Reinforcement of wood with natural fibers

This paper describes an experimental programme which examines the reinforcement in flexure of timber beams with composite materials based on natural fibers in the form of fabrics made from hemp, flax, basalt and bamboo fibers. The industrial use of natural fibers has been continuously increasing since 1990s due to their advantages in terms of production costs, pollution emissions and energy consumption for production and disposal. The technique allows the reinforcement of the intrados of beams, avoiding the dismantling of the overlying part of the structure with significant savings in terms of costs and work time. The test program consists of three phases incorporating 45 beams. The bending tests on the wooden elements made it possible to measure the increase in capacity and stiffness resulting from the composite reinforcement. This was applied to beams, creating different arrangements and using different quantities (number of layers). Despite the diversity of the various tests carried out, the results obtained in some cases showed significant increases in terms of load-carrying capacity and in deflection ductility.     

Cellulosic fibers from rice straw and bamboo used as reinforcement of cement-based composites for remarkably improving mechanical properties

The cement-based composites reinforced with cellulosic fibers isolated from rice straw were fabricated by a slurry vacuum de-watering technique. The physical structures and mechanical properties of the composites with fiber contents ranging from 2% to 16% by weight (wt.%) were investigated. Moreover, the composites reinforced with bamboo cellulosic fibers and the control cement paste, sample without cellulosic fibers, were also fabricated as reference materials. As a result, the cement-based composites reinforced by cellulosic fibers showed a remarkable improvement in the mechanical properties. The measurements of the flexural strength and the fracture toughness of the optimal sample were found to be increased by 24.3% and 45 times, respectively. The bulk density of the composites was decreased by 12.4–37.3% as a result of the introduction of cellulosic fibers. Additionally, the field emission scanning electron microscope (FSEM) observations and energy dispersive spectroscopy (EDS) analyses revealed that the hydration products of Portland cement migrated to the fiber lumens, resulting in mineralizing the cellulosic fibers and decreasing the fracture toughness of the composites.     

Effect of surface modification of bamboo cellulose fibers on mechanical properties of cellulose/epoxy composites

Bamboo cellulose fibers were treated with NaOH aqueous solution and silane coupling agent, respectively, before they were applied into epoxy composites. The effect of surface modification on mechanical properties was evaluated by tensile and impact tests under controlled conditions. Compared with the untreated cellulose filled epoxy composites, the NaOH solution treatment increased the tensile strength by 34% and elongation at break by 31%. While silane coupling agent treatment produced 71% enhancement in tensile strength and 53% increase in elongation at break. The scanning electron microscopy (SEM) was used to observe the surface feature of the cellulose fibers and the tensile fractures as well as cryo-fractures of the composites. The Fourier transform infrared (FTIR) was employed to analyze the chemical structure of the cellulose fibers before and after modifications. The results indicated different mechanisms for the two modifications of cellulose. The NaOH solution partly dissolved the lignin and amorphous cellulose, which resulting in splitting the fibers into smaller size. This led to easier permeating into the gaps of the fibers for epoxy resin (EP) oligmer and forming effective interfacial adhesion. Based on the emergence of Si–O–C and Si–O–Si on the cellulose surface, it was concluded that the enhancement of mechanical properties after coupling agent modification could be ascribed to the formation of chemical bonds between the cellulose and the epoxy coupled with the coupling agent.     

Preparation and characterization of novel biodegradable composites based on acylated cellulose fibers and poly(ethylene sebacate)

The preparation and characterization of biodegradable composite materials with improved properties based on poly(ethylene sebacate) (PES) and acylated cellulose fibers is reported. These biocomposites showed improved mechanical properties, as evidenced by the increase in both elastic and Young moduli and in the tensile strength, and also showed low water sensitivity and a high biodegradability rate. These novel biocomposites were prepared essentially from renewable resources and therefore constitute an important contribution to the development of the area of sustainable composite materials.     

A novel process to improve yield and mechanical performance of bamboo fiber reinforced composite via mechanical treatments

The aims of the present study are to produce bamboo fiber reinforced composite (BFRC) with high yield and to investigate the mechanical properties of BFRC comparing with those of commercial bamboo scrimber (BS) and laminated bamboo lumber (LBL). A novel process was developed for production of BFRC using oriented bamboo fiber mat (OBFM) made by a pilot machine. The yield and the mechanical properties of BFRC were investigated and analyzed in comparing with those of raw bamboo and other bamboo-based composites. The results show that the novel process produces 92.54% yield of OBFM due to without any chemical and special removing of inner and outer layer of bamboo during processing. In addition, all the mechanical properties and the variability of BFRC were significantly enhanced comparing with those of raw bamboo and other bamboo-based composites.     

Flexural properties of fully biodegradable alpha-grass fibers reinforced starch-based thermoplastics

This work has the aim of study the flexural properties of alpha-grass reinforced starch-based composites. The composite materials contain alpha-fibers in the range from 5 to 35 wt%. The reinforcing fibers were submitted to an alkali treatment to create a good interphase between the fibers and the matrix. It was observed that a mild 2.5 h cooking process was enough to create a good interphase, while longer periods rendered lesser improvements. The surface charges of the fibers and the matrix were determined by polyelectrolyte titration, and it was found that after the alkaline treatment both were similar. The composite materials were injection molded and tested under flexural conditions. All the flexural properties of the studies composites increased linearly with the reinforcement contents. The micromechanics of the flexural modulus and strength were studied and compared with that of tensile modulus and strength. It was established that the efficiency factors for the tensile and flexural properties were statistically similar. Three different methods were used to compute the intrinsic flexural strength from the available data. Finally the Weibull theory was used to study the best prediction of the standard deviation of the intrinsic flexural modulus.     

Effect of alumina,silk and ceria short fibers in reinforcement of Bis-GMA/TEGDMA dental resin

The present study is focused to investigate influence of short fibers such as Alumina Microfibers (AMFs), Silk Microfibers (SMFs) and Ceria Nanofibers (CNFs) as reinforcements in Bis-GMA/TEGDMA resin towards development of composite dental filler. Morphologies of AMFs, SMFs, CNFs and their representative fracture surfaces of the reinforced dental resins/composites were examined by SEM. X-ray Diffraction Analysis was done to analyse the phase of the fibers used in this study and degree-of-conversion of the fiber incorporated base resin was studied by FTIR. Viscosity study of fiber resin mixture, depth of cure and mass change behaviour of the fibers resin composites in artificial saliva were done to analyse the flow ability and physical properties of the fiber resin composites. Mechanical properties of the composites were tested by a universal testing machine. This study demonstrated that incorporation of 10% AMFs, 5% SMFs, and 3.33% CNFs individually in Bis-GMA/TEGDMA dental resin resulted in similar degree of conversion compared to the control. Also the fiber reinforced composites (10% AMFs, 5% SMFs, and 3.33% CNFs) demonstrated significant improvement in mechanical properties compared to Bis-GMA/TEGDMA resin (Control). However, depth of cure was significantly reduced due to incorporation of fibers in the resin. The reinforcement effect of AMFs, SMFs in dental resin was superior due to their uniform distribution and good interfacial bonding between fibers and resin matrix. In case of CNFs, rapid increase in viscosity during mixing of fibers with resin and inhomogeneous mixing were the major problem encountered during formulation, which was mainly associated with high surface to volume ration of the nanofibers. The resultant composite containing CNFs had less improvement in mechanical properties which may be due to less fiber content, formation of agglomerates and improper distribution of the fibers in the composite which subsequently resulted in reduction of adhesive strength.     

Gel-spun fibers from magnesium hydroxide nanoparticles and UHMWPE nanocomposite: The physical and flammability properties

In this study, magnesium hydroxide [Mg(OH)₂] nanoparticles were successfully incorporated into ultrahigh molecular weight polyethylene (UHMWPE) to produce nanocomposite fibers with reduced flammability. To improve the dispersibility in processing solvent (decalin) and the compatibility with UHMWPE, the Mg(OH)₂ nanoparticles were firstly modified with oleic acid. Fourier transform infrared spectroscopy indicated that the oleic acid was covalently bonded on the surface of Mg(OH)₂ nanoparticles and formed a coating layer. The modified Mg(OH)₂ could disperse evenly and stably in decalin. The UHMWPE/decalin solutions with different percentages of Mg(OH)₂ were gel-spun and then hot drawn to yield UHMWPE nanocomposite fibers. The addition of Mg(OH)₂ nanoparticles altered the mechanical properties of the fibers by affecting their maximum draw ratio. High loading of the nanoparticles would decrease both strength and modulus of the fibers. The thermal gravimetric analysis (TGA) and differential thermal analysis (DTA) under air flow demonstrated their flammability could be remarkably reduced. The starting thermal decomposition temperature was increased by 30 °C. Their combustion peak temperatures were also increased significantly.     

Development of a biocomposite based on green epoxy polymer and natural cellulose fabric (bark cloth) for automotive instrument panel applications

Natural fiber reinforced composites have attracted interest due to their numerous advantages such as biodegradability, dermal non-toxicity and with promising mechanical strength. The desire to mitigate climate change due to greenhouse gas emissions, biodegradable resins are explored as the best forms of polymers for composites apart from their synthetic counterparts which are non-renewable. In this study biodegradable bark cloth reinforced green epoxy composites are developed with view of application to automotive instrument panels. The optimum curing temperature of green epoxy was shown to be 120 °C. The static properties showed a tensile strength of 33 MPa and flexural strength of 207 MPa. The dynamic mechanical properties, frequency sweep showed excellent fiber-matrix bonding of the alkali treated fabric with the green epoxy polymer with glass transition temperature in the range of 160 °C–180 °C. Treatment of the fabric with alkali positively influenced the mechanical properties of the fabric reinforced biocomposites.     

Fully biodegradable natural fiber composites from renewable resources: All-plant fiber composites

Wood flour can be converted into thermoplastics through proper benzylation treatment, which introduces large benzyl group onto cellulose and partially deteriorates the ordered structure of the crystalline regions. By changing a series of parameters, like reaction temperature, concentration of aqueous caustic solution, species of phase transfer catalyst, etc., the extent of benzyl substitution is regulated within a wide range so that a balanced thermal formability and mechanical performance of the modified wood flour is obtained. By using the properly plasticized China fir sawdust as the matrix, both discontinuous and continuous sisal fibers are compounded to produce composites from renewable resources, respectively. These all-plant fiber composites are characterized by moderate mechanical properties and full biodegradability, and might act as alternative to petro-based materials in terms of structural applications.     

Bounds on the Elastic Brittle solution in bodies reinforced with FRP/FRCM composite provisions

In the paper the behavior of composite-reinforced masonry structures is discussed. One focuses on the problem of the reinforced structure under different modeling of the basic material keeping into account possible strength in tension of the masonry material. Actually the presence of the reinforcement requires the development of a specialized treatment, that is presented in the paper, with the purpose of exploring the dependence of the solution on more or less refined hypotheses about the masonry material in the presence of reinforcement. The original set up leads to the formulation of new bounding theorems for masonry structures reinforced by composites, with the masonry possibly modeled by an elastic brittle assumption.     

Wood based PLA and PP composites: Effect of fibre type and matrix polymer on fibre morphology,dispersion and composite properties

This study presents a comparison of the effect of various wood fibre types in polylactic acid and polypropylene composites produced by melt processing. The study also reveals the reinforcing effect of pelletised wood fibres compared to conventionally used wood flour or refined fibres. Composites containing 30 wt.% of chemical pulps, thermomechanical pulp and wood flour were produced by compounding and injection moulding. Fibre morphologies were analysed before and after melt processing. The dispersion of the fibres and mechanical performance of the composites were also investigated. Fibre length was reduced during melt processing steps, reduction being higher with longer fibres. Wood fibres provided clearly higher plastic reinforcement than wood flour. Comparing the wood fibre types, TMP fibres provided the highest improvement in mechanical properties in polylactic acid composites with uniform fibre dispersion. In polypropylene composites, fibre selection is not as crucial.     

Hybrid multi-scale epoxy composite made of conventional carbon fiber fabrics with interlaminar regions containing electrospun carbon nanofiber mats

Herein we report the development and evaluation of hybrid multi-scale epoxy composite made of conventional carbon fiber fabrics with interlaminar regions containing mats of electrospun carbon nanofibers (ECNs). The results indicated that (1) the interlaminar shear strength and flexural properties of hybrid multi-scale composite were substantially higher than those of control/comparison composite without ECNs; in particular, the interlaminar shear strength was higher by ∼86%; and (2) the electrical conductivities in both in-plane and out-of-plane directions were enhanced through incorporation of ECNs, while the enhancement of out-of-plane conductivity (∼150%) was much larger than that of in-plane conductivity (∼20%). To validate the data reduction procedure, a new shear stress formula was formulated for composite laminates, which took into account the effect of layup and inter-layers. The study suggested that ECNs could be utilized for the development of high-performance composites, particularly with the improved out-of-plan properties (e.g., interlaminar shear strength).     

Pullout resistance of straight NiTi shape memory alloy fibers in cement mortar after cold drawing and heat treatment

In our study, we found cold drawing to be an effective method for enhancing the pullout resistance of NiTi shape memory alloy (SMA) fibers in concrete. The pullout resistance was observed to be dependent on the contact pressure and friction coefficient at the interface between the fibers and the mortar matrix. The drawing process increased the stiffness and yield stress of the fibers and consequently increased the contact pressure at the interface between the fibers and the mortar matrix. Moreover, heat treatment of the fibers after cold drawing was found to noticeably recover the fiber diameter, thereby significantly enhancing the pullout resistance. The enhancement of the interfacial bond strength by heat treatment verified the crack-closing capabilities of SMA-fiber-reinforced cement composites.     

The effect of alkaline treatment on mechanical properties of kenaf fibers and their epoxy composites

In this work, kenaf fibers were pre-treated in a NaOH solution (6% in weight) at room temperature for two different periods (48 and 144 h). The chemical treatment of kenaf fibers for 48 h allowed to clean their surface removing each impurity whereas 144 h of immersion time had detrimental effect on the fibers surface and, consequently, on their mechanical properties.Untreated and NaOH treated kenaf fibers (i.e. for 48 h) were also used as reinforcing agent of epoxy resin composites. The effect of the stacking sequence (i.e. using unidirectional long fibers or randomly oriented short fibers) and the chemical treatment on the static mechanical properties was evaluated showing that the composites exhibit higher moduli in comparison to the neat resin. As regards the strength properties, only the composites reinforced with unidirectional layers show higher strength than the neat resin. Moreover, the alkali treatment increased the mechanical properties of the composites, due to the improvement of fiber–matrix compatibility.The dynamic mechanical analysis showed that the storage and the loss moduli are mainly influenced by the alkali treatment above the glass transition temperature. Moreover, the alkali treatment led to a notable reduction of tan δ peaks in addition to significant shifts of tan δ peaks to higher temperatures whereas the stacking sequence did not influence the trends of storage modulus, loss modulus and damping of the composites.     

Reinforcement of ramie fibers on regenerated cellulose films

All-cellulose composite films reinforced with ramie fibers were prepared from aqueous NaOH–urea solvent system via a simple pathway. The structure and physical properties of the modified ramie fibers and composite films were characterized by scanning electron microscope (SEM), wide angle X-ray diffraction (WAXD), Fourier transform infrared spectrometer, ultraviolet–visible spectroscope, thermogravimetry, biodegradation tests and tensile tests. The results revealed that a good compatibility existed between the modified ramie fibers and cellulose matrix. The all-cellulose composite films exhibited high tensile strength, good optical transmittance, thermal stability, and biodegradability. The tensile strength and elastic modulus of the composite films increased with an increase of the ramie fibers. These high-strength biodegradable films prepared by a “green” pathway have potential applications as packaging materials and biomaterials.     

Influence of various starch/hemp mixtures on mechanical and acoustical behavior of starch-hemp composite materials

The starch-hemp composite materials are manufactured from the natural raw materials (water, starch and hemp shives) and a new durable material for construction and building. In this work, experimental investigation was carried out to study the mechanical and acoustical performance of starch-hemp composite materials. The starch-hemp composite materials specimens with five Hemp/Starch ratios (H/S = 6, 8, 10, 12 and 14), were manufactured by using the optimal binder and two hemp shives (0–15 mm and 0–20 mm). Density of the starch-hemp composite materials varies with the H/S ratio. The dry density for the starch-hemp composite materials is lower, between 163.6 kg/m³ and 169.1 kg/m³ in case of the hemp shives 0–15 mm and between 168.1 kg/m³ and 174.3 kg/m³ for the hemp shives 0–20 mm. The relation between stress and strain of the composite materials is not linear. The ultimate compressive stress can reach 0.55 MPa and the compressive strain is up to 30%. The results obtained by test show that the tensile strength depends strongly on the Hemp/Starch ratio and the hemp shives sizes. The variation of elasticity modulus and Poisson's ratio in function of the H/S ratio was also analyzed in this paper. The mechanism of the cracks or failure of the specimens was studied by using ARAMIS optical system. The study on acoustical behavior shows that the starch-hemp composite materials are a good sound absorber material for medium and high frequencies with a value around 0.7. The influence of the H/S ratio on the absorption coefficient is small. The results show that the starch-hemp composite materials have a good mechanical and acoustical performance and can be used as building materials.     

Effects of carbon nanotube functionalization and annealing on crystallization and mechanical properties of melt-spun carbon nanotubes/poly(ethylene terephthalate) fibers

To generate poly(ethylene terephthalate) (PET) fibers with enhanced mechanical properties, we prepared melt-spun PET fibers that incorporated pristine, acid-treated, and functionalized multi-walled carbon nanotubes (MWNTs) with 2-phenylethyl alcohol and 4-phenyl-1-butanol. The incorporation of MWNTs into the melt-spun fibers resulted in increased crystallization of PET but lower breaking stress than that of pure PET fibers, even in those containing well-dispersed functionalized MWNTs. The breaking stress of drawn composite fibers was also lower than that of pure PET fibers prepared at the same draw ratio. However, the annealing of melt-spun fibers enhanced the mechanical properties and crystallization, and the annealing effect was more dominant for composite fibers with functionalized MWNTs. These findings indicate that the presence of well-dispersed MWNTs disturbs the crystallization and orientation of PET molecules in highly stressed fibers, which differs from MWNT-induced crystallization of PET molecules in relaxed fibers.     

Mechanical behavior of self-assembled carbon nanotube reinforced nylon 6,6 fibers

The versatile electrospinning technique was used to successfully align and disperse multiwalled carbon nanotubes (MWCNT) in nylon 6,6 matrix to obtain composite fibers. The morphology of the composite fibers and the dispersion of the CNTs within the fibers were analyzed using scanning electron microscopy (SEM) and transmission electron microscopy (TEM), respectively. TEM analysis revealed that the CNTs were well-dispersed, separated and aligned along the fiber axis. The thermal and mechanical properties of the composite fibers were characterized as a function of weight fraction of the CNTs. Incorporation of the CNTs in the fibers resulted in an increase in glass-transition temperature (T_g) by ∼7 °C, indicating that the addition of CNTs has restricted the mobility of the polymer chains and provided confinement to neighboring molecular chains. Tensile and nanoindentation experiments were performed to investigate the mechanical deformation behavior of the composite fibers. The results suggested that incorporation of high strength and high aspect ratio CNTs into the fiber matrix enhanced significantly the stiffness and strength of nylon 6,6 fibers. An understanding of the structure–property relationships can provide fruitful insights to develop electrospun fibers with superior properties for miniaturized and load-bearing applications.     

Polymer matrix,polymer ribbon-reinforced transparent composite materials

Composites of a polymer–matrix reinforced by polymer ribbon monofilaments are investigated as mechanically robust, transparent composite materials. Transparent nylon monofilaments are mechanically worked to form flattened nylon ribbons, which are then combined with index-matched epoxy resin to create transparent composites. A range of optical and mechanical experiments are performed on composites and surrogate systems in order to quantify properties and guide system design. The results show that these polymer–polymer composites provide good transparency over a wide temperature range, and superior ballistic penetration resistance compared to monolithic transparent polymers.