Investigation of the long term effects of moisture on carbon fibre and epoxy matrix composites

The aim of this work is to investigate the long term effects of moisture on the interface between a carbon fibre and an epoxy matrix. High modulus carbon fibres were used to prepare single fibre model composites based on an epoxy resin. The samples were immersed in the seawater and demineralised water and their moisture uptake behaviour was monitored. The equilibrium moisture content and diffusion coefficients for the samples were determined. DSC has been used to analyse the moisture effects on glass transition temperature and thermal stability of the pure epoxy specimens. These results showed a reduction in the glass transition temperature (T_g) after moisture absorption. Tensile tests were also carried out for the epoxy specimens and a general decrease in the mechanical properties of the epoxy matrix was observed. Raman spectroscopy was used to observe the effects of moisture on the axial strain of the carbon fibre within the composite and stress transfer at the interface as a function of exposure time. The results show that the decrease in the mechanical and interfacial properties of the model composites under the seawater immersion is more significant than under demineralised water immersion.     

Chemical treatments on plant-based natural fibre reinforced polymer composites: An overview

This paper provides a comprehensive overview on different surface treatments applied to natural fibres for advanced composites applications. In practice, the major drawbacks of using natural fibres are their high degree of moisture absorption and poor dimensional stability. The primary objective of surface treatments on natural fibres is to maximize the bonding strength so as the stress transferability in the composites. The overall mechanical properties of natural fibre reinforced polymer composites are highly dependent on the morphology, aspect ratio, hydrophilic tendency and dimensional stability of the fibres used. The effects of different chemical treatments on cellulosic fibres that are used as reinforcements for thermoset and thermoplastics are studied. The chemical sources for the treatments include alkali, silane, acetylation, benzoylation, acrylation and acrylonitrile grafting, maleated coupling agents, permanganate, peroxide, isocyanate, stearic acid, sodium chlorite, triazine, fatty acid derivate (oleoyl chloride) and fungal. The significance of chemically-treated natural fibres is seen through the improvement of mechanical strength and dimensional stability of resultant composites as compared with a pristine sample.     

Thermal and dynamic mechanical analysis of polystyrene composites reinforced with short sisal fibres

The thermal behaviour of polystyrene composites reinforced with short sisal fibres was studied by means of thermogravimetric and dynamic mechanical thermal analysis. The thermal stability of the composites was found to be higher than that of sisal fibre and the PS matrix. The effects of fibre loading, fibre length, fibre orientation and fibre modification on the dynamic mechanical properties of the composites were evaluated. Fibre modifications were carried out by benzoylation, polystyrene maleic anhydride coating and acetylation of the fibre and the treatments improved the fibre-matrix adhesion. PS/sisal composites are thermally more stable than unreinforced PS and sisal fibre. The addition of 10% fibre considerably increases the modulus but the increase is found to level off at higher fibre loadings. The T_g values of the composites are lower than that of unreinforced PS and may be attributed to the presence of some residual solvents in the composites entrapped during the composite preparation. The treated-fibre composites show better properties than those of untreated-fibre composites. The Arrhenius relationship has been used to calculate the activation energy of the glass transition of the composites. A master curve is constructed based on time-temperature superposition principle.     

Mechanical properties of chemically-treated hemp fibre reinforced sandwich composites

In this study, hemp fabrics were used as reinforcements with polyester resin to form composite skins while short hemp fibres with polyester as a core for making composite sandwich structures. To improve the fibre matrix adhesion properties, alkalisation, silane and acetylation treatments on the fibres surface were carried out. Examinations through fourier transform infrared (FTIR) spectroscopy, scanning electron microscope (SEM), differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were conducted to investigate the physical and thermal properties of the fibres. Mechanical properties such as flexural and compressive strengths of the sandwich structures made by treated and untreated hemp fibres were studied. Based on the results obtained from the experiments, it was found that the fibre treated with alkalic solution and post-soaked by 8% NaOH exhibited better mechanical strength as compared with other treated and untreated fibre composite samples. Besides, DSC and TGA analysis showed that the thermal stability of all treated fibre was enhanced as compared with untreated samples.     

LDPE–wood composites utilizing degraded LDPE as compatibilizer

The effect of degraded low-density polyethylene (dLDPE) as compatibilizer on the morphology and properties of low-density polyethylene (LDPE)/wood flour (WF) composites was investigated. The formation of functional groups on the degraded polyethylene chains enables the dLDPE to be used as a compatibilizer. The SEM images show smooth surfaces with fewer voids and fibre pullout for the dLDPE modified composites. The carbonyl index of the dLDPEs increased up to 7 weeks degradation, while the molecular weight decreased significantly. In the dLDPE treated composites a nucleating effect of the fibres gave rise to increased LDPE melting and crystallization enthalpies. There was no significant improvement in the thermal stability of the dLDPE treated composites. The presence of dLDPE observably influenced the viscoelastic properties and mechanical properties of the composites. It was found that the higher carbonyl index dLDPEs are more efficient compatibilizers in LDPE/WF composites, despite their significantly reduced molecular weights.     

Artichoke (Cynaracardunculus L.) fibres as potential reinforcement of composite structures

The aim of this paper is to examine the use of artichoke fibres as potential reinforcement in polymer composites. The fibres are extracted from the stem of artichoke plant, which grows in Southern Sicily. In order to use these lignocellulosic fibres as potential reinforcement in polymer composites, it is fundamental to investigate their microstructure, chemical composition and mechanical properties.Therefore, the morphology of artichoke fibres was investigated through electron microscopy, the thermal behaviour through thermogravimetric analysis and the real density through a helium pycnometer. The chemical composition of the natural fibres in terms of cellulose, lignin, and ash contents was determinated by using standard test methods.Finally, the mechanical characterization was carried out through single fibre tensile tests, analysing the results through statistical analysis.     

Morphological,thermal and mechanical characterization of okra (Abelmoschus esculentus) fibres as potential reinforcement in polymer composites

Okra technical fibres are extracted from the stem of a plant of the Malvaceae family (Abelmoschus esculentus), which is originally from Egypt, but is also cropped in Southern Asia and elsewhere for nutritional purposes. Their use as potential reinforcement in polymer composites requires the understanding of their microstructure and mechanical properties. This work investigates the morphology of the technical fibres through optical and electron microscopy and their thermal behaviour through thermogravimetric analysis. Single fibre tensile tests were performed in order to obtain their mechanical properties and the results were analyzed through a two-parameter Weibull distribution. The fracture modes of okra fibres were also addressed.     

The effects of the injection moulding temperature on the mechanical properties and morphology of polypropylene man-made cellulose fibre composites

Composites made of polypropylene and man-made cellulose fibres that are intended for injection moulding applications show potential for use in sustainable and light weight engineering with high energy absorption capacity. Due to the thermal sensitivity of the cellulose fibres, process parameters play an important role during the injection moulding process. A polypropylene and a man-made cellulose fibre were chosen for this investigation. Effective melt temperatures between 200 °C and 269 °C were used to process the compounds into test specimens. Tensile, impact and colorimetric tests, as well as an SEM analysis, and a measurement of the fibre length distribution were carried out in order to characterise the mechanical and optical properties of the composites. It was observed that the fibre length becomes shorter above 256 °C and elongation at break and Charpy strength (notched) of the composites already decrease at lower temperatures than tensile strength. A direct correlation between mechanical properties and discoloration was not observed. Therefore, melt temperatures up to 250 °C are suitable for these composites.     

Polypropylene composites with enzyme modified abaca fibre

Abaca fibre reinforced PP composites were prepared using a high speed mixer followed by injection moulding with 30 wt.% of fibre load. Prior to composite production, the fibres were modified by fungamix and natural enzyme. The effects of modification of the fibre were assessed on the basis of morphology and thermal resistance and as well as on mechanical, thermal and environmental stress corrosion resistance properties of the resulting composites. Coupling agent (MA-PP) was also used with unmodified abaca fibre to observe the coupling agent effect on resulting composites properties. The moisture absorption of the composites was found to be reduced 20–45% due to modification. Tensile strength found to be 5–45% and flexural strengths found to be 10–35% increased due to modification. Modified fibre composites found to better resistance in acid and base medium.     

Characteristics of Hermès flax fibres as a function of their location in the stem and properties of the derived unidirectional composites

The tensile mechanical properties of flax fibres from the Hermès variety are estimated according to their diameter and their location in the stems. The large scattering of these properties is ascribed to the variation of the fibre size along its longitudinal axis, as revealed by SEM observations. The higher values of the mechanical properties for the fibres issued from the middle of the stems are associated with the chemical composition of their cell walls. The mechanical properties of unidirectional flax fibre/epoxy matrix composites are studied as a function of their fibre content. The properties of the composites are lower than those expected from single fibre characteristics.     

Tailoring of the interfacial properties of polymeric single fibre-reinforced epoxy composites by non-oxidative plasma treatments

The interfacial properties of epoxy composites reinforced with a single, plasma-treated fibre of either poly(p-phenyleneterephthalamide) (PPTA) or poly(p-phenylenebenzobisoxazole) (PBO) have been investigated with a focus on evaluating the effect of two non-oxidative (He and N₂) microwave plasma treatments on interfacial adhesion properties. Tensile testing of single filaments revealed that their tensile strength does not diminish with the plasma treatments, despite the fact that their surface properties have been both physically and chemically modified. Interfacial characterisation by Raman spectroscopy indicated that the quality of adhesion was substantially enhanced following exposure of the fibres to microwave plasma treatment in either pure helium or pure nitrogen flows for just one minute. Such improvement was higher than that attained when O₂ was used for blowing the plasma, under the same operational conditions. Moreover, no swelling effect was observed by AFM after exposure of the He or N₂ plasma-treated fibres to ambient conditions for as much as 24 h.     

A review on basalt fibre and its composites

In recent years, both industrial and academic world are focussing their attention toward the development of sustainable composites, reinforced with natural fibres. In particular, among the natural fibres (i.e. animal, vegetable or mineral) that can be used as reinforcement, the basalt ones represent the most interesting for their properties. The aim of this review is to illustrate the results of research on this topical subject. In the introduction, mechanical, thermal and chemical properties of basalt fibre have been reviewed. Moreover, its main manufacturing technologies have been described. Then, the effect of using this mineral fibre as reinforcement of different matrices as polymer (both thermoplastic and thermoset), metal and concrete has been presented. Furthermore, an overview on the application of this fibre in biodegradable matrix composites and in hybrid composites has been provided. Finally, the studies on the industrial applications of basalt fibre reinforced composites have been reviewed.     

Characterisation of the water absorption,mechanical and thermal properties of recycled cellulose fibre reinforced vinyl-ester eco-nanocomposites

Natural fibres are cheaper and have a lower density compared to synthetic reinforcement products and therefore offer benefits for use in commercial applications. The physical and mechanical properties of these ‘eco-composites’ can be further enhanced through the addition of nanoclay. This paper reports on the fabrication of vinyl ester, eco-composites and eco-nanocomposites and characterises these samples in terms of water absorption, strength, toughness, and thermal properties. Weight gain and FTIR spectrum analysis indicated that 5% nanoclay addition gave favourable reduction in the water absorption behaviours of the samples. Nanoclay addition strengthened fibre–matrix adhesion leading to improved strength properties in the eco-nanocomposites. However, SEM images of fracture surfaces revealed that nanoclay addition limited toughness mechanisms of fibre pull-out and fibre debonding leading to sample brittleness. Eco-nanocomposites were still found to have favourable thermal stability and flammability results.     

Polyolefin composites with curaua fibres: Effect of the processing conditions on mechanical properties,morphology and fibres dimensions

Composites of polypropylene (PP) and high density polyethylene (HDPE) reinforced with 20 wt.% of curaua fibres were prepared using a twin-screw extruder and the effect of screw rotation speed (SRS) was evaluated by measuring the output, the mechanical properties of the composites, the morphology and the fibre dimensions. Increase in SRS causes a decrease in length, diameter and aspect ratio of the fibres in both composites, due to the high shear forces acting in the molten polymer and transferred to the fibres. Consequently, the reinforcement effect of the fibres decreased, as evidenced by the flexural and tensile mechanical properties of the composites. Additionally, polymeric matrices undergoes thermo-mechanical degradation during processing, this also contributed to the changes in the mechanical properties. Comparison between the matrices showed that PP composites are less affected by changes in SRS, suffering fewer changes in fibre dimensional parameters and in the mechanical properties than HDPE composites.     

The effect of organic and inorganic fibres on the mechanical and thermal properties of aluminate activated geopolymers

The addition of fibres to a brittle matrix is a well-known method to improve the flexural strength. However, the success of the reinforcements is dependent on the interaction between the fibre and the matrix. This paper presents the mechanical and microstructural properties of PVA and basalt fibre reinforced geopolymers. Moreover low density and thermal resistant materials used as insulating panels are known be susceptible to damage due to their poor flexural strength. As such the thermal and fire resistance properties of foamed geopolymers containing fibre reinforcement were also investigated.The results highlight that the presence of PVA fibres greatly increased the flexural strength and the toughness of the geopolymer composite, while the presence of basalt fibres improved the flexural behaviour of the composite after high temperature exposure.     

Tensile properties of chemically treated hemp fibres as reinforcement for composites

Natural fibres, unlike synthetic fibres fabricated in-house, grow naturally. Their geometrical and physical properties are highly affected by environmental issues such as climate change. For instance, inconsistent cross-sectional areas and shapes along the length of a natural fibre can result from environmental changes. These irregularities in natural fibres affect the ultimate load that can be carried by these fibres in structural engineering applications. In this study, the tensile properties of single hemp fibres were measured by taking into account, the variations in fibre diameters. Alkali, acetyl and silane treatments of fibres were carried out to obtain a better surface finish. The treatment effects on the fibres with respect to tensile properties were discussed. The relationship between tensile properties of treated fibres and the variation of their diameters was also studied. It was found that the tensile strength of chemically-treated fibres was lower than that of untreated fibres.     

Advantages of regenerated cellulose fibres as compared to flax fibres in the processability and mechanical performance of thermoset composites

Man-made cellulosic fibres (MMCFs) have attracted widespread interest as the next generation of fibre reinforced composite. However, most studies focused entirely on their performance on single fibre level and little attention has been paid to their behaviour on a larger application scale. In this study, MMCFs were utilized as reinforcement in unidirectionally (UD) manufactured thermoset composites and compared to several commercial UD flax fibre products. Specimens were prepared using a vacuum bag based resin infusion technique and the respective laminates characterized in terms of void fraction and mechanical properties. MMCF laminates had comparable or better mechanical performance when compared to flax fibre laminates. Failure mechanisms of MMCF laminates were noted to differ from those of flax-reinforced laminates. The results demonstrate the potential of MMCFs as a viable alternative to glass fibre for reinforcement on a larger scale of UD laminates. These results were utilized in the Biofore biomaterial demonstration vehicle.     

A study of the effect of surface treatments on the tensile strength of flax fibres: Part I. Application of Gaussian statistics

Natural fibres are considered to be a potential alternative to glass fibres for use in composites applications. However, although natural fibres have many advantages their compatibility with most thermoplastics is rather poor. Surface treatments, although having a negative impact on economics, may be applied to overcome the problem of interfacial adhesion. Unfortunately, natural fibres being very different materials from man-made fibres, are very prone to degradation and/or structure alteration during the application of surface treatments. This study focuses on the effect of two surface treatments (acetylation and stearation) upon the tensile strength of flax fibres. The results are discussed in terms of Gaussian statistics and it was found that the treatments did not significantly change the flax fibre tensile strength. In addition SEM examination of the fractured surfaces revealed that acetylated fibres exhibit a different mode of failure from the other fibres, suggesting that the treatment altered the bulk properties along with the surface properties.     

Green composites based on polypropylene matrix and hydrophobized spend coffee ground (SCG) powder

Green composites were prepared with polypropylene matrix and 20 wt.% spent coffee ground (SCG) powder for uses as a wood plastic composite (WPC). The effects of hydrophobic treatment with palmitoyl chloride on SCG powder is compared with conventional surface treatment based on silanization with (3-glycidyloxypropyl) trimethoxysilane and the use of a maleated copolymer compatibilizer (polypropylene-graft-maleic anhydride, PP-g-MA) in terms of mechanical properties, morphology, thermal properties and water uptake. Composites were previously mixed in a twin-screw co-rotating extruder and subsequently subjected to injection moulding. The comparative effect of the different surface treatments and or compatibilizers on mechanical performance was studied by flexural, impact tests and dynamic mechanical thermal analysis (DMTA-torsion); in addition, the stabilizing effect of SCG was revealed by differential scanning calorimetry (DSC) and thermogravymetric analysis (TGA). As one of the main drawbacks of wood plastic composites and natural fibre reinforced plastics is the moisture gain, water uptake tests were carried out in order to quantify the effectiveness of the hydrophobization process with palmitoyl chloride. Results show a slight increase in flexural modulus for composites with both untreated and treated/compatibilized SCG powder (20 wt.%). As expected, thermal stability is improved as indicated by an increase of more than 8% in the onset degradation temperature by DSC if compared to unfilled polypropylene. Fracture analysis by scanning electron microscopy (SEM) shows better particle dispersion for PP-SCG composites with hydrophobized SCG with palmitoyl chloride treatment; in addition a remarkable decrease in water uptake is observed for composites with hydrophobized SCG.     

Experimental characterization of voids in high fibre volume fraction composites processed by high injection pressure RTM

Replacing autoclave processes is a well-known industry drive in the composites community. One of the most recognized candidates for this replacement is high injection pressure resin transfer moulding (HIPRTM), because it is both an out of autoclave process and because the high processing pressures can, hypothetically, reduce the size of voids, thereby reducing void content. In order to clarify this issue, this paper presents our results on the size distribution and total void fraction of composites containing high fibre volume fractions (>60%) composites produced by HIPRTM. To substantiate this work we present a comparative study considering both autoclave and RTM at lower pressure/fibre volume fractions. Results show that HIPRTM is able to produce high fibre volume fraction parts at very low void content (<0.05%) and is comparable to autoclave results. Future work should study the mechanical properties of these laminates in order to clarify further the limits of HIPRTM.