A recent review of recycled carbon fibre reinforced polymers: Recycling methods and their mechanical and electrical performance

The fire resistance of carbon fiber reinforced polymer (CFRP) composite is a huge challenge for strengthening and repairing structures in civil engineering. In the present study, the effects of temperature exposure on the mechanical, thermal and microstructure properties of carbon fibers (~700°C) and CFRP plates (~300°C) with the bisphenol‐A epoxy matrix (E51) and hydantoin epoxy matrix (HY) were investigated. The thermal decomposition, void content, surface morphology, and internal microstructures were obtained to reveal the degradation mechanism of CFRP plates. Exposure at elevated temperatures brought about the obvious degradation of the tensile strength of carbon fiber and CFRP plate, and the final tensile strength retention rates of CFRP plates were 72.6% with the low glass transition temperature of CFRP (E51) and 72.3% with the high glass transition temperature of CFRP (HY), respectively. The allowable maximum exposure temperature was estimated to be 200°C to guarantee the desirable tensile stiffness for the CFPR plates. Oxidation of carbon fiber has an inhibition effect on the decomposition of E51 resin in CFRP (E51). By comparison, the decomposition of HY resin and oxidation of carbon fibers were independent in CFRP (HY). The increased void content of CFRP plates at elevated temperatures aggravated the debonding of fiber/resin interface and weakened the synchronous load‐bearing capacity between the fibers, which led to the decrease of mechanical properties. Finally, the rule‐of‐mixture and the elastic mechanics theory were applied to obtain the life prediction model of tensile strength of CFRP plate at elevated temperatures. According to the model, the limit exposure temperatures of tensile strength were 483°C for CFRP (E51) and 466°C for CFRP (HY), respectively.

Due to their extraordinary properties, Carbon Fiber Reinforced Polymers (CFRPs) are used in a growing number of fields (automotive, military, aircraft, aerospace, wind turbines, sport, civil infrastructure and leisure). Since the matrix in CFRPs is polymer-based, these composites have poor resistance to fire; additionally, when exposed to high temperatures, they can burn or lose their thermo-mechanical stability. Moreover, the recent huge and continuous development of CFRPs opened the question related to their disposal and total dependence on fossil resources. This thesis focussed on epoxy-based CFRPs. In more detail, commercial epoxy resins have been modified and replaced with bio-based alternatives, and short recycled carbon fibers composites have been produced. Two new bentonite-based organoclays were prepared with low cost reactants and mild reactions conditions and used to modify the flame behaviour of a commercial epoxy resin. The epoxy-modified resin flame behaviour was evaluated by cone-calorimeter and some significant improvements with just a 3 %wt loading level of organoclay were obtained. Furthermore, the possibility to recover and reuse carbon fibers by pyrolysis of CFRPs waste was studied: a validation of the recycling conditions and the treatments required to reuse recycled carbon fibers were assessed in order to obtain clean fibers and promote fiber/matrix adhesion in epoxy composites. Recycled carbon fiber were then used in a lab-scale composite manufacturing process and comparable mechanical properties for virgin and recycled short carbon fiber composites were achieved when an optimized coupled pyrolysis/oxidative process to CFRPs waste is applied. Finally, more sustainable CFRPs have been produced and characterized coupling highly bio-based epoxy systems, appropriately modified and optimized, and recycled carbon fibers. This latter work represents the first attempt aimed at replacing petroleum- BPA-based epoxy resins and high cost short virgin carbon fibers in the future CFRPs manufacturing processes.

A means of enhancing electrical and thermal conductivities of carbon fibre reinforced polymer (CFRP) composites is investigated for the purpose of reducing damage when electric current and/or heat is introduced into a CFRP structure. The addition of commercially available graphene oxide (GO) nano-flakes dispersed into an epoxy resin is studied; quantities up to 6.3 vol% are used in a vacuum infusion process with carbon fibre fabric to form CFRP laminates. Measurements of the anisotropic electrical and thermal conductivity of the laminate were conducted on CFRP specimens with and without the GO nano-flakes. It is shown that the electrical conductivity in the through-thickness direction increased markedly, reaching values up to 0.18 S/cm, when 6.3 vol% of GO was added into the epoxy, showing a threefold increase compared to the neat CFRP. Similar improvement was also found in the thermal through-thickness conductivity for the same filler content, where the laminate exhibited identical values in both transverse and through-thickness directions. However, the properties transverse to the fibres were not greatly affected by the GO addition. To assess the effect of the GO on the mechanical properties, interlaminar shear strength tests were conducted that showed that the addition of the GO significantly enhanced the through-thickness shear strength.

This paper focuses on modelling the deflection of Carbon Fibre Reinforced Polymer (CFRP) and aluminium panels when subjected to forces generated by a lightning current flowing through them. Accurate quantification of the deflection of a CFRP plate under a lightning strike is important as this may exacerbate deterioration of the CFRP material and compromise its mechanical and electrical properties. In order to predict plate deflection accurately, there are several parameters that need to be taken into account within an electromechanical model, and these include; current amplitude and injection point, CFRP sample geometry and layup, electrical conductivity, and mechanical properties (Young's modulus and Poisson's ratio). In this paper, models of a set of quasi-isotropic layers in a multi-layup CFRP plate and an aluminium plate were constructed. The simulation results of the modelled plates show that deflection increases with current amplitude as higher stresses are generated. However, the deflection decreases as the number of plies and material stiffness increase. Over the parameter range that was considered in this study, no significant change in sample deflection was seen with electrical conductivity. The results also confirmed that deflection is largest at the current injection point and that the greatest deflection occurs when the injection point is at the centre point of the sample.

Facile preparation, closed-loop recycling of multifunctional carbon fiber reinforced polymer composites

Carbon fiber reinforced polymer (CFRP) composites are used in aerospace sector due to their high specific stiffness and light weight, and are a better alternative for aluminum, Invar, and Kovar alloys. However, lower electrical and thermal conductivity of CFRP was the main restriction for extensive use in aerospace sector as compared to aluminum alloys. High aspect ratio, high mechanical stability, high electrical as well as thermal conductivity of carbon nanotubes (CNTs) makes them excellent filler material in CFRP to enhance its electrical, mechanical, and thermal properties. CNTs add synergetic effect to CFRP giving improved CNT-CFRP composites in terms of electrical and mechanical properties. Single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs) are used as filler for fabricating conductive CFRP. Selection of type of CNT (SW/MW-CNT), percentage of CNTs to be used as filler, extent of dispersion in thermosetting resin and carbon fiber layers, governs the properties of CNT-CFRP composites. Objective of the current study was to find optimum percentage of SWCNT and MWCNT to get highly electrically conductive CNT-CFRP composites with ease of manufacturing.

Carbon fiber-reinforced polyamide composites with efficient stress transfer via plasma-assisted mechanochemistry

Cost-effective rehabilitation and/or strengthening of steel structures currently demanded by the telecommunications industry and transportation departments. Rehabilitation is often required due to crosssection losses resulting from corrosion damage and strengthening may be required due to changes in the use of a structure. Current strengthening techniques, have several disadvantages including their cost, poor fatigue performance and the need for ongoing maintenance due to continued corrosion attack. The current research program makes use of new high modulus types of carbon fiber for strengthening steel structures. The research program, currently in progress, includes phases to determine the appropriate resin and adhesive for wet lay-up of carbon fiber reinforced polymer (CFRP) sheets and bonding of CFRP strips, respectively. Test results of three scaled monopoles showed significant stiffness increases prior to yield. A significant stiffness as well as ultimate strength increase was found for the first steel-concrete composite girder tested in the program. Surface preparation of the steel must be undertaken to enhance the formation of chemical bonds between the adherend surface and the adhesive. This requires a chemically active surface that is free from contaminants. The most effective means of achieving this is by grit blasting (Hollaway and Cadei, 2002). For the CFRP strip, it is usually desirable that the strip would be fabricated with a peel-ply on one or both surfaces. However, for the small amount of CFRP produced for this program it was not economical to manufacture the CFRP strips with peel plys. As such, the procedure recommended by Hollaway and Cadei (2002) was followed, whereby the strips were abraded on the side to be bonded with sandpaper and cleaned with a solvent, which was methanol in this study. 1.2.3 Previous Work Previous work has shown the effectiveness of the technique in improving the ultimate strength of steel-concrete composite girders, although little enhancement to the stiffness has been shown. Sen et al. (2001) strengthened steel-concrete composite girders that were initially loaded past the yield strength of the tension flange. Ultimate strength increases were possible, however stiffness increases were small particularly for the thinner of the two types of CFRP laminate strips studied. It was noted that even for these specimens, the increase in the elastic region of the strengthened members might allow service load increases. Tavakkolizadeh and Saadatmanesh (2003a) also noted considerable ultimate strength increases and insignificant elastic stiffness increases. Potential to increase the elastic stiffness increase by strengthening with many plys of CFRP strips was discounted, since as the number of plys increase, the efficiency for utilizing the CFRP decreased. However, for girders that simulated corrosion damage with notches of the tension flange, Tavakkolizadeh and Saadatmanesh (2003c) found that elastic stiffness increases were possible. Vatovec et al. (2002) performed tests on square tubular steel sections that were 152 mm in depth with a span of 3048 mm. After some early trials, the tubes were filled with concrete to prevent premature local buckling of the tubes. The reported increases of strength varied from 6 to 26 percent depending on the configuration and number of plys used. No meaningful difference in stiffnesses between the unstrengthened tubes and strengthened tubes could be found and it was claimed that strengthened steel elements could not develop the full ultimate tensile or compressive strength of the CFRP due to premature delamination. Current techniques for strengthening and rehabilitation of steel structures often require bolting or welding steel plates to the existing structure. Welding is often not desirable due to the poor fatigue performance of welded connections. In contrast, the fatigue performance of repairs made to cracked steel cross-girders by bonding with CFRP has been shown to be effective up to 20 million cycles (Bassetti et al. 2000). For notched tensile specimens subjected to fatigue loading, Gillespie et al. (1997) has shown that CFRP patches applied across the notch have the effect of reducing the stress concentration at the notch, thereby substantially increasing the life of the specimen due to the slower rate of crack propagation. This finding was confirmed for notched flexural specimens subjected to fatigue loading (Tavakkolizadeh and Saadatmanesh, 2003b). The durability of CFRP materials bonded to metallic surface has to be carefully considered due to the potential for galvanic corrosion to occur if three conditions are met: an electrolyte (such as salt water) must bridge the two materials, there must be electrical connection between the materials and there must be a sustained cathodic reaction on the carbon (Francis, 2000). Brown (1974) studied the corrosion of different metals connected to CFRP by adhesive bonding or bolting. For the specimens connected by adhesive bonding there was no accelerated corrosion attack. This behavior was claimed to be due to the insulating behavior of most structural adhesives in not allowing electrical contact between the two materials. Tavakkolizadeh and Saadatmanesh (2001) provided the most comprehensive study of galvanic corrosion between steel and CFRP to date. Thicker epoxy films between the steel and CFRP surfaces were shown to significantly slow the corrosion rate of steel. The proceeding study proposed placing a layer of non-conductive GFRP as an insulating layer between the steel and CFRP interface. However, Tucker and Brown (1989) have found that glass fibers placed within a carbon fiber composite result in the blistering of the composite by creating conditions favorable for the development of a strong osmotic pressure within the composite. Clearly, water being drawn within the bond line by osmotic pressure is not favorable for maintaining a durable bond. 1.3 Carbon Fiber Material The work presented in this paper makes use of two types of carbon fiber with properties given in Table 1. The high modulus carbon fiber used, was in the form of unidirectional tow sheets or CFRP laminate strips. These sheets had a width of 330 mm and are suitable when a wet lay-up process is necessary to conform to the exact surface configuration of the structure. The same fiber was also pultruded into unidirectional CFRP laminate strips using Resolution Performance Products Epon 9310 epoxy resin with Ancamine 9360 curing agent at a fiber volume content of 55 percent. These strips were expected to be more suitable for field applications where a greater degree of strengthening is required and flat uniform surfaces are available for bonding. An intermediate modulus fiber was also pultruded using the same epoxy and to the same fiber volume fraction. The properties of the CFRP strips, as determined by the manufacturer, are provided in Table 2. Table 1. Fiber properties for two types of fiber used in the ex-

Recycling of carbon fiber composites in carbon-bonded alumina refractories

The rapid increase in the application of carbon fiber reinforced polymer (CFRP) composite materials represents a challenge to waste recycling. The circular economy approach coupled with the possibility of recovering carbon fibers from CFRP waste with similar properties to virgin carbon fibers at a much lower cost and with lower energy consumption motivate the study of CFRP recycling. Mechanical recycling methods allow the obtention of chopped composite materials, while both thermal and chemical recycling methods aim towards recovering carbon fibers. This review examines the three main recycling methods, their processes, and particularities, as well as the reuse of recycled carbon fibers in the manufacture of new composite materials.

The impressive mechanical properties of carbon fibre reinforced polymer (CFRP) have stimulated research interest in the application of CFRP for the retrofitting and strengthening of ageing infrastructures. However, some obstacles such as debonding and the low shear strength of the bond, particularly at temperature about the glass transition temperature (Tg) of bond adhesive, limit the use of CFRP with steel structures. Nanofillers such as carbon nanotubes (CNTs) have the potential to improve the mechanical and thermo-mechanical properties of epoxy resin systems for this purpose. The objectives of the present work were thus threefold: (1) to understand the mechanical properties of CNTs via buckling analysis using molecular dynamics simulations, (2) to fabricate and characterize CNT reinforced epoxy composites to improve their mechanical and thermo-mechanical properties, and investigating the effect of ultrasonication energy, CNT geometry, dispersant type, and type of epoxy on the final properties, and (3) to apply CNT reinforced epoxy resins as structural adhesives in CFRP strengthened steel structures subjected to moderately elevated temperatures. The results show that the compression load capacity of short multi-walled CNTs (MWCNTs) was greater than that of long MWCNTs. It was observed that the variation of the buckling strain of short MWCNTs was inversely proportional to the number of nanotube walls. For slender MWCNTs, the buckling strains fluctuated as the number of walls increased. The strain increased for beam-like buckling mode, decreased for shell-like buckling mode and was approximately constant for shell-beam-like buckling mode. Increase in the length of MWCNT also led to a significant decrease of the buckling strain for short MWCNTs. However, chirality had no significant effect on the buckling strain of MWCNTs, nor did it alter the buckling mode of short MWCNTs. To obtain effective dispersion of CNTs in an epoxy matrix, high shear mixing, ultrasonication treatments and surfactants were found to be essential. It was shown that it is important to use a set of compatible key parameters such as sufficient sonication energy, a strong dispersant, together with the appropriate CNTs. The results show that using ductile epoxy with 3 wt.% CNT masterbatch could enhance Young’s modulus by 20%, tensile strength by 30%, flexural strength by 15%, and Tg by 34% of neat epoxy. The degree of CNT dispersion is not only a matter of the copolymer surfactant concentration; the copolymer adsorption morphology on the surface of CNTs also plays a role. Three adsorption morphologies, i.e. random, hemi-micelle, and cylindrical morphology, of BYK 9076 copolymer on the CNT surface were observed at different copolymer/CNT ratios. It was found that hemi-micelle morphology could prevent the agglomeration of CNTs when CNT concentration increased up to 8.7 mg/ml, whereas a cylindrical morphology was more efficient and stable in providing dispersion of a higher concentration of CNTs. It was found that the failure mode in both double strap joints with neat epoxy and/or CNT-epoxy was a combination of steel-adhesive interface failure, cohesive failure, epoxy-CFRP interface failure and CFRP delamination. Joints bonded with CNT-epoxy adhesive possessed an effective bond length of about 60 mm, whereas the effective bond length of joints bonded with neat epoxy was about 70 mm. Increasing the test temperature caused a transition of failure mode from the epoxy-CFRP interface to the steel-epoxy interface and to the cohesive layer in joints with neat epoxy. The cohesive failure could be avoided in the joints with CNT-epoxy. Observations from scanning electron microscopy revealed that CNTs bridge the cracks in epoxy matrix, providing a reinforcing effect. Overall, CNT-epoxy resin systems can provide a significant increase (about two-fold) in bond strength at moderately elevated temperatures compared with neat epoxy.

During Carbon Fibre Reinforced Polymers (CFRPs) manufacturing, large quantities of scrap are being produced and usually disposed to landfill or incinerated, resulting in a high environmental impact. Furthermore, CFRP parts that have been damaged or reached their end-of-life, follow the same disposal route and because of this, not only the environment is affected, but also high added-value materials, such as carbon fibres (CFs) are lost without further valorisation. Several recycling technologies have been suggested, such as pyrolysis, to retrieve the CF reinforcement from the CFRPs. However, pyrolysis produces CFs that have residual resin and pyrolytic carbon at their surface. In order to retrieve clean long fibres, oxidation treatment in high temperatures is required. The oxidation treatment, however, has a high impact on the mechanical properties of the reclaimed CFs; therefore, an optimised pyrolysis procedure of CFRPs and post-pyrolysis treatment of reclaimed fibres (rCFs) is required. In this study, CFRPs have been subjected to pyrolysis to investigate the reclamation of CF fabrics in their primal form. The temperature of 550 °C was selected as the optimum processing temperature for the investigated composites. A parametric study on the post-pyrolysis treatment was performed in order to remove the residues from the fabrics and at the same time to investigate the CFs reusability, in terms of their mechanical and surface properties.

A fast and effective numerical method to predict mechanical properties of carbon fibre reinforced polymer (CFRP) composites, even elastic properties, is complicated due to the mismatch of mechanical properties among the constituents. Furthermore, it is not possible to completely characterise the influence of multiple parameters including mechanical and structural parameters on the bulk properties of CFRP by experiments. In this study, a three-phase finite-element model consisting of matrix, carbon fibre and interface was developed to predict the elastic mechanical behaviour of unidirectional CFRP. The elastic properties in terms of two Young's moduli, two Poisson's ratios and a shear modulus were calculated by means of a homogenisation method. High-accuracy Kriging surrogate models were constructed to fast-calculate the elastic responses for a large number of samples. Combining Kriging and high-dimensional model representation (HDMR) methods, a global sensitivity analysis was performed to study how the microscopic parameters influence the elastic responses to get a deeper understanding of elastic property-structure relationship. Eleven parameters, including mechanical and geometry properties of constituent phases, were chosen as inputs. Independent and cooperative effects of input parameters on the elastic properties of the studied composites were surveyed via first- and second-order sensitivity indices, respectively. An importance ranking of these parameters for each elastic response was derived directly by these indices. The procedure proposed in this work could serve as a theoretical guide for further design optimisation of CFRP.

Effect of EPD coated silanized graphene oxide on carbon fiber reinforced plastic: An emphasis on mechanical properties at cryogenic temperatures

Carbon fibre reinforced polymers (CRFP) are extensively used in many industrial applications thanks to its mechanical properties and its low weight. Nevertheless, the orthotropic character of CRFP highly reduces its applications. The transversal electrical conductivity in CRFP is two orders poorer than in the longitudinal direction. To improve their electrical properties, this work proposes the use of polycrystalline boron doped diamond (BDD) as coating of the carbon fibres (CF). BDD coating is deposited on CF surface using microwave plasma enhanced chemical vapor deposition (MPCVD) system. The BDD coating forms a rigid conductive coating around the CF as a core–shell structure. Here, an electrical characterization of both, 12,000 filaments (a tow) and a single coated filament, are carried out in the longitudinal and cross-section directions. Macro, micro and local analysis using the Kelvin method, Conductive Atomic Force Microscopy (C-AFM), and Scanning Microwave Impedance Microscopy (sMIM), were carried out to evidence the improvement of the electrical properties. Macro measurement reveals that the BDD coating decreases to half the resistivity of the CF. The BDD coating raises the local electrical conductivity of the CF by an order of magnitude with respect to the uncoated ones. sMIM maps identified BDD locations in ring-like configurations.