene commingled wrap yarns in which all flax fibres were twistless. Composites made from the wrap yarn demonstrated significant improvement of flexural modulus. Most currently available low cost natural fibres, such as decorticated hemp, cannot be efficiently made into yarns because of their lack of cohesion. Adding polypropylene fibres to decorticated hemp improved textile processing performance. The polypropylene fibres served as a carrier for the natural fibres during processing and became the polymer matrix in the final composites.     

Manufacturing and mechanical properties of soy-based composites using pultrusion

Two primary cost driving factors for the composites industry are raw materials and labor. Inexpensive alternative epoxy resin systems based on epoxidized soyate resins are developed for fiber reinforced composite applications. This research investigated on the manufacturing and mechanical characterization of fiber/epoxy composites using chemically modified soy-based epoxy resins. Co-resin systems with up to 30 wt% soyate resins were used to manufacture composites through pultrusion. Mechanical tests show that the pultruded composites with soy based co-resin systems possess comparable or improved structural performance characteristics such as flexural strength, modulus, and impact resistance. Maximum mechanical properties enhancement is demonstrated by the enhanced epoxidized allyl soyate (EAS) formulation. Further property improvement is obtained through using a two-step prepolymer process. The EAS holds great potential as partial supplement for polymer and composites applications from renewable resources.     

The manufacture and mechanical testing of thermosetting natural fibre composites

High volume fraction hemp and flax fibre composites were manufactured using low viscosity epoxy and phenolic resins. Using 80% volume fraction of flax fibres in epoxy resin, composites with a mean stiffness of 26 GPa and a mean strength of 378 MPa were produced. By reducing processing damage of the plant fibres mechanical properties could be increased by 40%. Strips of retted fibre tissue were found to be just as effective for reinforcement as fibre bundles and individual fibres. Phenolic resin and decorticated flax fibres produced very poor composites. Using 40% volume fraction of fibres the mean stiffness was 3.7 GPa and the mean strength was 27 MPa. Two fibre pre-treatments were devised to improve adhesion with resins. The first, 6 M urea was used only in natural fibre-epoxy composites where it increased the stiffness but not the strength. The second pre-treatment was a 50% PVA solution, which was cured prior to the addition of space filling resin. The PVA treatment improved the stiffness and strength of both natural fibre-epoxy composites and natural fibre-phenolic composites.     

绿色复合材料的研究进展

天然植物纤维具有价廉质轻、比强度高和自然降解等优良特性,其与生物降解树脂进行复合,能开发出环境友好、可自然降解的绿色复合材料.本文评述了该领域的研究进展,重点是竹纤维、各种麻纤维以及甘蔗渣纤维等天然纤维与不同的生物降解树脂的绿色复合材料的制备,不同成型方法的探讨,材料各种力学性能的分析比较,以及为了增强材料的力学性能而进行的纤维改性,并展望了绿色复合材料的未来的研究趋势.     

Mechanical properties of biaxial weft-knitted flax composites

There has been an increasing interest of using natural fibers to replace glass fibers for composite reinforcement nowadays. However, most of the developments have focused on the random discontinuous fiber composite system. In this study, low cost flax continuous yarn was used to make non-crimp fabric for composite reinforcement based on a biaxial weft-knitted structure. A modified flat knitting machine was developed to produce this kind of reinforcement. Both NaOH treated and untreated reinforcements were used for fabricating the composite samples through the vacuum assisted resin transfer molding process (VARTM). The mechanical properties of the flax yarn, reinforcement and composites were tested and the effect of the NaOH treatment was discussed. The results revealed that although the NaOH treatment resulted in the reduction of the mechanical properties of both yarn and reinforcement, the mechanical properties of the composites could be considerably improved by the NaOH treatment of the reinforcement. The study has provided a simpler way of using low cost natural fiber yarns made from flax long-fiber bundles to produce high performance composites.     

Multi-scale study of the adhesion between flax fibers and biobased thermoset matrices

The environmental impact of composite materials made with a thermoset matrix can be reduced in two ways. First, glass fibers can be replaced by natural fibers. Second, petrochemical components from the matrix can be replaced by biobased renewable equivalents. The quality of the interface between the matrix and the fibers has a strong influence on the composite mechanical properties. In this study, tensile performances of flax fibers and commercially partly biobased epoxy and polyester matrices have been investigated and corresponding unidirectional composites were elaborated. Their mechanical performances are in accordance with fiber and matrices properties, taking into account fiber dispersion. Then, at the microscopic scale, the debonding test was used; a great adhesion between flax fiber and thermoset matrices was highlighted. Finally, tensile tests on ±45° laminates were carried out to create an in-plane shear at the macroscopic scale. Interestingly, the results obtained at the macroscopic scale are well correlated to the ones given by the debonding test at the microscopic scale.     

Why are designers fascinated by flax and hemp fibre composites?

Flax and hemp fibres have been increasingly used as reinforcement in polymer composites. First, an overview is given of the technical arguments which convinced designers of consumer goods to use these fibres. In particular, their composites show higher specific stiffness than glass fibre composites in both tension and plate bending and only slightly lower values than carbon fibre composites in plate bending. Moreover, flax and hemp fibres possess a much higher vibration damping capacity, making them excellent candidates for applications in sporting goods or musical instruments. Secondly, the paper describes how designers relate to the non-technical characteristics of these natural fibres. Many concrete examples are given from different application domains in consumer goods: sports, mobility, music and sound, furniture and interior design. The fascination of designers for these bio-based materials combined with the recent introduction in the market of new, composites-oriented preforms of flax and hemp fibres, is rapidly increasing the number and variety of composite products using flax and hemp fibres as reinforcement.     

Properties of cellulosic fibre reinforced plaster: influence of hemp or flax fibres on the properties of set gypsum

In the last few years, eco friendly materials have become an important part of the building materials market. Natural fibres are already used in various types of materials, like plastics, concrete and lime-based products. They demonstrate different attributes like the combination of good mechanical, thermal and acoustic properties that allow these types of materials to be used for different applications. The main drawback associated with plaster is its brittleness, especially under tensile stress. Therefore, it is interesting to investigate different methods that could potentially enhance the mechanical properties of plaster. Adding fibres to gypsum to obtain a composite material is one way to improve the behaviour of the product, especially after the failure of the matrix. The aim of this work was to the study the effects of adding natural fibres, namely hemp and flax fibres, on the setting time of plaster and the mechanical properties of the composite matrix. It was shown that hemp delayed the setting of plaster, unlike flax. The initial and final setting times almost doubled when hemp was added in a plaster matrix, whereas flax fibres did not drastically change them. Different chemical treatments of hemp were tested and the impact on the setting time was measured. The setting times of both composites made with hemp and flax were reduced once the fibres were treated (25–40% reduction), compared to the setting time of the calcium sulphate hemihydrate alone. The mechanical properties of the composite materials are also discussed. The behaviour of plaster was modified from brittle to a non-linear one when fibres were added, and even at small levels of addition, flax fibres allowed slightly higher values of flexural strength to be reached.     

Determination of mechanical properties of intra-layer abaca–jute–glass fiber reinforced composite

Composites made with natural fibers are finding applications in a wide variety of engineering fields due to their low cost and eco-friendly nature. This paper deals with the fabrication and evaluation of hybrid natural fiber composite using jute and abaca fibers along with glass fibers. Each composite is made up of five layers with three layers of jute and abaca enclosed by two layers of glass fibers. The composites are manufactured with three different fiber orientations and the compositions are varied in three different proportions. The fabricated composite samples are tested to investigate their various mechanical properties. From the test results, it is observed that fiber orientation plays a vital role in determining the mechanical properties of the composite. Morphological analysis is done using Scanning Electron Microscope (SEM).     

Natural and man-made cellulose fibre-reinforced poly(lactic acid) (PLA) composites: An overview about mechanical characteristics and application areas

The paper describes the production and the mechanical characteristics of composites made completely of renewable raw materials. Composites of different kinds of natural fibres like cotton, hemp, kenaf and man-made cellulose fibres (Lyocell) with various characteristics were processed with a fibre mass proportion of 40% and poly(lactic acid) (PLA) by compression moulding. Additionally, composites were made of fibre mixtures (hemp/kenaf, hemp/Lyocell). The composites were tested for tensile strength, elongation at break, Young’s modulus and Charpy impact strength. Their characteristics varied markedly depending on the characteristics of the raw fibres and fibre bundles and fibre mixtures used. While kenaf and hemp/PLA composites showed very high tensile strength and Young’s modulus values, cotton/PLA showed good impact characteristics. Lyocell/PLA composites combined both, high tensile strength and Young’s modulus with high impact strength. Thus, the composites could be applied in various fields, each meeting different requirements.     

Mechanical performance of natural fiber-reinforced composites for the strengthening of masonry

The growing concerns regarding the environmental impact generated by the use of inorganic materials in different fields of application increased the interest towards products based on materials with low environmental impact. In recent years, researchers have turned their attention towards the development of materials obtained from renewable sources, easily recoverable or biodegradable at the end of use. In the field of civil structures, a few attempts have been done to replace the most common composites (e.g. carbon and glass fibers) by materials less harmful to the environment, as natural fibers.This work presents a comprehensive experimental research on the mechanical performance of natural fibers for the strengthening of masonry constructions. Flax, hemp, jute, sisal and coir fibers have been investigated both from physical and mechanical points of view. The fibers with better performance were tested together with three different matrices (two of organic nature) in order to produce composites. These experimental results represent a useful database for understanding the potentialities of natural fibers as strengthening systems.     

Porous polyurethane composites with natural fibres

In the work the methodology and results of the investigations that concern rigid polyurethane foams modified with natural fibres and oil-based polyol are presented. The goal of the investigations was to obtain the cellular, polyurethane composites with the heat insulating and mechanical properties similar or better as in the case of the reference material. The obtained polyurethane composites had apparent densities about 40 kg/m³. The modified composites contained the considerable part of biodegradable components on the base of renewable raw materials. The influence of the rapeseed oil-based polyol, flax and hemp fibres of different length on the cell structure, closed cells content, apparent density, thermal conductivity and compression strength of the rigid polyurethane composites are analyzed. In the case of application of fibre in the amount of 5% php (per hundred polyols) the foam composites with the highest values of compressive strength and the lowest thermal conductivity were obtained.     

Defects in natural fibres: their origin, characteristics and implications for natural fibre-reinforced composites

This article reviews defects in natural fibres and how, ultimately, they affect the properties of composite materials reinforced with such fibres. Under ideal circumstances, certain natural fibre like flax and hemp can display excellent tensile mechanical properties. However, the potential of the fibre is generally not realised in natural fibre-reinforced composites. Partly, this poor performance can be explained by the presence of defects in the fibres known variously as dislocations, kinks or microcompressions. After briefly considering the chemistry and structure of plant fibres, the properties of selected natural fibres are reviewed. The origin of defects and the impact that processing has on their presence is then considered. The effect that defects have on the mechanical properties of bast fibre and their susceptibility to chemical degradation is also reviewed. Finally, the effect that dislocations have on the properties of composites reinforced with natural fibres is discussed and areas of potential further research needed are highlighted.     

Konstruktionswerkstoffe aus nachwachsenden Rohstoffen (BioVerbunde)

Structural materials from renewable resources (Biocomposites) In view of the resources steadily running short, while ecological damages are increasing, and apart from energy saving effects by lightweight constructions, the aspects of the exploitation of raw materials and the recovery after the end of the lifetime of products have to be more and more taken into consideration. Making use of conventional, i.e. petrochemically based plastics and fibre reinforced polymers, the production, use and recovery are often very difficult and demand considerable technical resources. An answer to solve all these problems may be given by natural fibre reinforced biopolymers based upon renewable resources, in the following called biocomposites. By embedding plant fibres, e. g. from flax, hemp or ramie (cellulose fibres) into biopolymeric matrices, e. g. derivatives from cellulose, starch, shellac or plant oils, fibre reinforced polymers are obtained that can environmentally friendly be integrated into natural cycles, e. g. by CO₂‐neutral incineration (including recovery of energy) by classic recycling, and possibly by composting.     

Effect of operating parameters and chemical treatment on the tribological performance of natural fiber composites: A review

The demand for high-performance engineering products made from natural resources is increasing because of the low-cost, low-density, biodegradability, renewable nature and lighter than synthetic fibers. With these characteristics, the tribological performance of natural fiber composite has become an important element to be considered in most industrial and manufacturing functions. This paper presents an overview of the factors that influence the tribological performance of natural fiber composites, which include applied load, sliding distance, sliding velocity and fiber orientation. Influences of chemical treatment is also reviewed and illustrated through scanning electron microscope (SEM) observations. This review will focus on kenaf fibers (KFs) and oil palm fibers (OPFs) which have been widely exploited over the past few years among the available natural resources. The results show that the operating parameter, fiber orientation and chemical treatment has significant effects on the tribological performance of natural composite. A clear understanding of the factors that affect the tribological performance is very essential in performance improvement on natural fibers reinforced polymer composite for potential applications.     

Mechanical and interfacial properties of wood and bio-based thermoplastic composite

The aim of this investigation was to study a new family of wood polymer composites with thermoplastic elastomer matrix (pebax® copolymers) instead of commonly used WPC matrices. These copolymers are polyether-b-amide thermoplastic elastomers which present an important elongation at break and a melting point below 200 °C to prevent wood fibers degradation during processing. Moreover these polymers are synthesized from renewable resources and they present a hydrophilic character which allow them to interact with wood fibers. We have used two pebax® grade with different hardness and three types of wood fibers, so the influence of the matrix and wood fibers characteristics were evaluated. Composites were produced using a laboratory-size twin screw extruder to obtain composite pellets prior to injection moulding into tensile test samples. We have evaluated fibers/matrix interaction by differential scanning calorimetry (DSC), infrared spectroscopy (IRTF) and scanning electron microscopy (SEM). Then, the mechanical properties, through tensile test, were assessed. We also observed fibers dispersion into the matrix by tomography X. DSC, IRTF and SEM measurements confirmed the presence of strong interface interactions between polymer and wood. These interactions lead to good mechanical properties of the composites with a reinforcement effect of wood fibers due also to a good dispersion of fibers into the matrix without agglomerate.     

Natural fiber-reinforced thermoplastic starch composites obtained by melt processing

Thermoplastic starch (TPS) from industrial non-modified corn starch was obtained and reinforced with natural strands. The influence of the reinforcement on physical–chemical properties of the composites obtained by melt processing has been analyzed. For this purpose, composites reinforced with different amounts of either sisal or hemp strands have been prepared and evaluated in terms of crystallinity, water sorption, thermal and mechanical properties. The results showed that the incorporation of sisal or hemp strands caused an increase in the glass transition temperature (T_g) of the TPS as determined by DMTA. The reinforcement also increased the stiffness of the material, as reflected in both the storage modulus and the Young’s modulus. Intrinsic mechanical properties of the reinforcing fibers showed a lower effect on the final mechanical properties of the materials than their homogeneity and distribution within the matrix. Additionally, the addition of a natural latex plasticizer to the composite decreased the water absorption kinetics without affecting significantly the thermal and mechanical properties of the material.     

Chemical modification of flax reinforced polypropylene composites

This paper presents an experimental study on the static and dynamic mechanical properties of nonwoven based flax fibre reinforced polypropylene composites. The effect of zein modification on flax fibres is also reported. Flax nonwovens were treated with zein coupling agent, which is a protein extracted from corn. Composites were prepared using nonwovens treated with zein solution. The tensile, flexural and impact properties of these composites were analysed and the reinforcing properties of the chemically treated composites were compared with that of untreated composites. Composites containing chemically modified flax fibres were found to possess improved mechanical properties. The viscoelastic properties of composites at different frequencies were investigated. The storage modulus of composites was found to increase with fibre content while damping properties registered a decrease. Zein coating was found to increase the storage modulus due to enhanced interfacial adhesion. The fracture mechanism of treated and untreated flax reinforced polypropylene composites was also investigated from scanning electron microscopic studies.     

Harakeke (phormium tenax) fibre–waste plastics blend composites processed by screwless extrusion

Most plastic packaging products are discarded after a single use resulting in an abundant supply of waste plastics. Natural fibres are light in weight, abundant and inexpensive. The stiffness and strength of polymers have been shown to improve with the incorporation of natural fibres. Hence, composite materials made of natural fibres and waste plastics would result in the reduction of plastic wastes and the use of fibres from renewable resources. Composite specimens and sheets consisting of fibres from the New Zealand flax plant, phormium tenax or Harakeke as it is known in Maori, and waste plastics have been produced through screwless extrusion followed by injection moulding. The tensile and impact properties of these composites have been determined. The formability of these composites has been assessed through thermoforming.     

Triglyceride-based thermosetting resins with different reactive diluents and fiber reinforced composite applications

Recently, there has been a growing research interest on renewable composite due to sustainability concerns. This work demonstrates the possibilities of bio-based reactive diluents and thermoset resin systems in composite applications from an engineering point of view. Formulating bio-based resins and monomers with different reactive diluents can tailor the physical and mechanical properties of the polymer system, allowing them to be suitable for different applications. In addition to the traditional reactive diluents, bio-based methacrylated fatty acid (MFA) was used in the formulations. Increasing the MFA content increases the toughness and the bio-based content of thermosetting polymers while reducing the hazardous air pollutant emissions. Composites panels with fiber glass and natural fiber reinforcement using selected bio-based resin formulations showed good mechanical properties and exhibited similar physical performance to parts made with commercial petroleum derived resins.