Methods for Enhancing the Electrical Properties of Epoxy Matrix Composites

: This paper presents ways to modify epoxy resin matrix composites to increase their electrical conductivity. Good electrical properties are particularly important for materials used in the construction of vehicles (cars, trains, airplanes) and other objects exposed to lightning (e.g., wind turbines). When the hull plating is made of an electrical conductor (e.g., metal alloys) it acts as a Faraday cage and upon lightning discharge the electrical charge does not cause damage to the structure. Epoxy-resin-based composites have recently been frequently used to reduce the weight of structures, but due to the insulating properties of the resin, various modiﬁcations must be applied to improve the conductivity of the composite. The methods to improve the conductivity have been categorized into three groups: modiﬁcation of the matrix with conductive ﬁllers, modiﬁcation of the composite reinforcement, and addition of layers with increased electrical conductivity to the composite.


Introduction
Polymer composites are at present increasingly used as structural materials replacing metal alloys due to their high strength-to-weight ratio and corrosion resistance.However, the disadvantage of these types of material is their low electrical conductivity, so they do not protect against lightning and do not provide good protection against electromagnetic fields.These properties are particularly important for the aerospace industry due to the high exposure to lightning strikes [1][2][3], which in the case of materials with low electrical conductivity can cause severe damage [4].
Lightning also carries the risk of damaging the delicate electrical equipment inside the aircraft due to the strong electromagnetic field when lightning comes in contact with the fuselage surface [3,5].Resistance to electromagnetic beams is also important for materials for military applications.Since World War II, designers have been searching for increasingly better materials capable of absorbing the electromagnetic radiation emitted by radars used to detect enemy vehicles [6].Weapons emitting electromagnetic fields, which can cause the complete immobilization of combat vehicles by destroying electrical equipment necessary for their proper functioning, also pose a great threat to the army [7][8][9][10].
Epoxy/carbon (EP/CF) composites are most commonly used as a base for improving electrical conductivity because of their high strength and good resistance to lightning, even without modification [11][12][13][14].However, even such composites deviate significantly from the electrical properties of metals, which are classified as electrical conductors, that is, materials whose conductivity is greater than 10 3 S/cm.The conductivity of epoxy resin, depending on its type, ranges from 10 −17 to 10 −12 S/cm, so it is classified as an insulator, i.e., a material with a conductivity below 10 −12 S/cm, while carbon fiber is about 6 × 10 2 S/cm, which allows it to be classified as a semiconductor, i.e., a material whose conductivity is between 10 −12 and 10 3 S/cm [15].The EP/CF composite, on the other hand, has a conductivity of about 10 −4 -10 −2 S/cm [11,14], that is, it also classifies as a semiconductor [15].Thus, to minimize the damage due to lightning discharge, it would be necessary to improve the conductivity of both the matrix and the composite reinforcement.
Therefore, the motivation of this review is to point out to researchers the opportunity that could be brought about by the simultaneous modification of both the epoxy matrix and its reinforcement, as well as the incorporation into the structure of the composite of special layers of materials with reduced resistivity in order to obtain a composite, which, because of its electrical conductivity, could be classified as a conductor.Recent reviews on the topic of increasing the electrical conductivity of epoxy composites focus mainly on presenting the modification of the polymer matrix itself [16][17][18][19][20][21] neglecting the topics related to the other materials included in the composites, which in most cases constitute a larger mass share in the composite structure.This review therefore aims to identify different approaches to modifying the electrical properties of epoxy composites in order to interest researchers in using them simultaneously for further research on this topic.The review therefore focuses simultaneously on three areas of composite modification: the epoxy matrix, the fiber reinforcement, and the use of additional conductive layers that can be incorporated into the composite structure.

Modification of Polymer Matrix to Improve Composite Conductivity
One of the most popular methods to improve the electrical conductivity of composites is the modification of the matrix.Currently, additives in the form of conductive polymers (most commonly polyaniline) [22][23][24][25] and carbon fillers (graphite, graphene, conductive carbon black, carbon nanotubes) are mainly used [14,[26][27][28][29][30][31][32][33][34][35][36].At the same time, the use of metal modifiers of epoxy resin is abandoned, because, firstly, they significantly increase the weight of the whole composite, and, secondly, their nanoparticles are undesirable due to their toxic properties [37][38][39][40][41] and possibility of carcinogenicity [42].However, recent studies show that there is an opportunity to use metal nanoparticles as modifiers of carbon fillers [43][44][45]; limiting them in this manner and incorporating them into the structure of other molecules increases their overall size, thus reducing negative effects on living organisms.
Researchers in their publications [22,23] have presented a review of the most commonly used conductive polymers for improving electromagnetic radiation and lightning resistance.They show that one of the better polymers used for this purpose is polyaniline (PANI), due to its low cost and ease of synthesis and modification.
Katunin et al. [24] studied an epoxy resin modified by the addition of different polyaniline contents (20-70 percentage by volume (vol.%)) for subsequent use as a matrix for epoxy/carbon composites.It was found that the highest possible PANI content in the epoxy resin allowing its use in the composite molding process is 50 vol.%.The electrical conductivity results for compositions containing 30 vol.% and 50 vol.%were compared.The result for the former was below the measurement range, while for 50 vol.%content of PANI a conductivity of 0.440 S/cm was obtained.The lightning strike resistance of the material was also investigated and better results were obtained than for typical carbonfiber-reinforced composites.
Wei et al. [26] studied the effect of synergistic properties when more than one carbon filler is added to the epoxy matrix.They checked the electrical conductivity properties at a filler content of 1 percent by weight (wt.%) in the resin by adding graphite nanoparticles (GNPs), carbon black (CB) and carbon nanotubes (CNTs) at different weight ratios.The measurements determined that the composites containing GNP and CB at a ratio of 9:1 (CNT0.9CB0.1)had the best electrical conductivity, and by adding CNT to this composi-tion a significant improvement in conductivity was observed at a GNP/CB/CNT mass ratio of 7:1:2 (GNP0.7CB0.1CNT0.2).Conductivity was investigated for three composites: CNT/EP, CNT0.9CB0.1/EPand GNP0.7CB0.1CNT0.2/EP,at a total filler content in the resin of up to 3 wt.%.The electrical percolation threshold was obtained already at 0.2 wt.% GNP0.7CB0.1CNT0.2,when for CNT and CNT0.9CB0.1 it was 1 wt.% and 0.5 wt.%, respectively.From SEM studies, it was found that the synergistic effect of the conductive properties of the composites was not only due to the unique geometrical structure of each filler, but also as a result of the combination of narrow and wide gaps between graphite planes by spherical carbon black particles and long and flexible carbon nanotubes.
The use of unmodified graphite as an epoxy resin filler was tested by Radouane et al. [27].They checked its effect on conductivity at contents ranging from 3 to 20 vol.% and for the highest content they obtained a result of 1.57 × 10 −5 S/cm.After converting the vol.% to wt.% content based on the resin and graphite densities reported by the researchers, it appears that as much as 33 wt.% graphite must be added to the epoxy resin to achieve this conductivity result.A similar conductivity result was shown by researchers Meng et al. [28] with about 7 wt.%graphene in the epoxy resin, a volume that is 65% smaller.On the other hand, Wang et al. [29] obtained a conductivity about 20,000 times higher than the result for an epoxy composite containing 20 vol.% graphite in their study with twice the volume content of graphene.Thus, the results of recent studies clearly indicate that graphene is a much better filler for modifying the electrical conductivity of an epoxy matrix than graphite.With a much lower content of graphene in the composite, it is possible to obtain a much higher conductivity, so that the increase in matrix viscosity can be reduced by using less filler to obtain the desired electrical properties.This property is particularly important when using such a resin and filler system in the manufacture of fiber composites by the infusion method.
Graphene oxide has recently been increasingly used to improve the electrical conductivity of polymer matrices.Senis et al. [30] studied the effect of epoxy resin modification with graphene oxide on the properties of carbon-fiber-reinforced composite.As a result of this study, they found that the composition containing 6.3 vol.% graphene oxide showed the best properties.An increase in electrical conductivity in comparison with the unfilled epoxy resin was observed: by 25% for measurements along the reinforcement plane and by 230% for electrical conductivity measured perpendicular to this plane.Thus, these results are much better than those obtained for graphene as presented in the previous paragraph.At a similar filler volume content in the resin for graphene oxide, conductivities four orders of magnitude higher were obtained compared to the modification with graphene [28].Modifications of epoxy resin with chemically reduced graphene oxide (CRGO) were also carried out [31][32][33].The results of conductivity tests of epoxy composites modified with this filler presented here allow one to consider it as another good modifier of electrical properties of polymeric composites.Kernin et al. [31], with the addition of only 0.5 wt.% of CRGO, obtained the conductivity of the modified composite at a level of 10 −3 S/cm, while Han et al. [32], increasing its content to about 4 wt.%,obtained conductivity values over 100 times higher.Gao et al. [33], on the other hand, showed that CRGO together with nickel chains can serve as a modifier for epoxy microcellular foams.By using an asymmetric design of the conductive structure consisting of a layer rich in nickel chains and a layer rich in CRGO, they obtained an epoxy foam characterized both by significantly increased conductivity (10 −3 S/cm) and, due to the spatial structure of such a composite, also by increased electromagnetic-radiation-shielding efficiency.
The effect of the addition of carbon fillers in the form of carbon black, carbon nanotubes and exfoliated graphite (EG) was also studied by Kuzhir et al. [34].They checked the electrical, electromagnetic, mechanical and thermal properties of epoxy composites modified with these additives (in amounts ranging from 0.25 wt.% to 2 wt.%).They obtained the best improvement in Young's modulus of 17.5% for the composite filled with EG; however, above a content of this filler of 1.5 wt.%, a decrease in modulus was observed due to its inaccurate dispersion in the resin.The percolation threshold was reached the fastest for the modification with CNTs (at 0.25 wt.% content), while the composite containing 2 wt.%EG showed the best electrical conductivity value.Figure 1 shows the dependence of the effect of the amount of filler on the conductivity of the modified epoxy resin using carbon black as an example.Analyzing the graph, it can be observed that the highest increase in the conductivity value is obtained at a filler content of 1 wt.%.At higher amounts the growth rate of this parameter gradually decreases.In the case of electromagnetic radiation resistance tests, the epoxy composite containing 2 wt.%EG also showed the best properties.
nanotubes and exfoliated graphite (EG) was also studied by Kuzhir et al. [34].They checked the electrical, electromagnetic, mechanical and thermal properties of epoxy composites modified with these additives (in amounts ranging from 0.25 wt.% to 2 wt.%).They obtained the best improvement in Young's modulus of 17.5% for the composite filled with EG; however, above a content of this filler of 1.5 wt.%, a decrease in modulus was observed due to its inaccurate dispersion in the resin.The percolation threshold was reached the fastest for the modification with CNTs (at 0.25 wt.% content), while the composite containing 2 wt.%EG showed the best electrical conductivity value.Figure 1 shows the dependence of the effect of the amount of filler on the conductivity of the modified epoxy resin using carbon black as an example.Analyzing the graph, it can be observed that the highest increase in the conductivity value is obtained at a filler content of 1 wt.%.At higher amounts the growth rate of this parameter gradually decreases.In the case of electromagnetic radiation resistance tests, the epoxy composite containing 2 wt.%EG also showed the best properties.Researchers also examined the effect of the addition of carbon nanofibers (CNFs) on improving the electrical conductivity properties in epoxy/carbon composites (EP/CF) [14] and carbon nanotubes in epoxy/glass composites (EP/GF) [35].An electrical conductivity of less than 10 −10 S/m was obtained for the unfilled epoxy/glass composite while 0.034 S/m was obtained for the epoxy/carbon composite.It was observed that using carbon fiber as reinforcement was a more favorable choice due to its better properties.When the matrix was modified with both CNTs and CNFs, a significant improvement in electrical conductivity was obtained compared to the unmodified epoxy composites.This value increased by six orders of magnitude (from 10 −10 S/m to 10 −4 S/m) at a filler content of 0.1 wt.% for EP/GF, while a twentyfold increase in conductivity (from 0.034 S/m to 0.68 S/m) was observed for EP/CF at a filler content of 0.5 wt.%.In the case of the epoxy/carbon composite, a further increase in the carbon nanofiber content resulted in a slight decrease in electrical conductivity, which may be due to agglomeration of CNTs and the formation of filler-free spaces in the matrix.
It is also possible to use metals to improve the conductive properties of carbon fillers.Such studies were recently conducted by Qian [43], Kandare [44] and Zhang et al. [45] based on modifications using silver nanoparticles.The first two groups of researchers focused on modifying graphene while the third group focused on graphene oxide.All studies showed significant improvement in electrical conductivity compared to unmodified carbon fillers.Qian et al. [34] used 20 vol.% graphene modified with silver nanoparticles for modification, which allowed them to obtain a very high conductivity for Plot of the dependence of carbon-black-weight percentage on the conductivity of the epoxy resin modified with it (source: own work on the basis of [34]).
Researchers also examined the effect of the addition of carbon nanofibers (CNFs) on improving the electrical conductivity properties in epoxy/carbon composites (EP/CF) [14] and carbon nanotubes in epoxy/glass composites (EP/GF) [35].An electrical conductivity of less than 10 −10 S/m was obtained for the unfilled epoxy/glass composite while 0.034 S/m was obtained for the epoxy/carbon composite.It was observed that using carbon fiber as reinforcement was a more favorable choice due to its better properties.When the matrix was modified with both CNTs and CNFs, a significant improvement in electrical conductivity was obtained compared to the unmodified epoxy composites.This value increased by six orders of magnitude (from 10 −10 S/m to 10 −4 S/m) at a filler content of 0.1 wt.% for EP/GF, while a twentyfold increase in conductivity (from 0.034 S/m to 0.68 S/m) was observed for EP/CF at a filler content of 0.5 wt.%.In the case of the epoxy/carbon composite, a further increase in the carbon nanofiber content resulted in a slight decrease in electrical conductivity, which may be due to agglomeration of CNTs and the formation of filler-free spaces in the matrix.
It is also possible to use metals to improve the conductive properties of carbon fillers.Such studies were recently conducted by Qian [43], Kandare [44] and Zhang et al. [45] based on modifications using silver nanoparticles.The first two groups of researchers focused on modifying graphene while the third group focused on graphene oxide.All studies showed significant improvement in electrical conductivity compared to unmodified carbon fillers.Qian et al. [34] used 20 vol.% graphene modified with silver nanoparticles for modification, which allowed them to obtain a very high conductivity for modifying only the epoxy matrix; this was 2.13 × 10 2 S/cm.This is a result that gives hope that with the reinforcement of this composite with modified carbon fiber it will be possible to obtain conductivity results at the level of conductors.This supposition may also be confirmed by the study of Kandare et al. [44], who used only 1 vol.% of this filler when reinforcing an epoxy composite with unmodified carbon fiber.The conductivity result they obtained for the composite so prepared was ten times higher than the conductivity value obtained for the unmodified epoxy/carbon composites.

Reinforcement Modification and Its Effect on the Electrical Conductivity Properties of the Composite
Most commonly, glass or carbon fiber fabrics are used to reinforce polymer composites used in aviation.Porras and Mucha [46] investigated what effect CF fabric reinforced with different weights has on the electrical conductivity of epoxy-resin-matrix composites.
They investigated what effect the use of FG fabrics in the outer layers of the laminate has on this parameter.They prepared six laminates consisting of four reinforcement layers for the study.They were able to obtain the best electrical conductivity for the laminate consisting entirely of carbon fabric reinforcement with a higher weight (245 g/m 3 ).For the CF reinforced laminate of 160 g/m 3 , the electrical conductivity was almost two times lower.The laminates consisting of two layers of CF and two layers of GF (stacked relative to each other) had better properties when a higher weight carbon fabric was used.At the same time, these composites showed worse performance than those reinforced by carbon fiber alone.
The most popular filler currently used to modify the reinforcement of epoxy resin matrix composites is carbon nanotubes.A number of researchers have investigated the possibility of using them to modify various fibers, through the most commonly used are fiberglass [47] and carbon fiber [48] in aviation to cotton fiber [49] and basalt fiber [50].All researchers obtained epoxy composites with similar conductivities ranging from 10 −3 to 10 −1 S/cm but with significantly different filler contents used to modify the fibers.The best electrical conductivity using the least amount of carbon nanotubes (2.65 wt.%) was obtained by Kim et al. [50] modifying basalt fibers by soaking the reinforcement in an aqueous solution of CNTs with surfactant in the form of sodium dodecyl sulfate, which raises the possibility of further research in terms of using this reinforcement to produce composites with increased conductivity.In comparison, Xu et al. [49] had to use more than ten times (30 wt.%) the amount of CNTs to modify cotton fibers to obtain a conductivity similar to an epoxy composite reinforced with them.A similar content of reinforcement modifier was used by researchers [47,48] to modify glass fiber and carbon fiber, respectively, to obtain a conductivity value similar to composites.Considering the significantly better electrical properties of carbon fiber compared to glass fiber, this is a rather strange relationship.However, the weight content of the filler used by the researchers to modify the carbon fiber may be greatly inflated by the fact that they also used an additional modification with copper particles, which despite their excellent electrical conductivity also have a much higher density compared to carbon nanotubes.Using them in this case, therefore, does not seem to provide any measurable benefit for the electrical properties of the composite, due to the significant increase in weight.
Wu et al. [51] present in their recent study the possibility of using carbon fiber in the form of a soft mat formed by airflow-netting-forming technology and needle punching.The mat thus fabricated was immersed in a mixture of phenolic resin and ethanol for 1 min and then placed in an oven heated to 80 • C for 2 h to remove the ethanol and for 5 h at 175 • C to crosslink the resin.The mat thus prepared was graphitized and carbonized in an oven at 2400 • C to decompose the phenolic resin to carbon, which when incorporated into the carbon fiber structure increased the chances of contact between fibers, resulting in improvements in both the mechanical properties and conductivity of the reinforcement.The next step of modification of the prepared reinforcement was to immerse it for 1 h in graphite suspension in ethanol and evaporate the solvent again in an oven at 80 • C. The modified mat was then subjected to pressing under a pressure of 10 MPa in order to reduce its volume by half, and this was used to prepare composites based on an epoxy resin matrix.The composites obtained by the researchers exhibited a conductivity of 10 −2 S/cm at a modified reinforcement content of 33.23 wt.%.The electrical conductivity value of this composite is much lower than that of the conductors; however, the presented method of preparation and modification of carbon fibers provides a chance to use different types of modifiers simultaneously and obtain better conductivity values for the reinforcement.
Energies 2022, 15, 4562 6 of 18 Maity and Chatterjee [52] present in their publication the different approaches of researchers when modifying fabrics to improve the protective properties against electromagnetic radiation.They list three main ways to improve electrical conductivity by modifying fibers.The first is to apply electrically conductive particles, such as copper or silver, to the fabric surface as a conductive coating by vacuum evaporation of a solvent from a nanoparticle solution applied to the reinforcement surface.In this case, shielding takes place by the reflection of energy from the applied surface, which in many cases may cause unwanted interference.Another method mentioned by the authors is the modification of the reinforcement by incorporating metallic fibers of various forms and sizes into its structure.The last group of methods to improve electrical conductivity is to coat the fibers with a layer of conductive polymer-like polyaniline (PANI) or polyacetylene (PPy).Such modifications give a significant improvement in the electrical conductivity of the composite and increase the protection against electromagnetic (EM) radiation, which in this case is mostly absorbed by the polymer layer.
Modification of carbon fibers by applying copper particles to them using a cold spray process was investigated by Archambault et al. [53].The presented process of creating a metallic layer on the surface of a carbon fabric allows one to obtain a composite covered with a thin conductive layer (100 µm) with a high adhesion strength of 2.6 MPa ± 0.8 MPa and a low resistivity equal to 3.6 × 10 −8 Ωm ± 0.3 × 10 −8 Ωm.The cold spray process also makes it possible to create components with a metallic layer of various shapes and sizes that can be easily deformed.This provides the opportunity to use it for manufacturing aircraft components with complex geometries.
Another reinforcement modification can also be carried out by spinning carbon fibers with metal-coated yarns.Rehbein et al. [54] investigated how silver-coated polyamide yarn, which was used for spinning uncoated carbon fibers, would affect the electrical conductivity of composites reinforced with these fibers.A schematic of the composites prepared by the authors is shown in Figure 2.They consisted of alternating layers of carbon fabric with fiber orientations of −45 • and +45 • , with an additional silver-coated layer with a fiber orientation of 90 • separating them.The addition of spacers of the aforementioned uncarpeted carbon fibers increased the electrical conductivity of the tested composites to nearly 60,000 S/m.The high conductivity also resulted in a reduction in the lightning strike damage area by up to 90% at a depth of 1 mm.different types of modifiers simultaneously and obtain better conductivity values for the reinforcement.
Maity and Chatterjee [52] present in their publication the different approaches of researchers when modifying fabrics to improve the protective properties against electromagnetic radiation.They list three main ways to improve electrical conductivity by modifying fibers.The first is to apply electrically conductive particles, such as copper or silver, to the fabric surface as a conductive coating by vacuum evaporation of a solvent from a nanoparticle solution applied to the reinforcement surface.In this case, shielding takes place by the reflection of energy from the applied surface, which in many cases may cause unwanted interference.Another method mentioned by the authors is the modification of the reinforcement by incorporating metallic fibers of various forms and sizes into its structure.The last group of methods to improve electrical conductivity is to coat the fibers with a layer of conductive polymer-like polyaniline (PANI) or polyacetylene (PPy).Such modifications give a significant improvement in the electrical conductivity of the composite and increase the protection against electromagnetic (EM) radiation, which in this case is mostly absorbed by the polymer layer.
Modification of carbon fibers by applying copper particles to them using a cold spray process was investigated by Archambault et al. [53].The presented process of creating a metallic layer on the surface of a carbon fabric allows one to obtain a composite covered with a thin conductive layer (100 μm) with a high adhesion strength of 2.6 MPa ± 0.8 MPa and a low resistivity equal to 3.6 × 10 −8 Ωm ± 0.3 × 10 −8 Ωm.The cold spray process also makes it possible to create components with a metallic layer of various shapes and sizes that can be easily deformed.This provides the opportunity to use it for manufacturing aircraft components with complex geometries.
Another reinforcement modification can also be carried out by spinning carbon fibers with metal-coated yarns.Rehbein et al. [54] investigated how silver-coated polyamide yarn, which was used for spinning uncoated carbon fibers, would affect the electrical conductivity of composites reinforced with these fibers.A schematic of the composites prepared by the authors is shown in Figure 2.They consisted of alternating layers of carbon fabric with fiber orientations of −45° and +45°, with an additional silver-coated layer with a fiber orientation of 90° separating them.The addition of spacers of the aforementioned uncarpeted carbon fibers increased the electrical conductivity of the tested composites to nearly 60,000 S/m.The high conductivity also resulted in a reduction in the lightning strike damage area by up to 90% at a depth of 1 mm.

Application of Additional Conductive Layers in Multilayer Polymer Composites
During the last 5 years, the most common way to improve the electrical conductivity of polymer composites is to use layers of increased conductivity in their structure.They can be incorporated into the composite in the form of spacers or facing layers.Similarly to matrix modification or reinforcement, carbon [55][56][57][58][59][60][61][62] and metallic [63][64][65][66][67][68][69] additives are also used to improve electrical properties.For conductivity-enhanced layers with carbon fillers, carbon nanotubes are most commonly used [55][56][57][58][59]. Han et al. [55] investigated the discharge resistance of an EP/CF composite coated by a nanotube-containing composite bonded to it through an adhesive layer.Different compositions were used to bond the two composites: unfilled EP, EP with carbon nanotubes and EP with boron nitride (BN).How the thickness of the adhesive layer affects the area of the electric discharge damage was also checked.The composites bonded using EP/BN showed the best resistance.With a 100 kA simulated lightning discharge test, for a bonding layer thickness of 200 µm, they showed damage only on the surface of the composite and the area was almost 10 times smaller than the EP layer.The composite was also the only one, for a bonding layer thickness of 200 µm, to have a compressive strength after the discharge test similar to the pre-test result.The researchers also point out that while maintaining similar properties to copper-coated composites, it is 30% lighter than them.A similar way to improve the electrical properties of EP/CF composite was tested by Kumar et al. [56].He used laminates obtained from eight layers of epoxy/carbon prepregs and incorporating BP layers into their structure in different ways.The first one, designated BP0-CF, did not contain any modification; in the second one (BP1-CF) a BP layer was added on top of the composite; while the third one (BP4-CF), in addition to the top layer, contained three more layers located between the next four prepreg layers from the top.It has been shown that the addition of conductive layers in the form of BP between the reinforcement layers causes a decrease in the mechanical properties of the composite, but at the same time the composites are more resistant to lightning.This is mainly due to the prevention of pyrolysis phenomena on the laminate surface.However, the researchers intend to improve the mechanical properties of the modified composites by using thinner BP layers.
A conductive layer of carbon nanotubes can also be produced directly on the prepreg using a spray method [57].This consists of dispersing CNTs in methanol and then spreading the solution thus obtained on the surface of the prepreg through a spray gun.Subsequently, methanol was evaporated from the layer.A schematic of the fabrication of the composites by the authors is shown in Figure 3. Laminates consisting of nine layers modified in this way were fabricated by the vacuum bag method.The electrical resistance of these composites was reduced from 1.43 Ω to 0.98 Ω with the weight content of nanotubes in the laminate being only 0.047 wt.%.The fracture toughness was also improved, increasing by 22% and 47% at a nanotube weight content of 0.02 wt.% and 0.047 wt.%, respectively.Sobolewski and Dydek [58], on the other hand, used copolyamide fibers with 7 wt.%CNTs blended using an extruder, which were then cut and compressed into 15 g/m 2 and 25 g/m 2 nonwoven veils.The researchers examined the properties of EP/CF composites with 12 layers, where the first (PP_1) consisted of epoxy/carbon prepregs only; the second (PPv_1) with alternating layers of prepregs and nonwoven fabric (15 g/m 2 ); and the third (PPv_2) with two layers of nonwoven fabric (25 g/m 2 ) stacked on top of the laminate, followed by three more in the form of nonwoven fabric spacers between successive prepregs.The PPv_2 composite showed the highest conductivity (with a value of 560 S/m).Sobolewski and Dydek [58], on the other hand, used copolyamide fibers with 7 wt.%CNTs blended using an extruder, which were then cut and compressed into 15 g/m 2 and 25 g/m 2 nonwoven veils.The researchers examined the properties of EP/CF composites with 12 layers, where the first (PP_1) consisted of epoxy/carbon prepregs only; the second (PPv_1) with alternating layers of prepregs and nonwoven fabric (15 g/m 2 ); and the third (PPv_2) with two layers of nonwoven fabric (25 g/m 2 ) stacked on top of the laminate, followed by three more in the form of nonwoven fabric spacers between successive prepregs.The PPv_2 composite showed the highest conductivity (with a value of 560 S/m).This was 17.5 times higher than the reference composite PP_1 and 1.3 times higher than PPv_1.
Another team of researchers [59] studied EP/CF composites modified with conductive layers containing carbon nanotubes embedded in a phenolphthalein-modified polyetherketone (PEK-C) film.They investigated the lightning-strike resistance of these composites and compared these results with the properties obtained for commonly used laminates coated with a 60 µm thick silver layer.After modification with conductive layers, the EP/CF composite showed better lightning resistance properties.The surface of the impact damage decreased by 77% while the depth of damage decreased by 68%.Compared to silver-modified composites, these results were better for surface damage and slightly worse for depth damage.This is due to the better conductivity for the outer metal layer along the composite plane, while the nanotube layers sandwiched between the reinforcement improved the electrical conductivity both along and across the plane.
An unusual use of graphene as a conductive layer was presented by Zhang [60], Wang [61], Zhao [62] and their co-authors.The former [60] modified epoxy/carbon composites by introducing graphene into the outer layer in the form of a thin, flexible film 0.1 mm thick.This layer was fabricated by vacuum filtration of a graphene solution in distilled water.The graphene layer that remained on the filter paper was then covered with another filter paper, which was compressed using an aluminum roller in the next step.The final step was to remove the filter papers using tweezers.The laminates were fabricated by placing eight layers of prepregs on a layer of graphene and then cured in an oven.The composites exhibited reduced surface damage area by 94% and volume damage by 96%.They also exhibited better resistance to electromagnetic radiation.Wang et al. [61], on the other hand, fabricated a graphene oxide (RGO) layer using an infusion process.For this purpose, they dispersed RGO in epoxy powder resin using a ball mill.They then placed the mixture thus obtained on a layer of filter paper over which a preform consisting of eight layers of reinforcement was laid.The laminate forming process consisted of three stages (Figure 4).In the first, the air in the system was removed using a vacuum.In the second, the vacuum was removed and the system was heated to 120 °C to melt the resin and saturate the reinforcement layers.On the other hand, in the third step, vacuum was again created to remove the excess resin and produce a graphene oxide thin film on the laminate surface, and the composite was then heated to temperatures of 180 °C and 200 °C, respectively, to cure the matrix.The resulting laminates exhibited an improvement in electrical conductivity from 16 S/cm for the unmodified EP/CF composite to 440 S/cm.With such a significant improvement in conductivity at lightning discharge, electric current can spread more easily across the surface, making the composite less damaged.The lastmentioned group of researchers [62] used graphene in the form of a thin film, which was applied to a layer of epoxy resin mixed with graphite, used as a thermo-insulator.A tensile In the first, the air in the system was removed using a vacuum.In the second, the vacuum was removed and the system was heated to 120 • C to melt the resin and saturate the reinforcement layers.On the other hand, in the third step, vacuum was again created to remove the excess resin and produce a graphene oxide thin film on the laminate surface, and the composite was then heated to temperatures of 180 • C and 200 • C, respectively, to cure the matrix.The resulting laminates exhibited an improvement in electrical conductivity from 16 S/cm for the unmodified EP/CF composite to 440 S/cm.With such a significant Energies 2022, 15, 4562 9 of 18 improvement in conductivity at lightning discharge, electric current can spread more easily across the surface, making the composite less damaged.The last-mentioned group of researchers [62] used graphene in the form of a thin film, which was applied to a layer of epoxy resin mixed with graphite, used as a thermo-insulator.A tensile strength test after a simulated lightning discharge test for an EP/CF composite modified with such a layer showed a 20% improvement in this property compared to the unmodified composite.
Initially, metal spacers were used to improve the conductivity of polymer composites.Kawakami et al. [63] presented in his publication a study on an epoxy/carbon composite modified by adding an outer layer in the form of a copper mesh.A simulated lightning discharge test showed that the damage area at the lightning strike site decreased by more than 20 times after the copper mesh was applied.Modification of the EP/CF composite in the form of a metal layer was also used by Wang et al. [64].He used aluminum for this purpose, with which he covered the laminates in three different ways: covering only a small area of the surface near the edges and in the middle, covering the top surface of the composite completely, and adding a layer of glass fabric covered entirely with a metal layer.Through electrical discharge testing, he determined that doubling the aluminum layer resulted in nearly twice the area of composite failure for each of the compositions tested.However, the laminate that contained a layer of metal-coated glass fabric showed the best properties.For this composite, the area of surface damage from lightning strikes was reduced by 25 times compared to the unmodified EP/CF composite.
Metals can also be incorporated into the laminate structure in the form of nanowires.Guo and Xiaosu [65,66] et al. studied conductive layers fabricated using silver nanowire, the fabrication of which was described in their work by Zhang et al. [67].They tested two methods of distribution of the mentioned nanoparticles: in a film made of phenolphthaleinmodified polyetherketone (PEK-C) and by applying them to a nylon veil by immersing it in a solution with nanowires.In both cases, the incorporation of such a prepared conductive layer into the composite structure resulted in a significant decrease in resistivity.For the case in which silver nanowires were applied to the nylon layer, it was observed that with increasing number of immersions the surface density of such spacer increases linearly, while the resistivity decreases exponentially.To better illustrate the effect of nanowires on the electrical conductivity of nylon, Figure 5 shows the direct dependence of the number of immersions of the veil on its surface resistivity.By analyzing the results, the preparation method can be chosen to obtain satisfactory electrical properties with a slight increase in mass.The interlaminar fracture toughness of the fabricated laminates was also improved.was observed that with increasing number of immersions the surface density of such spacer increases linearly, while the resistivity decreases exponentially.To better illustrate the effect of nanowires on the electrical conductivity of nylon, Figure 5 shows the direct dependence of the number of immersions of the veil on its surface resistivity.By analyzing the results, the preparation method can be chosen to obtain satisfactory electrical properties with a slight increase in mass.The interlaminar fracture toughness of the fabricated laminates was also improved.Dong [68] and Guo [69] and their co-authors tested the possibility of combining metals with carbon fillers to improve the conductivity of composites.The first group of researchers [68] studied nickel-coated carbon nanotubes (Ni-CNTs).They fabricated epoxy/carbon composites consisting of 32 layers, with different arrangements of carbon fabric layers and prepregs, to whose surface a Ni-CNT layer was applied by spraying a solution of this filler in ethanol.Conductivity measurements showed that the addition of layers modified with the Ni-CNT filler resulted in an increase in the electrical conductivity Dong [68] and Guo [69] and their co-authors tested the possibility of combining metals with carbon fillers to improve the conductivity of composites.The first group of researchers [68] studied nickel-coated carbon nanotubes (Ni-CNTs).They fabricated epoxy/carbon composites consisting of 32 layers, with different arrangements of carbon fabric layers and prepregs, to whose surface a Ni-CNT layer was applied by spraying a solution of this filler in ethanol.Conductivity measurements showed that the addition of layers modified with the Ni-CNT filler resulted in an increase in the electrical conductivity of the composite by two orders of magnitude compared to the unmodified EP/CF composite.On the other hand, a group of researchers led by Guo [64] used a nonwoven fabric consisting of chopped nickel-coated carbon fibers (Ni-CFNV).They compared the properties of modified EP/CF composites using this nonwoven fabric to those modified with copper foil.The composite containing a face layer made of a nonwoven, higher-weight Ni-CFNV showed the best results, both for conductivity and mechanical strength tested after lightning strike tests.
Kumar et al. [70], on the other hand, applied a film of polyaniline (PANI), an electrically conductive polymer, to an epoxy/carbon composite.The authors premise was to produce a composite that could successfully replace those containing a metal film layer, using the same structure at a much lower overall weight.The weight was significantly reduced due to the lower density of the polymer and the lack of need for additional layers in the form of glass fiber, which are usually used to protect the metal from galvanic corrosion.Two composites were fabricated for the study, one coated with polyaniline film and the other without this layer.The investigated conductivity along the direction perpendicular to the reinforcement increased more than 450 times (from 0.22 S/m to 100 S/m) with respect to the unmodified composite after the application of the polymer layer.The addition of the PANI film also resulted in a significant improvement in lightning strike resistance.After a 100 kA electrical discharge test, the flexural strength of this composite was as high as 99% of the pre-test result, while it dropped to 36.6% for the composite without the conductive layer.

Summary
Analyzing the publications cited above, it can be concluded that the most popular way to modify the electrical conductivity properties of composites is to use in them additional layers with increased conductivity.The advantages of creating such hybrid composites include: 1.
much easier process of laminate preparation (no problem of increasing matrix viscosity when modifying it with a large amount of fillers) 2.
possibility to compose the composite from layers with different functional properties 3.
possibility to modify only one part of the laminate (if the composite is used as a protection against atmospheric discharges, it is enough to add conductive layers only in the upper part of the laminate) Modification of the entire epoxy matrix used to produce composites carries the aforementioned risk of excessively increasing the viscosity of the composition.Therefore, it cannot be used to produce laminates, e.g., by the infusion method, where at excessive viscosity there is a risk that the resin will not saturate the composite reinforcement completely.This method of fabrication may also cause a filtration effect of the fillers used to modify the matrix on the fibrous reinforcement.However, this method of increasing the conductivity allows for satisfactory electromagnetic radiation shielding effects because it takes place throughout the composite and not just on individual layers.
Increasing the electrical conductivity of the reinforcement itself, on the other hand, has an overall beneficial effect on the conductivity of the overall composite.However, in this method we are limited by the small amount of fillers we can use for modification.
We are noticing many more publications on the topic of improving the electrical conductivity of epoxy-resin-matrix-polymer composites.However, in order not to increase the volume of this publication too much, the rest of the researchers findings on the conductivity of the composites are presented in the form of Table 1.It contains a description of the fillers used for modification, the reinforcement used and the electrical conductivity results along with literature references to the full papers.
List of abbreviations for Table 1: It can be seen from Table 1 that the most commonly used filler for modifying epoxy resin to increase its electrical conductivity is graphene.Even with a small amount of this filler, a significant decrease in resistivity can be achieved.It can also be seen that the most common method to improve conductivity is to modify the matrix itself.Only in about 30% of publications do the researchers use fiber reinforcements for the production of composites, among which carbon fiber was used most often.This has to do with their very good electrical conductivity, which was mentioned earlier in this work.It can also be noted that, at equal concentrations of the same fillers, different groups of researchers obtained different resistivity results of the composites.The main reason for these differences is probably the way the samples were prepared for testing and the measurement errors resulting from the use of apparatus with different measurement accuracy.
Therefore, Table 1 presented in this work is only a starting point in the search for the results we are interested in, and more detailed information on the physical properties of the modifiers used as well as on the methods of sample preparation and testing should be found in individual literature references.
Only one composite obtained a conductivity value above 10 3 S/cm which means that out of all the composites cited in this review, only one can be classified as a conductor.Therefore, there is still much room for improvement in the electrical properties of composites based on epoxy resin matrix.

Figure 1 .
Figure1.Plot of the dependence of carbon-black-weight percentage on the conductivity of the epoxy resin modified with it (source: own work on the basis of[34]).

Figure 2 .
Figure 2. Schematic of layering in the composite with fiber orientation (source: own work on the basis of [54]).

Figure 3 .
Figure 3. Schematic of composite fabrication with additional conductive layer formed by spray application of filler solution (source: own work on the basis of [57]).

Figure 3 .
Figure 3. Schematic of composite fabrication with additional conductive layer formed by spray application of filler solution (source: own work on the basis of [57]).

Figure 4 .
Figure 4. Schematic of the resin film infusion (RFI) process to accumulate RGO on the carbon-fiberreinforced polymer (CFRP) surface (source: own work on the basis of [61]).

Figure 4 .
Figure 4. Schematic of the resin film infusion (RFI) process to accumulate RGO on the carbon-fiberreinforced polymer (CFRP) surface (source: own work on the basis of [61]).

Figure 5 .
Figure5.Plot of the dependence of the number of immersions of a nylon veil in a solution with nanowires on its surface resistivity (source: own work on the basis of[67]).

Figure 5 .
Figure5.Plot of the dependence of the number of immersions of a nylon veil in a solution with nanowires on its surface resistivity (source: own work on the basis of[67]).

Table 1 .
Composition of the studied composites along with their conductivity results based on literature.