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This work evaluates a new eco-friendly surface treatment method for coir fibres based on sodium bicarbonate (NaHCO3). The fibres are treated with 10 wt% NaHCO3 solution for different periods of time (24, 96 and 168 h). A 2141 Full Factorial Design is established to investigate the effect of the type of matrix (epoxy and polyester) and treatment time (24, 96 and 168 h) on the mechanical properties of coir fibre composites. X-Ray diffraction (XRD) analysis reveals that the treatment does not promote a complete change of cellulose-I into cellulose-II. However, the degradation of hemicellulose and pectin of the fibre surface is evidenced by the thermogravimetric (TGA) curves. The chemical treatment contributes to increase the fibre density and to reduce its thermal stability. Single fibre tensile tests reveal a gradual increase in the elastic modulus attributed to the treatment time factor. Epoxy composites presented higher tensile and flexural strength in comparison to polyester composites, which exhibit higher tensile and flexural modulus. The alkaline treatment does not affect the mechanical strength of the composites. However, higher tensile and flexural moduli are obtained when the treatment time increases to 96 or 168 h. The proposed treatment proves to be feasible and efficient to increase the stiffness of coir fibre composites, besides being less damaging to the environment after disposal when compared to strongly alkaline solutions such as NaOH.

In recent years, green composites based on thermoplastic matrices from renewable sources, and reinforced with natural fibres, have gained significant importance in different industrial applications, due to lower environmental impacts and production costs than traditional composites. This work investigates the manufacturing process, fibre/matrix integration and mechanical properties of a novel environmentally friendly green composite with a recyclable biobased polymer from a renewable source and a biodegradable natural fibre. Untreated woven sisal fibres reinforced post-consumer green polyethylene composites were evaluated in terms of flexural, tensile and impact properties. Traditional and green high-density polyethylene (HDPE), originated from sugarcane ethanol, were utilised as matrices of the investigated composites. Woven sisal fibres were arranged in two different stacking sequences, i.e. [0°/90°] and [± 45°], being incorporated into the HDPE with a mass percentage proportion of 30/70 (fibre/matrix). A low-cost manufacturing process based on the hot compression moulding was used to produce the composites. The results were analysed by a factorial design to identify the effects of polyethylene type and the use of woven sisal fibres, considering the [0°/90°] and [± 45°] orientations. Thermal gravimetric analysis was used to verify the thermal stability of the sisal fibre. The topographic surface of sisal fibres was observed by scanning electron microscopy. The results showed that the use of green polyethylene reinforced with untreated woven sisal fibres achieved higher flexural modulus (35%), flexural strength (13%), tensile strength (39%) and ultimate strain (68%) than traditional polyethylene without reinforcement. The green composite presented promising mechanical results to replace materials from non-renewable sources and can reduce manufacturing costs of the final product. These composite materials can be efficient for structural applications such as insulated panels, drywall and partitions for furniture.

A full factorial design (2 231 ) has been used to investigate the effect of the use of sodium hydroxide fibre treatment, Portland cement and uniaxial pressure on the physical and mechanical properties of hybrid short coir fibre reinforced composites (HSCoirFRCs). The response variables considered in this work were the apparent density, porosity, tensile and flexural strength , the modulus of elasticity and the Charpy impact resistance. The alkali treatment contributed not only to reduce the apparent porosity , but also to increase the mechanical properties of the HSCoirFRCs. A reduction of the impact resistance and an increase of the apparent density was also identified after treatment. Cold pressing significantly affected the physical and mechanical properties of the HSCoirFRCs. Higher pressure levels enhanced the wettability of the fibres and, consequently, the mechanical performance of the composites. The incorporation of cement microparticles as a second reinforcement phase was however not effective, leading to decreased strength and an increased apparent density of the materials. The HSCoirFRC structure can be considered an economical and sustainable alternative for future secondary structural parts in lightweight transport applications.

The hybrid configuration of bio-reinforced composites has established a new extended boundary for the development of pro-ecological technologies due to light weight, moderate specific strength, low cost, environmental benefits, and potential applications of natural components. This work investigates the physical and mechanical properties of hybrid composites made of sisal/glass fibres and Portland cement inclusions. A full factorial design was generated to identify the effects of the stacking sequence and cement particles on the flexural strength, flexural stiffness, apparent density, apparent porosity and water absorption of the composites. The significant contributions of these main factors and their interactions were determined via Design of Experiments (DoE) and Analysis of Variance (ANOVA). The fracture features and damage mechanisms of hybrid composite were also reported. The inclusion of cement microparticles led to an increased apparent porosity, as well as enhanced water absorption, flexural stiffness and flexural strength of the hybrid composites. The mechanical properties were strongly dependent on the fibre stacking sequence, which accounts for approximately 98% of the effects observed. Moreover, the stacking sequence affected the damage mechanism of the bio-composites. Finally, the replacement of glass fibres by unidirectional sisal reinforcements may potentially improve the specific properties in structural applications with an environmental sustainable footprint.

Surface modification induced by sodium carbonate on coir fibres was studied in view of its application as a reinforcing agent in polyester and epoxy polymer fibre-reinforced composites. Coir fibres were treated with a 10 wt% sodium carbonate solution for different periods of time (24, 96 and 168 h) at room temperature. The surface treatment was evaluated by scanning electron microscopy, X-ray diffraction and thermogravimetric analysis. Tensile, flexural and impact properties of treated and untreated coir fibre-reinforced composites were compared. Coir-reinforced epoxy and polyester composites were manufactured by the uniform dispersion of randomly oriented coir fibres within the polymeric matrix. Tensile and flexural modulus of coir-reinforced polyester composites increased nearly 28% and 25%, respectively after 96 h of coir treatment. An analogous increase of 31% and 17% was obtained for coir-reinforced epoxy composites. In addition, coir-polyester composites achieved superior tensile (∼17%) and flexural (∼5%) modulus and impact strength (∼193%) compared with coir-epoxy-derived composites. In contrast, coir-epoxy composites led to superior tensile and flexural strength. The experimental results revealed that sodium carbonate solution effectively removes hemicellulosic compounds, promotes swelling and increases superficial roughness of the fibres, improving the mechanical properties (modulus and strength) of the coir-reinforced composites studied.

This work investigates the effect of using functionalised poly-diallyldimethylammonium chloride (PDDA) silica micro and nanoparticles on the apparent density, tensile and flexural strength and modulus of hybrid glass fibre reinforced composites. The work is carried out using a Design of Experiment (DoE) approach on a population of 60 composite specimens produced according the size of the nanoparticles and their weight fraction. A 2% of weight fraction of functionalised silica nanoparticles is shown to reduce the flexural strength and density of the composites, but to maximise significantly both the tensile, flexural modulus and tensile strength of the hybrid glass fibre materials.

Fibre metal laminates (FMLs) have been widely used to manufacture airframe components. This work describes novel sisal fibre reinforced aluminium laminates (SiRALs) that have been prepared by cold pressing techniques and tested under tensile, flexural and impact loading. The pristine sisal fabric and the sisal fibre reinforced composites (SFRCs) were also tested to understand the difference in mechanical performance of the sisal fibre metal laminates. The SiRALs achieved not only the highest modulus and strength, but also the highest specific properties. The mean specific tensile strength and modulus of the SIRALs reached increases of 132% and 267%, respectively, when compared to the sisal fibre reinforced composites (SFRCs). Moreover, the mean specific flexural strength and modulus of the SiRALs were significantly higher than SFRCs, revealing increases of 430% and 973%, respectively. A delamination fracture mode was noted for SiRALs under bending testing. The SiRALs can be considered promising and sustainable composite materials for structural and multifunctional applications.

In today's modern world the composite materials have been widely used not only for aerospace/aeronautics, but also automotive, sports and construction industries. The flexural strength of laminated composites depends on the characteristics of dispersive and matrix phases, considering the presence of tensile and co mpressive loadings. For this reason, the addition of particles into the matrix phase has been investigated to enhance its stiffness and consequently the elastic modulus of the composite. Th is work investigates a glassfibre co mposite reinforced with silicon carbide part icles. The following responses were evaluated: apparent density, water absorption, flexural strength, modulus of elasticity. The results were compared to those manufactured with no silicon carbide particles. It was observed that the silicon carbide particles provided the increase of the apparent density and flexu ral strength of the composites.