Counting carbon fibres by electrical resistance measurement

Electrical Impedance Measurement has been used to measure the diameter of single carbon fibres to within 3% of the actual value measured by Scanning Electron Microscopy (SEM). The precision of the technique developed also allows for the accurate determination of the number of fibres present in a carbon fibre bundle, such data are important for the calculation of fibre tensile strength from the tensile force applied to carbon fibre bundles. The impedance of a single carbon fibre and carbon fibre bundles of up to 20 fibres have been measured, with results showing good agreement with theoretical values. The impedance of multiple lengths of carbon fibres ranging from 80 to 300 mm has also been studied, with the impedance being directly proportional to the fibre length, as per electrical theory. This technique will be suitable for determining the number of fibres in a virgin or recycled carbon fibre bundle.     

Evaluating the mechanical properties of reinforced LDPE composites made with carbon fibres recovered via solvothermal processing

Carbon fibre was recovered from a thermoset composite via a solvo-thermal process and used as reinforcement in low density polyethylene (LDPE). The oxidized recovered carbon fibres have shown better properties than original non-oxidized fibres. The best interactions between the continuous and dispersed phases were found using 3-aminopropyl-trimetoxysilane and experimentally synthesized polyalkenyl-polymaleic anhydride based polymers. The tensile strength of the prepared composites nearly doubled when 3-aminopropyl-trimetoxysilane was used as compatibilizer, in comparison to the composites prepared without additives. Based on infrared analysis, a chemical reaction has been proposed between –COOH groups of compatibilizers and the –OH groups of the carbon fibre surface for the best composites.     

The effect of recycling on the mechanical response of carbon fibres and their composites

The development of processes for recycling carbon–fibre composite waste rises a question yet to be answered: how good can the performance of recycled composites be? This paper analyses fibres reclaimed in commercial facilities, and compares the performance of subsequently manufactured recycled woven composites to that of the virgin precursor (with the same fibre architecture). Different pyrolysis cycles resulted into different compromises between complete resin removal and full fibre strength retention. At the composite level, this paper shows how strength varies with the reclamation cycle, re-impregnation process and loading direction, while stiffness remains virtually unaffected. It is shown that composite tensile strength is favoured by gentle pyrolysis cycles generating little fibre damage, while compressive strength is fully retained after more aggressive cycles which completely remove the matrix. This work proves that the mechanical response of recycled composites can rival that of virgin precursors, while highlighting the benefits of application-driven optimisation of reclamation processes.     

Effects of functional group of carbon nanotubes on mechanical properties of carbon fibers

The effects of surface functionalization of carbon nanotubes (CNTs) on the mechanical properties of carbon fibers (CFs) have been investigated. The surface functionalization of CNTs was carried out with a diazonium reagent. Compared to pure PAN, only the fluoro phenyl functionalized CNTs (F-Ph-CNT) incorporated PAN composites showed a significant increase up to 22 °C of T_g and displayed the second peak due to the interfacial interaction between F-Ph-CNT and PAN. Among the samples, 0.5wt% of F-Ph-CNT reinforced CFs exhibited a 46% increase in tensile strength (4.1 GPa) and a 37% increase in modulus (302 GPa), respectively compared to that of pure CFs.     

Effect of coupling agents on reinforcing potential of recycled carbon fibre for polypropylene composite

Polypropylene (PP) composites reinforced with recycled carbon fibre have been prepared through extrusion compounding and injection moulding. The reinforcing potential of the recycled fibre was increased by improving the interfacial adhesion between the fibre and PP matrix and this was done by the addition of maleic anhydride grafted polypropylene (MAPP) coupling agents. Three MAPP couplers with different molecular weights and maleic anhydride contents were considered. The effects on the mechanical properties of the composite were studied, and scanning electron microscopy (SEM) was used to study the fracture morphology of the tensile specimens. It was observed that with the addition of MAPP the interfacial adhesion was improved as fewer fibres were pulled-out and less debonding was seen. A microbond test was performed and a significant improvement in interfacial shear strength was measured. This resulted in composites with higher tensile and flexural strengths. The maximum strength was achieved from MAPP with the highest molecular weight. Increased modulus was also achieved with certain grades of MAPP. It was also found that the composite impact strength was improved significantly by MAPP, due to a higher compatibility between the fibre and matrix, which reduced crack initiation and propagation.     

Tribological behaviour and wear of carbon nanotubes grafted on carbon fibres

Carbon nanotubes (CNTs) grafted on fibres are widely used to reinforce composites in order to improve their mechanical properties. This study concerned the tribological properties of CNTs grafted on carbon fibres by the flame method. The aim of this study was to determine whether CNTs on fibres suffer damage under stress, similar to those applied during composite manufacturing, which can damage composite properties, particularly fibre/matrix adhesion. For this purpose, reciprocating friction tests were performed to examine the resistance of CNTs and highlight a wear mechanism. The results showed that the presence of CNTs increased the coefficient of friction in the first friction cycles and then decreased it to close to the COF of the fibre without CNTs. The wear mechanism showed that after a small number of friction cycles, the CNTs were flattened out and formed a transfer film.     

Electrostatic self-assembled carbon nanotube/nano carbon black composite fillers reinforced cement-based materials with multifunctionality

Electrostatic self-assembled carbon nanotube (CNT)/nano carbon black (NCB) composite fillers are added into cement mortar to fabricate smart cement-based materials. The grape bunch structure of CNT/NCB composite fillers is beneficial for dispersing CNT/NCB in cement mortar matrix and achieving cooperative improvement effect. The mechanical, electrically conductive, and piezoresistive behaviors of the cement mortar are investigated. The CNT/NCB composite fillers can effectively enhance the flexural strength and electrical conductivity of cement mortars, and endow stable and sensitive piezoresistivity to cement mortar at a low filler content. However, they weaken the compressive strength of cement mortar to some extent. The percolation threshold zone of cement mortar with CNT/NCB composite fillers ranges in the amount of 0.39–1.52 vol.%. The optimal content of CNT/NCB composite fillers is 2.40 vol.% for piezoresistivity and the stress and strain sensitivities can reach 2.69% MPa⁻¹ and 704, respectively.     

Investigation of carbon nanotube reinforced aluminum matrix composite materials

We have increased the tensile strength without compromising the elongation of aluminum (Al)–carbon nanotube (CNT) composite by a combination of spark plasma sintering followed by hot-extrusion processes. From the microstructural viewpoint, the average thickness of the boundary layer with relatively low CNT incorporation has been observed by optical, field-emission scanning electron, and high-resolution transmission electron microscopies. Significantly, the Al–CNT composite showed no decrease in elongation despite highly enhanced tensile strength compared to that of pure Al. We believe that the presence of CNTs in the boundary layer affects the mechanical properties, which leads to well-aligned CNTs in the extrusion direction as well as effective stress transfer between the Al matrix and the CNTs due to the generation of aluminum carbide.     

Impact of electrochemical cycling on the tensile properties of carbon fibres for structural lithium-ion composite batteries

Carbon fibres are particularly well suited for use in a multifunctional lightweight design of a structural composite material able to store energy as a lithium-ion battery. The fibres will in this case act as both a high performance structural reinforcement and one of the battery electrodes. However, the electrochemical cycling consists of insertions and extractions of lithium ions in the microstructure of carbon fibres and its impact on the mechanical performance is unknown. This study investigates the changes in the tensile properties of carbon fibres after they have been subjected to a number of electrochemical cycles. Consistent carbon fibre specimens were manufactured with polyacrylonitrile-based carbon fibres. Sized T800H and desized IMS65 were selected for their mechanical properties and electrochemical capacities. At the first lithiation the ultimate tensile strength of the fibres was reduced of about 20% but after the first delithiation some strength was recovered. The losses and recoveries of strength remained unchanged with the number of cycles as long as the cell capacity remained reversible. Losses in the cell capacity after 1000 cycles were measured together with smaller losses in the tensile strength of the lithiated fibres. These results show that electrochemical cycling does not degrade the tensile properties which seem to depend on the amount of lithium ions inserted and extracted. Both fibre grades exhibited the same trends of results. The tensile stiffness was not affected by the cycling. Field emission scanning electron microscope images taken after electrochemical cycling did not show any obvious damage of the outer surface of the fibres.     

Properties of composites of carbon nanotube fibres

Composites have set the standard for high strength materials for several decades. With the discovery of nanotubes, new possibilities for reinforced composites have arisen, with potential mechanical properties superior to those of currently available materials. This paper reports the properties of epoxy matrix reinforced with fibres of carbon nanotubes (CNTs) which, in many ways, are similar to standard composites reinforced with commercial fibres. The composites were formed by the back diffusion of the uncured epoxy into an array of aligned fibres of CNTs. The fibre density and volume fraction were measured from thermogravimetric analysis (TGA). Properties in tension and compression were measured, and the level of fibre–matrix interaction analysed fractographically. The results show the significant potential for this route to CNT reinforcement.     

Epoxy composites containing CFRP powder wastes

The increasing utilisation of carbon materials increases the waste generation. Therefore, it is necessary to analyse recycling alternatives. In this research, carbon powder wastes obtained from the cutting process of laminate composites have been incorporated into epoxy matrix phase in order to improve the mechanical characteristics. Physical and mechanical properties, hardness, abrasion, erosion and thermal behaviour have been analysed. Results show that carbon powder wastes incorporated to new epoxy matrix phases act basically as reinforcement. This allows for the recycling of the residues as well as improves some properties of the composites.     

Regenerating the strength of thermally recycled glass fibres using hot sodium hydroxide

Results are presented from the ReCoVeR project on the regeneration of the strength of thermally conditioned glass fibres. Thermal recycling of end-of-life glass fibre reinforced composites or composite manufacturing waste delivers fibres with virtually no residual strength or value. Composites produced from such fibres also have extremely poor mechanical performance. Data is presented showing that a short hot sodium hydroxide solution treatment of such recycled fibres can more than triple their strength and restore their ability to act as an effective reinforcement in second life composite materials. The implications of these results for real materials reuse of recycled glass fibres as replacement for pristine reinforcement fibres are discussed.     

Effect of superheated steam treatment of carbon fiber on interfacial adhesion to epoxy resin

To elucidate the effect of superheated steam (SHS) treatment of carbon fiber on the adhesion to epoxy resin and surface states, virgin unsized carbon fiber was exposed to SHS with or without N₂ in the temperature range of 500–800 °C. The interfacial shear strength (IFSS) between the carbon fiber and epoxy resin was successfully improved by SHS treatment with N₂, and the IFSS of fiber treated above 700 °C was the same as or higher than that of a commercial sized fiber. SHS treatment without N₂ resulted in an increase of total acidic groups on the fiber surface accompanied with the increase of phenolic hydroxyl groups, whereas that with N₂ resulted in a simultaneous increase of total acidic and basic functional groups. The significant improvement in the IFSS after SHS treatment with N₂ is considered to be due to the increase of basicity on the fiber surface.     

Preparation and characterization of poly(3-octylthiophene)/carbon fiber thermoelectric composite materials

Poly(3-alkylthiophene) (P3AT) with a high Seebeck coefficient has recently been reported. However, P3AT/inorganic conductive composites exhibit relatively poor thermoelectric performance because of their low electrical conductivity. In this work, carbon fiber sheets with a high electrical conductivity were chosen as the inorganic phase, and poly(3-octylthiophene)(P3OT)/carbon fiber composites were prepared by casting P3OT solution onto the carbon fiber sheets. The carbon fiber sheets incorporated into the composites can provide good electrical conductivity, and P3OT can provide a high Seebeck coefficient. The highest power factor of 7.05 μW m⁻¹ K⁻² was obtained for the composite with 50 wt% P3OT. This work suggests a promising method for preparing large-scale thermoelectric composites with excellent properties.     

Recycled carbon fibre-reinforced polypropylene thermoplastic composites

Comingled carbon fibre (CF)/polypropylene (PP) yarns were produced from chopped recycled carbon fibres (reCF) (20 mm in length, 7-8 μm diameter) blended with matrix polypropylene staple fibres (60 mm in length, 28 μm diameter) using a modified carding and wrap spinning process. Microscopic analysis showed that more than 90% of the reCF were aligned along the yarn axis. Thermoplastic composite test specimens fabricated from the wrap-spun yarns had 15-27.7% reCF volume content. Similar to the yarn, greater than 90% of the reCF comprising each composite sample made, showed a parallel alignment with the axis of the test specimens. The average values obtained for tensile, and flexural strengths were 160 MPa and 154 MPa, respectively for composite specimens containing 27.7% reCF by volume. It was concluded that with such mechanical properties, thermoplastic composites made from recycled CF could be used as low cost materials for many non-structural applications.     

Design of composite materials with improved impact properties

Composites have been widely used in applications where there is a risk of impact, due to the excellent properties these materials display for absorbing impact energy. However, composites during impact situations typically generate an enormous number of small pieces, due to the energy absorption mechanism of these materials, a mechanism which does not include plastic deformation. This can prove dangerous in sports competitions, where the small fragments of the original structure may harm competitors.This study was designed to explore the possibility of incorporating a material which, whilst maintaining a high level of energy absorption without any plastic deformation mechanism, was able to maintain its original form, or at least significantly reduce the number of pieces generated after impact.The addition of a polyamide layer, NOMEX^®, to a monolithic fabric laminate was investigated in this paper. The process of fabrication is described and the different properties of the material under consideration: interlaminar fracture toughness energy (G_IC), indentation (id) and delamination after impact (A_i) and compression after impact (σ_CAI), were measured and compared with those of the original monolithic fabric.     

Enhancement of carbon nanotube fibres using different solvents and polymers

Liquid infiltration is an efficient way to densify carbon nanotubes (CNTs) and was used to strengthen CNT fibres in the method of array spinning. Rather than the volatility, the dipole moment of solvent plays a more important role in determining the densification level. The fibres densified by highly polar but non-volatile solvents such as N,N-dimethylformamide, dimethyl sulphoxide, and N-methyl-2-pyrrolidone were 100–200 MPa stronger than those by ethanol and acetone. Ethylene glycol is the most efficient solvent due to its two polar OH groups and improved the fibre strength to 1.45 GPa. Long chain or cross-linked polymers like polyvinyl alcohol, polyimide, and bismaleimide (BMI) were introduced into CNT fibres by infiltration with aid of polar solvents. These polymers reinforced the fibres significantly, as they can connect non-neighboring CNTs and benefit the load transfer. The strongest CNT/BMI fibre was 2.38 GPa in strength and 110 GPa in modulus.     

Mechanical studies of single glass fibres recycled from hydrolysis process using sub-critical water

The study reports mechanical performance of the recycled glass fibres produced from a water-based solvolysis technology, known as the hydrolysis process. The chemical reaction was carried out using sub-critical water to dissolve polyester resin and recover the glass fibres from composites. The effect of temperatures, times, catalyst and water amount on mechanical properties of the recovered glass fibres were investigated. Scanning electron microscopy (SEM), energy dispersive X-ray (EDX) and time-of flight secondary ion mass spectroscopy (ToF-SIMS) analyses were also employed to examine the fibre surface associated with the polyester resin eliminated level after the hydrolysis reaction. The results revealed that by carefully adjusting the hydrolysis parameters the tensile strength and failure strain of the recycled fibres decrease by approximately 40-70% in comparison with virgin fibres while Young’s moduli remain similar. The relationship between the hydrolysis conditions, recovered fibres and mechanical performance was discussed in this study.     

Carbon fiber extraction from waste CFRP by microwave irradiation

The present paper proposes an effective method to extract carbon fibers from waste CFRPs with low energy consumption and low processing time. Carbon fibers were extracted from waste CFRPs by irradiating microwaves under different atmospheres. The effect of the atmosphere and field intensity of irradiated microwaves on the efficiency of extraction of carbon fibers was investigated. The mechanism of extraction through microwave irradiation was also studied. Finally, the tensile strength of extracted carbon fibers was investigated and compared with that of carbon fibers extracted using conventional methods. Test results showed that the carbon-fiber extraction through microwave irradiation can be considered to occur in three stages. First, the carbon fibers in CFRP were heated through the antenna effect by microwave irradiation. Then, the gasification of resin was promoted by the heated carbon fibers. Finally, the gasified resin was decomposed by spark glow plasma between carbon fibers.     

Assessment of carbon nanotube yarns as reinforcement for composite overwrapped pressure vessels

Carbon nanotubes (CNTs) are one-dimensional nanomaterials with outstanding electrical and thermal conductivities and mechanical properties. Recent advances in CNT manufacturing have made bulk forms such as yarns, tapes and sheets available in commercial quantities to permit the evaluation of these materials for aerospace use. The high tensile properties of CNT composites can be exploited in tension-dominated applications such as composite overwrapped pressure vessels (COPVs). To investigate their utility in this application, aluminum (Al) rings were overwrapped with thermoset/CNT yarn, thermally cured under a vacuum bag, and their mechanical properties measured. Fabrication parameters such as CNT/resin ratio, tension applied during winding, and the number of CNT yarn layers were investigated to determine their effects on the mechanical performance of overwrapped Al rings. Mechanical properties of the CNT composite overwrapped Al rings (CCOARs) were measured under static and cyclic loads at room, elevated, and cryogenic temperatures to evaluate their performance relative to bare Al rings. The ultimate load carried by the composite overwrap in the CCOARs increased with increasing number of wraps. The wet winding process for the CCOAR fabrication improved load transfer compared to the dry winding process due to enhanced interfacial adhesion between the CNT yarn and the applied resin. Wet winding Al rings with CNT yarn/thermoset overwraps resulted in ∼11% increase in weight relative to the bare ring and increased the room temperature breaking load by over 200%.