Free-standing graphene films embedded in epoxy resin with enhanced thermal properties
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The poor thermal conductivity of polymer composites has long been a deterrent to their increased use in high-end aerospace or defence applications. This study describes a new approach for the incorporation of graphene in an epoxy resin, through the addition of graphene as free-standing film in the polymeric matrix. The electrical and thermal conductivity of composites embedding two different free-standing graphene films was compared to composites with embedded carbon nanotube buckypapers (CNT-BP). Considerably higher thermal conductivity values than those achieved with conventional dispersing methods of graphene or CNTs in epoxy resins were obtained. The characterisation was complemented with a study of the structure at the microscale by cross-sectional scanning electron microscopy (SEM) images and a thermogravimetric analysis (TGA). The films are preconditioned in order to incorporate them into the composites, and the complete manufacturing process proposed allows the production and processing of these materials in large batches. The high thermal conductivity obtained for the composites opens the way for their use in demanding thermal management applications, such as electronic enclosures or platforms facing critical temperature loads.
KeywordsFunctional composites Nanocomposites Polymer-matrix composites (PMCs) Thermal properties Scanning electron microscopy (SEM)
Thermal management of aerospace structures is extremely important for many applications including space or defence platforms, re-entry vehicles, propulsion systems, electronics or high energy systems. Next-generation aerospace structures could potentially use more thermally conductive materials to direct heat flow in satellites, thermal protection systems, near-propulsion structures, electronic boxes or radiators. The inherent poor thermal conductivity of polymer composites, which are the most widely used materials in aerospace structures due to their high strength-to-weight ratio, is a significant drawback in such kind of applications.
The use of graphene as a thermal filler to increase the thermal conductivity of composites has been extensively studied due to its superior thermal properties. High in-plane thermal conductivity, great flexibility, and low density are the main properties that make graphene-related materials a good candidate for creating advanced functional materials .
Reduced graphene oxide (rGO) and graphene nanoplatelets (GNPs) are generally selected from among the variety of graphene-based materials for their potential use in polymer composites for thermal conductivity enhancement. In the last years, several research groups have reported enhanced thermal conductivity values for dispersed graphene/epoxy resins through blending methods [2, 3]. These methods include conventional mixing methods such as three roll milling or calendaring and high shear mixing processes. The analysis of these results shows that thermal conductivity of polymer-based composites is enhanced with the increased loading of fillers. However, the incorporation of high concentrations of graphene is difficult and hard to perform, due to the increase in the polymer viscosity.
Changqing Liu et al.  achieved thermal conductivity values of 1.26 W/mK of graphene epoxy composites with 10 vol% while Yunfei Sun et al.  achieved a thermal conductivity of 6.1 W/mK, with a maximum loading of 20 wt% rGO to the epoxy resin. On the other hand, the influence of GNPs on the thermal properties of epoxy composites has also been widely published . Yuan-Xiang Fu et al.  reported that graphene nanoplatelets can effectively enhance the k of epoxy matrices, 4.01 W/mK for a maximum filler loading of 10 wt%, which is more than 22 times the thermal conductivity of the pure epoxy resin. Yi Wang  outlined that the thermal conductivity of the epoxy composites with 8 wt% GNPs was 1.18 W/mK. However, Haddon and his co-authors  reached a thermal conductivity of 6.44 W/mK increasing the load of graphite nanoplatelets up to 34%. Therefore, the improvement of thermal conductivity in polymer composites is strongly influenced by the filler loading and also by the filler dispersion in the matrix . It is known that the thermal transport in graphite, graphene and their derivatives is dominated by acoustic phonons , meaning that the process used for its embedding and the resulting interfaces are crucial for the thermal transport. In this context, the use of free-standing graphene films can overcome the problem of high loadings and can be handled in a reproducible way. A free-standing graphene film consists of a highly dense network structure of graphene usually manufactured by vacuum filtration or the direct evaporation method [12, 13]. In general, these processes require further thermal treatment that can reduce oxygen content and restore C sp2 bonds. Other post-treatment methods such as high-pressure compression are conducted in order to remove air pores from graphene films. The different fabrication and post-treatment methods lead to different thermal behaviour. It was found that free-standing graphene films can exhibit thermal conductivities in a wide range of 30–3300 W/mK . The highest value of 3300 W/mK was reported by Gee et al. . It corresponds to graphene films prepared through an electrochemical and filtration process.
Besides, huge attention has been paid to the effect of the graphene content on the electrical conductivity of epoxy nanocomposites. The typical percolation threshold values that were reported in literature vary from 1 to 15 wt% for GNP composites obtaining conductivity values from 10−3 to ~ 10−1 S/m [16, 17]. Such large variations of percolation threshold values show that the graphene content, aspect ratio, dimensions and geometrical arrangement, and also composite processing conditions are important.
Hao Hou  managed to prepare an epoxy composite embedded with a quasi-isotropic graphene film by a simple two-step process: vacuum filtration of small graphene sheets to obtain the film followed by the infiltration of epoxy resin. At a low graphene loading of 5.5 wt%, graphene/epoxy in- and through-plane thermal conductivities of 10.0 and 5.4 W/mK, respectively, were reported. The An F. group  described a thermal conductivity of 35.5 W/mK through-thickness while in-plane is 17.2 W/mK at 19 vol% for epoxy containing a densely packed and vertically aligned graphene film. Solutions based on vertically aligned graphene are reporting high through-thickness thermal conductivity of epoxy composites; Zhang et al. present values up to 384.9 W/mK in through-thickness and 0.18 W/mK in-plane at 44 vol% graphene . Unfortunately, the processes proposed are not easily scaled up.
So far, there are very few reports on graphene films embedded in epoxy composites for thermal management applications . In this work, three free-standing films based on carbon nanostructures—commercial graphene film made by Nanografi, carbon nanotube buckypapers (CNT-BP) manufactured at Tecnalia and thermally-reduced graphene oxide (GO) film produced by Graphenea Nanomaterials S.A.—were embedded in a commercial liquid epoxy-based resin (Resoltech 1800/1805) to enhance their thermal conductivity. The composites were prepared by manual impregnation of the carbon films with the uncured liquid epoxy resin.
This paper is organised as follows: characterisation of the carbon-based films and fabrication of the nanocomposite followed by the comparison of the electrical and thermal conductivity values of the different composites fabricated with the carbon-based films.
2 Materials and methods
2.1 Carbon-based films
Free-standing films obtained from graphene or graphene oxide (GO) have attracted interest due to their high thermal conductivity, superior electrical conductivity and excellent mechanical properties. Two free-standing graphene films—graphene film made by Nanografi (with a density of 1.82 g/cm3 and 35 μm thick) and graphene oxide (GO) film with a thermal treatment at 200 °C for 18 h made by Graphenea (with a density of 1.87 g/cm3 and 20 μm thick)—were compared to carbon nanotube buckypaper (CNT-BP) manufactured by Tecnalia (with a density of 0.9 g/cm3 and 52 μm thick).
The graphene films fabricated using different initial graphene solutions exhibited different macroscale properties: morphology, colour and flexibility.
Graphenea has reported thermal conductivity values of the rGO films post-processed at different temperatures (from 200 to 1000 °C) . The room temperature in-plane thermal conductivity increases from 2.9 W/mK for the reference GO film to 61 W/mK for the rGO film annealed at 1000 °C. Reduced GO films become more conductive but also more brittle as the post-processing temperature increases and, as a consequence, the mechanical strength is significantly reduced. For that reason, rGO film thermally treated at a moderately low temperature of 200 °C was selected for further integration into the composites.
Electrical resistivity. The electrical resistivity of these materials was measured by the Van der Pauw method using 4-probe measurement. A Keithley 2410 was used as a source of DC current
Thermal conductivity was measured by Hot disk sensors TPS 2500 S
Porosity. The porosity was characterised by mercury intrusion porosimetry, using an Autopore IV Micromeritics Hg Porosimeter.
Properties of free-standing graphene films and CNT BP used
Free-standing graphene film
Electrical conductivity S/m
Thermal conductivity (W/mK)
290 × 290
3.75 × 105
rGO (T = 200 °C) (Graphenea)
140 × 90
5.1 × 102
45 × 45
5.0 × 103
Both the electrical and thermal conductivity of the GNP film are significantly higher than the CNT-BP and the rGO film values.
Porosity values of free-standing graphene films
Mean pore diameter
Thermal treatment of rGO film at 200 °C (Graphenea)
2.2 Fabrication of nanocomposites
Electrical sheet resistance of free-standing graphene perforated films
Electrical sheet resistance (ohm/sq)
45 mm × 45 mm
Y (config 1) sample1
Y (config 1) sample2
Y (config 1) sample3
Y (config 2) sample1
Y (config 2) sample2
Y (config 2) sample3
Y (config 3) sample1
Y (config 3) sample2
Y (config 3) sample3
45 mm × 45 mm
Y (config. 1)
Y (config. 1)
Y (config. 1)
Thermal treatment of rGO films at 200 °C 18 h
140 × 90 mm
Y (config 1)
Y (config 1)
Y (config 1)
Y (config 3)
Y (config 3)
Y (config 3)
It can be observed that the perforation vaguely modified the electrical resistance of the films. Assuming that the thermal behaviour will not be affected by the perforations either, configuration 3 was chosen for both GNP and rGO films. Configuration 1 was selected for CNT BP according to previous results from TECNALIA in terms of good impregnation and integration in the composite .
The compacted structure of the sample avoided the resin infiltration through it (Fig. 7b).
The infiltration of the resin is of utmost importance for the optimisation of electrical properties. A continuous electrical network is mandatory to reach electrical percolation threshold, whereas to improve the thermal conductivity, other mechanisms occur like phonon scattering which can result in thermal conductance through electrically insulating films.
Electrical and thermal conductivity of the prepared composites (with GNP, rGO and CNT-BP films) was measured and comparatively analysed. Based on those results, GNP-based composites have been further analysed in terms of thermal stability.
3.1 Electrical conductivity
Sheet resistance values of graphene films before and after impregnating in the 1800 resin
Sheet resistance of the bare films (Ohm/sq)
Sheet resistance of film 1800 resin composites (Ohm/sq)
GNP film from Nanografi
rGO film from Graphenea
CNT BP (Tecnalia)
The composites based on GNP film, not well impregnated by the resin, presented a very high electrical resistance, as well as the composites based on reduced graphene oxide films. The pictures show that a continuous electrical path has not been obtained in neither of the two graphene film composites. However, the impregnated buckypaper composite presented an electrical sheet resistance of 8.87 Ohm/sq. The good integration of CNT-BP material in the resin allowing a continuous network is behind these results.
3.2 Thermal conductivity
The effect of resin thickness and the total graphene loading was analysed in relation to thermal conductivity. The reference value of the neat resin is 0.23 W/mK. The thermal conductivity of the free-standing films embedded in epoxy resin samples after the curing process was measured in a TPS 2500 S Hot Disk Analyser. Several thicknesses of the composites result in different graphene concentrations that were analysed to understand the influence of the content of carbon nanostructures in the composite. The graphene contents were calculated by the weight change before and after impregnation.
Thermal conductivity of free-standing films embedded in epoxy resin
Thermal conductivity (W/mK)
Graphene/CNT content in the resin (wt%)
Neat resin Resoltech (1800/1805)
0.227 ± 0.002
GNP film Nanografi
450 ± 50
GNP film in epoxy resin Resoltech 1800/1805
9 ± 1
GNP film in epoxy resin Resoltech 1800/1805
15 ± 2
GNP film in epoxy resin Resoltech 1800/1805
20 ± 3
rGO film Graphenea
10 ± 1
rGO film in epoxy resin Resoltech 1800/1805
CNT- BP Tecnalia
8 ± 1
CNT- BP in epoxy resin Resoltech 1800/1805
1 ± 0.5
The thermal conductivity value of the GNP film dramatically decreased (from 450 ± 50 to 9 ± 1 Wm/K) when it was combined with the polymer. The rGO film presented the same trend (from 10 to 0.5 W/mK) whereas CNT BP showed a lower decrease (8 ± 1 to 1 Wm/K).
The composite based on the rGO film presented a very low improvement in thermal conductivity, from 0.23 of the neat resin up to 0.5 W/mK with an rGO content of 8 wt%. In the case of CNT-BPs, the increase was more notable, 1 Wm/k was obtained for a 5 wt% CNT content incorporated in the resin.
Overall, the results obtained are very promising, 20 W/mK is high if compared with the thermal conductivities of plastics in general (< 0.5 W/mK). These can also be compared with nanocomposites obtained by adding graphene in powder directly in the matrix (not as a film), where maximum values of 6.4 W/mK are reported in the prior art. Novel solutions based on vertically aligned graphene nanoplatelets report thermal conductivities as high as 35.5 W/mK through-thickness and 17.2 W/mK in-plane . However, these solutions require difficult to scale-up processes.
3.3 Thermal stability of GNP film composites
Weight loss of GNP/epoxy nanocomposite at 480 and 680 °C
Weight loss of GNP/epoxy nanocomposite (wt%)
GNPs 9.5% in resin
GNPs 19% in resin
GNPs 30% in resin
Therefore, the thermogravimetric analysis of the studied GNPs/epoxy composites has shown that the presence of graphene films embedded in the resin induces an increase in thermal stability of the nanocomposite. It has also been shown that the mechanisms that rule the thermal behaviour are different to the electrical conductivity mechanisms in a composite. The phonon scattering can happen through insulating polymer layers; however, an electrical network is necessary for electron conduction .
This work gives a novel and simple approach for the fabrication of grapheme-filled polymer composites increasing the graphene loading to obtain a high thermal conductivity. The fabrication method can be easily scaled up to allow the production and processing of these materials in large batches.
Graphene composites obtained by integrating graphene free-standing films show a significant improvement in thermal conductivity values when compared to conventional methods of integrating graphene in epoxy resins. The methodology developed introduces clear benefits compared to competing technologies in terms of performance as a result of the incorporation of high graphene contents into a composite without the high viscosity issues encountered using liquid technologies, as well as in the up-scaling of the manufacturing process.
The results obtained are very promising. It has been demonstrated that the thermal conductivity of the polymer increases by two orders of magnitude (from 0.2 to 20 W/mK) with the introduction of a graphene loading of 30 wt% as graphene-film, improving also the thermal stability of the composite.
The high thermal conductivity of graphene/epoxy resins enables them to be used in thermal management applications as efficient heat-dissipating structural materials in future structures and systems where temperature loads are critical. Military applications requiring multifunctional structures where thermal management is needed will benefit from the use of free-standing graphene films embedded in polymer matrixes.
Compliance with ethical standards
Copyright @ 2019- European Defence Agency. All rights reserved. The opinions expressed herein reflect the author’s view only. Under no circumstances shall the European Defence Agency be held liable for any loss, damage, liability or expense incurred or suffered that is claimed to have resulted from the use of any of the information included herein.
Conflict of interest
The authors declare that they have no conflicts of interest.
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