Search

Diglycidylether of vanillyl alcohol (DGEVA), in combination with mechanically recycled carbon fibers (RCFs), was used to make, via Radical-Induced Cationic Frontal Photopolymerization (RICFP), fully sustainable and bio-based conductive composites with good electrical conductivity and consequent Joule effect proprieties. Three different fiber lengths, using three different sieve sizes during the mechanical recycling process (0.2, 0.5, and 2.0 mm), were used in five different amounts (ranging from 1 to 25 phr). The samples were first characterized by dynamic mechanical thermal analysis (DMTA), followed byelectrical conductivity and Joule heating tests. More specifically, the mechanical properties of the composites increased when increasing fiber content. Furthermore, the composites obtained with the longest fibers showed the highest electrical conductivity, reaching a maximum of 11 S/m, due to their higher aspect ratio. In this context, the temperature reached by Joule effect was directly related to the electrical conductivity, and was able to reach an average and maximum temperatures of 80 °C and 120 °C, respectively, just by applying 6 V.

Graphene nanoplatelets (GNPs)/epoxy composites have been fabricated via gravity molding. The electrical and thermal properties of the composites have been studied with variable GNP type (C300, C500, and C750, whose surface areas are ~300, 500, and 750 m2/g, respectively), GNP loading (5, 10, 12, and 15 wt.%), and dispersion time via ultrasonication (0, 30, 60, and 120 min). By increasing the time of sonication of the GNP into the epoxy matrix, the electrical conductivity decreases, which is an effect of GNP fragmentation. The best results were observed with 10–12% loading and a higher surface area (C750), as they provide higher electrical conductivity, thereby preserving thermal conductivity. The influence of sonication over electrical conductivity was further analyzed via the study of the composite morphology by means of Raman spectroscopy and X-ray diffraction (XRD), providing information about the aspect ratio of GNPs. Moreover, electromagnetic shielding (EMI) has been studied up to 4 GHz. Composites with C750 and 120 min ultrasonication show the best performance in EMI shielding, influenced by their higher electrical conductivity.

The present study focuses on the multifunctional capabilities of carbon nanotube (CNT)-reinforced vitrimers. More specifically, the thermomechanical properties, the Joule effect heating capabilities, the electrical conductivity, the shape memory, and the chemical recycling capacity are explored as a function of the CNT content and the NH2/epoxy ratio. It is observed that the electrical conductivity increases with the CNT content due to a higher number of electrical pathways, while the effect of the NH2/epoxy ratio is not as prevalent. Moreover, the Tg of the material decreases when increasing the NH2/epoxy ratio due to the lower cross-link density, whereas the effect of the CNTs is more complex, in some cases promoting a steric hindrance. The results of Joule heating tests prove the suitability of the proposed materials for resistive heating, reaching average temperatures above 200 °C when applying 100 V for the most electrically conductive samples. Shape memory behavior shows an outstanding shape fixity ratio in every case (around 100%) and a higher shape recovery ratio (95% for the best-tested condition) when decreasing the NH2/epoxy ratio and increasing the CNT content, as both hinder the rearrangement of the dynamic bonds. Finally, the results of the recyclability tests show the ability to regain the nanoreinforcement for their further use. Therefore, from a multifunctional analysis, it can be stated that the proposed materials present promising properties for a wide range of applications, such as Anti-icing and De-icing Systems (ADIS), Joule heating devices for comfort or thermotherapy, or self-deployable structures, among others.

The Structural Health Monitoring (SHM) capabilities of a well-studied self-healing epoxy resin based on disulfide bonds, through the addition of carbon nanotubes (CNTs), are studied. Since these materials demonstrated, in recent works, a high dependency of the dynamic hardener content on the repair performance, this study aimed to analyze the effect of the vitrimeric chemistry on the electromechanical properties by studying different 2-aminophenyl disulfide (2-AFD) hardener and CNT contents. The electrical conductivity increases with both the CNT and AFD contents, in general. Moreover, an excess of AFD close to the stoichiometric ratio with a low CNT content improved the tensile strength by 45%, while higher AFD contents promoted its detriment by 41% due to a reduced crosslinking density. However, no significant difference in the mechanical properties was observed at a higher CNT content, regardless of the AFD ratio. The developed materials demonstrate a robust electromechanical response at quasi-static conditions. The sensitivity significantly increases at higher AFD ratios, from 0.69 to 2.22 for the 0.2 wt.%. CNT system, which is advantageous due to the enhanced repair performance of these vitrimeric materials with a higher hardener content. These results reveal the potential use of self-healing vitrimers as integrated SHM systems capable of detecting damages and self-repairing autonomously.

The use of polymer and polymer-based composites has increased over the last decades [...]

Self-healing materials have attracted great interest in recent years. Particularly, the use of thermoset/thermoplastics blends has emerged as a good option with relatively low activation temperatures and potential infinite healing cycles. Nevertheless, a methodical study of healing conditions and evaluation is still required for further industrial development. The effect of activation temperature and crack morphology in polycaprolactone (PCL)/epoxy blends are explored. For this purpose, PCL content was varied (5, 10, and 15 wt %) with contents lower than critical composition. Therefore, the morphology of all studied blends is the epoxy matrix with a separated PCL phase. In this sense, an increase in PCL content leads to a reduction in the Tg, due to the partial PCL miscibility, and the presence of larger PCL domains. It was observed that a higher temperature (150 °C) and PCL content led to a more efficient self-healing process because of both the lower viscosity of the melted PCL at higher temperatures and the presence of larger PCL reservoirs when increasing the PCL content. Crack morphology influence was studied by inducing cracks with different tools: a custom crack machine with a cutting blade, a scalpel, and a pin. The results show that the recovery was better when the cracks were smaller and shallower, that is, with the pin. In addition, the healing efficiency by means of both parameters, crack volume and depth change, showed more similar results in slimmer cracks, due to a lower crack width-to-depth ratio.

The influence of the average surface area of different graphene nanoplatelets (GNP) on the thermo-electrical behaviour, associated with Joule heating, and the attenuation of electromagnetic signals of epoxy composites has been studied, analysing the effect of the morphology obtained as a function of the dispersion time by ultrasonication and the GNP content added. Gravity moulding was used as the first stage in the scaling-up, oriented to the industrial manufacture of multilayer coatings, observing a preferential self-orientation of nanoparticles and, in several conditions, a self-stratification too. The increase of sonication time during the GNP dispersion provides a decrease in the electrical conductivity, due to the GNP fragmentation. Instead, the thermal conductivity is enhanced due to the higher homogeneous distribution of GNPs into the epoxy matrix. Finally, the lower surface area of GNPs reduces the thermal and electrical conductivity due to a greater separation between nanosheets. Regarding the study of the attenuation of electromagnetic waves, it has been discovered that in the frequency range from 100 Hz to 20 MHz, this attenuation is independent of the direction of analysis, the type of graphene, the sonication time, and the state of dispersion of the nano-reinforcement in the matrix. Furthermore, it has also been observed that the conservation of the constant shielding values for the three types of GNPs are in a range of average frequencies between 0.3 and 3 MHz.

The present work studies the electrical and electrothermal properties of CNT/GNP-doped nanocomposites for optimizing their anti-icing and de-icing capabilities. Here, a comparison between 3D-printed circuits and coatings based on these materials is carried out. In this regard, the higher electrical conductivity that is achieved by the specimens when increasing the nanoparticle content and the higher cross-sectional area of the coatings with regard to the 3D-printed circuits induces a higher heat generated by the Joule’s effect. Moreover, the successful de-icing test performed by the specimen with the highest self-heating capability, evinces that the studied nanocomposites are suitable for de-icing purposes.

This paper presents a process where carbon fibers and hydrogen can be recovered simultaneously through a two-stage thermal treatment of an epoxy-carbon fiber composite. For this purpose, some pieces of epoxy resin reinforced with carbon fiber fabrics have been fabricated and, after curing, have been pyrolyzed in an installation consisting of two reactors. In the first one, the thermal decomposition of the resin takes place, and in the second one, the gases and vapors coming from the first reactor are thermally treated. Once this process is completed, the solid generated is oxidized with air to eliminate the resin residues and carbonaceous products from the fibers surface. The recovered carbon fiber fabrics have been reused to make new cured parts and their electrical and mechanical properties have been measured. The results show that it is possible to obtain carbon fiber fabrics that can be processed as they leave the recycling process and that retain 80% of the tensile modulus, 70% of the flexural strength, and 50% of the interlaminar shear strength. At the same time, a gaseous stream with more than 66% by volume of hydrogen can be obtained, reaching a maximum of 81.7%.

In this paper, a novel carbon nanotube (CNT) polycaprolactone (PCL), epoxy, and glass fiber (GF) composite is reported. Here, the nanoreinforced composites show a flexural strength increase of around 30%, whereas the interlaminar shear strength increases by 10–15% in comparison to unenhanced samples. This occurs because the addition of the CNTs induces a better PCL/epoxy/GF interaction. Furthermore, the nanoparticles also give novel functionalities to the multiscale composite, such as strain and damage monitoring. Here, the electrical response of the tensile- and compressive-subjected faces was simultaneously measured during flexural tests as well as the transverse conductivity in interlaminar tests, showing an exceptional capability for damage detection. Moreover, it was observed that the electrical sensitivity increases with PCL content due to a higher efficiency of the dispersion process that promotes the creation of a more uniform electrical network.

This work consists of the development of an easy strategy to transform any structure into an efficient surface heater by the application of a low voltage over 3D printed nanocomposite circuits. To this end, the electrical conductivity and self-heating capabilities of UV-Assisted Direct Write 3D printed circuits doped with carbon nanotubes were widely explored as a function of the number of printed layers. Moreover, an optimization of the printing process was carried out by comparing the accuracy and printability obtained when printing with two different configurations: extruding and curing the ink in the same stage or curing the extruded ink in a second stage, after the whole layer was deposited. In this regard, the great homogeneity and repeatability of the heating showed by the four-layer printed circuits, together with their excellent performance for long heating times, proved their applicability to convert any structure to a surface heater. Finally, the deicing capability of the four-layer circuit was demonstrated, being able to remove a 2.5 mm thick ice layer in 4 min and 4 s.

Electrical and electromechanical properties of hybrid graphene nanoplatelet (GNP)/carbon nanotube (CNT)-reinforced composites were analyzed under two different sonication conditions. The electrical conductivity increases with increasing nanofiller content, while the optimum sonication time decreases in a low viscosity media. Therefore, for samples with a higher concentration of GNPs, an increase of sonication time of the hybrid GNP/CNT mixture generally leads to an enhancement of the electrical conductivity, up to values of 3 S/m. This means that the optimum sonication process to achieve the best performances is reached in the longest times. Strain sensing tests show a higher prevalence of GNPs at samples with a high GNP/CNT ratio, reaching gauge factors of around 10, with an exponential behavior of electrical resistance with applied strain, whereas samples with lower GNP/CNT ratio have a more linear response owing to a higher prevalence of CNT tunneling transport mechanisms, with gauge factors of around 3–4.

Electromechanical sensing devices, based on resins doped with carbon nanotubes, were developed by digital light processing (DLP) 3D printing technology in order to increase design freedom and identify new future and innovative applications. The analysis of electromechanical properties was carried out on specific sensors manufactured by DLP 3D printing technology with complex geometries: a spring, a three-column device and a footstep-sensing platform based on the three-column device. All of them show a great sensitivity of the measured electrical resistance to the applied load and high cyclic reproducibility, demonstrating their versatility and applicability to be implemented in numerous items in our daily lives or in industrial devices. Different types of carbon nanotubes—single-walled, double-walled and multi-walled CNTs (SWCNTs, DWCNTs, MWCNTs)—were used to evaluate the effect of their morphology on electrical and electromechanical performance. SWCNT- and DWCNT-doped nanocomposites presented a higher Tg compared with MWCNT-doped nanocomposites due to a lower UV light shielding effect. This phenomenon also justifies the decrease of nanocomposite Tg with the increase of CNT content in every case. The electromechanical analysis reveals that SWCNT- and DWCNT-doped nanocomposites show a higher electromechanical performance than nanocomposites doped with MWCNTs, with a slight increment of strain sensitivity in tensile conditions, but also a significant strain sensitivity gain at bending conditions.

A performance mapping of GNP/epoxy composites was developed according to their electromechanical and electrothermal properties for applications as strain sensors and Joule heaters. To achieve this purpose, a deep theoretical and experimental study of the thermal and electrical conductivity of nanocomposites has been carried out, determining the influence of both nanofiller content and sonication time. Concerning dispersion procedure, at lower contents, higher sonication times induce a decrease of thermal and electrical conductivity due to a more prevalent GNP breakage effect. However, at higher GNP contents, sonication time implies an enhancement of both electrical and thermal properties due to a prevalence of exfoliating mechanisms. Strain monitoring tests indicate that electrical sensitivity increases in an opposite way than electrical conductivity, due to a higher prevalence of tunneling mechanisms, with the 5 wt.% specimens being those with the best results. Moreover, Joule heating tests showed the dominant role of electrical mechanisms on the effectiveness of resistive heating, with the 8 wt.% GNP samples being those with the best capabilities. By taking the different functionalities into account, it can be concluded that 5 wt.% samples with 1 h sonication time are the most balanced for electrothermal applications, as shown in a radar chart.

The use of graphene nanoplatelets for several purposes such as barrier properties and structural health monitoring is widely studied in polymer-based coating technology. Nevertheless, their extremely good effect on wear resistance makes them particularly interesting for coating technology. The effect of graphene nanoplatelets addition on the wear behaviour of an epoxy resin was analysed. The effects of graphene nanoplatelets content (0–8 wt.%), morphology and functionalization in wear behaviour were analysed by evaluating the main wear parameters of the nanocomposites studied. The morphology of the wear surfaces was analysed by scanning electron microscopy (SEM) and 3D optical profilometry in order to evaluate the mechanism and severity of wear. The results showed that the wear behaviour of the epoxy resin improved considerably with the addition of graphene, especially for higher contents. Moreover, as the wear mechanism depends on the morphology of graphene nanoplatelets, it was concluded that the proper selection of graphene can be made depending on the chosen application.

Polycaprolactone (PCL) is being researched as a self-healing agent blended with epoxy resins by several reasons: low melting point, differential expansive bleeding (DBE) of PCL, and reaction induced phase separation (RIPS) of PCL/epoxy blends. In this work, PCL/epoxy blends were prepared with different PCL ratios and two different epoxy networks, cured with aliphatic and aromatic amine hardeners. The curing kinetic affects to the blend morphology, varying its critical composition. The self-healing behavior is strongly affected by the blend morphology, reaching the maximum efficiency for co-continuous phases. Blends with dispersed PCL phase into epoxy matrix can also show high self-healing efficiency because of the low PCL domains that act as reservoir of self-healing agent. In this last case, it was confirmed that the most efficient self-healable blends are one whose area occupied by PCL phase is the largest. These blends remain the good thermal and mechanical behavior of epoxy matrix, in contrast to the worsened properties of blends with bicontinuous morphology. In this work, the self-healing mechanism of blends is studied in depth by scanning electron microscopy. Furthermore, the influence of the geometry of the initial surface damage is also evaluated, affecting to the measurement of self-healing efficiency.

Mechanical and strain sensing capabilities of carbon nanotube (CNT) reinforced composites manufactured by digital light processing (DLP) 3D printing technology have been studied. Both CNT content and a post-curing treatment effects have been analyzed. It has been observed that post-curing treatment has a significant influence on mechanical properties, with an increase of Young’s modulus and glass transition temperature whereas their effect in electrical properties is not so important. Furthermore, the strain sensing tests show a linear response of electrical resistance with applied strain, with higher values of sensitivity when decreasing CNT content due to a higher interparticle distance. Moreover, the electrical sensitivity of bending tests is significantly lower than in tensile ones due to the compression subjected face effect. Therefore, the good gauge factor values (around 2–3) and the high linear response proves the applicability of the proposed nanocomposites in structural health monitoring applications.

Carbon fiber-reinforced plastic bonded joints with novel carbon nanotube (CNT) adhesive films were manufactured and tested under different aging conditions by varying the surfactant content added to enhance CNT dispersion. Single lap shear (SLS) tests were conducted in their initial state and after 1 and 2 months immersed in distilled water at 60 °C. In addition, their electrical response was measured in terms of the electrical resistance change through thickness. The lap shear strength showed an initial decrease due to plasticization of weak hydrogen bonds, and then a partial recovery due to secondary crosslinking. This plasticization effect was confirmed by differential scanning calorimetry analysis with a decrease in the glass transition temperature. The electrical response varied with aging conditions, showing a higher plasticity region in the 1-month SLS joints, and a sharper increase in the case of the non-aged and 2-month-aged samples; these changes were more prevalent with increasing surfactant content. By adjusting the measured electrical data to simple theoretical calculations, it was possible to establish the first estimation of damage accumulation, which was higher in the case of non-aged and 2-month-aged samples, due to the presence of more prevalent brittle mechanisms for the CNT-doped joints.

In this study, the hydrothermal resistance of an epoxy resin (aircraft quality) reinforced with graphene is analyzed. Different geometries and aspect ratios (thickness and lateral dimensions) of graphene nanoplatelets were studied. The addition of these graphene nanoplatelets induces important advantages, such as an increase of the glass transition temperature and stiffness and an enhancement of barrier properties of the epoxy matrix, in spite of the excellent behavior of pristine resin. The effectiveness of graphene nanoplatelets increases with their specific surface area while their dispersion degree is suitable. Thinner nanoplatelets tend to wrinkle, decreasing their efficiency as nanofillers. Graphene used as reinforcement not only reduces the absorbed moisture content but also decreases its effect on the thermal and mechanical properties related to the matrix.

Hybrid multi-material joints of titanium alloy (Ti6Al4V) bonded to carbon fibre reinforced polymer (CFRP) are really interesting for lightweight high-performance structures, i.e., aeronautical and marine applications. Co-bonding technique, in which the joining takes place at the same time as the manufacturing of the CFRP, could be a great solution for easy manufacturing fibre metal laminate (FML) and complex structures, or a time saving process. One of the biggest challenges in this type of multi-material structures is to be able to confirm the structural health in service. The present work deals with the use, as matrix, of an epoxy resin reinforced with multiwalled carbon nanotubes (MWCNT) for manufacturing of Ti-CFRP joints by co-bonding. The aim is the structural health monitoring (SHM) of these hybrid structures, enhancing their mechanical properties.The obtained results show an improvement of more than 140% in fracture resistance of the multi-material joints with optimal surface preparation, by incorporating CNT. Furthermore, electrical monitoring makes it possible to discern the quality of the manufactured joints, increasing their reliability and performance. Besides, the ability to localize crack propagation and stresses prior to joint failure has been confirmed, so CNT addition results in a powerful tool for SHM multi-material joints.

The repair performance of the so-called self-healing polymer systems has been extensively studied in recent works. However, the characterization methodology carried out to analyze the repair performance of these materials has not been widely discussed. Herein, a study of the influence of the characterization parameters on the self-healing capabilities of a 2-Aminophenyl disulfide (AFD)/epoxy system via convective heating is carried out. Results show a decrease in the self-healing efficiency proportional to the damage depth. Finally, increasing the healing temperature and time does not significantly affect the repair performance, obtaining similar self-healing efficiency values (>>95%) in all cases.