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Synthesis and characterization of the electrically conductive polymeric composite for lightning strike protection of aircraft structures
Accepted Manuscript Synthesis and characterization of the electrically conductive polymeric composite for lightning strike protection of aircraft structures Andrzej Katunin, Katarzyna Krukiewicz, Roman Turczyn, Przemysław Sul, Andrzej Łasica, Marcin Bilewicz PII: DOI: Reference:
S0263-8223(16)31476-3 http://dx.doi.org/10.1016/j.compstruct.2016.10.028 COST 7854
To appear in:
Composite Structures
Received Date: Accepted Date:
8 August 2016 11 October 2016
Please cite this article as: Katunin, A., Krukiewicz, K., Turczyn, R., Sul, P., Łasica, A., Bilewicz, M., Synthesis and characterization of the electrically conductive polymeric composite for lightning strike protection of aircraft structures, Composite Structures (2016), doi: http://dx.doi.org/10.1016/j.compstruct.2016.10.028
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Synthesis and characterization of the electrically conductive polymeric composite for lightning strike protection of aircraft structures Andrzej Katunin1*, Katarzyna Krukiewicz2, Roman Turczyn2, Przemysław Sul3, Andrzej Łasica3, Marcin Bilewicz4 1
Institute of Fundamentals of Machinery Design, Faculty of Mechanical Engineering, Silesian
University of Technology, Konarskiego 18A, 44-100 Gliwice, Poland 2
Department of Physical Chemistry and Technology of Polymers, Faculty of Chemistry,
Silesian University of Technology, M. Strzody 9, 44-100 Gliwice, Poland 3
Institute of Theory of Electrical Engineering, Measurement and Information Systems,
Faculty of Electrical Engineering, Warsaw University of Technology, Koszykowa 75, 00-662 Warsaw, Poland 4
Institute of Engineering Materials and Biomaterials, Faculty of Mechanical Engineering,
Silesian University of Technology, Konarskiego 18A, 44-100 Gliwice, Poland
*
corresponding author
[email protected], tel. +48 32 237 2741
Abstract In this work, the development and testing of the new all-polymeric conductive composite material dedicated for the manufacturing of matrices of carbon fiber-reinforced composites for lightning strike protection applications was described. The synthesis procedure described in the paper allows obtaining the highly-conductive composition of PANI/epoxy mixture, where the content of particular polymers was evaluated basing on numerical simulations of electrical percolation between conductive particles in the dielectric matrix. The performed physicochemical, mechanical and thermal tests reveal acceptable stability of the developed material for applications as a matrix of structural composites dedicated to the aircraft applications. Finally, the numerical and experimental evaluation of behavior of the developed composite material in the conditions of the lightning strike indicate several advantages of application of the developed material in the aircraft industry with respect to the traditional CFRP structures.
Keywords Electrically conductive polymeric composite; lightning strike protection; aircraft structures; polyaniline
1. Introduction A still increasing use of polymeric composites in manufacturing of aircraft structures contributes to the decreasing of overall mass of vehicles, which leads to decreasing of fuel consumption and reduction of emissions to the environment, and simultaneously retaining or even increasing their mechanical properties. The use of polymeric composites, besides their numerous advantages, has a drawback: the polymers used in aircraft industry are electrically insulating, which causes that such structures are compliant to the lightning strikes and derivative effects like extremal heating in the area of plasma channel, which leads to the thermal decomposition of a polymeric matrix and pyrolytic processes in this area and surrounding and ablation of the reinforcement; generation of an acoustic shock waves, which influence mechanically on the stroked composite structure, etc. [1-3]. Therefore, the appropriate lightning strike protection (LSP) solutions are necessary to prevent or at least lower the damaging of aircraft composite structures during the lightning strike. Current LSP solutions used in commercial aircraft composite structures are based on impregnation of meshes or foils made of highly conducting metals and alloys. Such solutions reveal significant decrease of damaged area after the lightning strike, which is justified by numerous theoretical and experimental studies [4-9], however, they cause increasing of mass of structural elements and significantly complicate the manufacturing process. It should be mentioned that the solutions based on impregnation of metallic meshes and foils allow for decreasing the damage extent, but do not eliminate the problem. Thus, the new solutions that allow increasing resistance to atmospheric discharges and simplify the manufacturing process are highly desired. Several attempts in LSP solutions have been made during last decades. These solutions cover using metallized sprays and coatings, metallization of reinforcing fibers and dispersion of micro- and nanoscale conducting particles (see the review in [3]). However, in the case of using metallized sprays and coating the content of metallic particles is quite high, while the LSP effectiveness is low, therefore such solution found an application primarily in electromagnetic shielding of aircraft [1]. Covering of the reinforcement by metallic layers or dispersing metallic particles in the polymeric matrix of a composite increases the mass of a structure, since the content of particles should be high (in order to form critical percolation cluster) [10,11]. Moreover, the problems with adhesion between metallic particles and polymer may appear. The last group of above-presented solutions is based on dispersion of carbon nanostructures (CNS) in the dielectric matrix [12-14], which is an effective alternative to the impregnated metallic meshes, i.e. CNS increase significantly the electrical conductivity
of a resulting material, however, such solutions are still too expensive to apply them into industrial manufacturing of structural elements of aircraft. Another solutions which may solve the above-mentioned problems is a possibility of using the intrinsically conducting polymers (ICPs) as a conducting filler of composites. Their application allows increasing electrical conductivity significantly, while the mass of the resulting composite structure remains similar. Several studies on using ICPs as conductive fillers for LSP applications have been performed to date. Jeon et al. [15] proposed composite made of polycarbonate (PC) and polyaniline (PANI), Jia et al. [16] investigated composites of epoxy resin and PANI-dodecylbenzenesulphonic acid, similar composites were studied by the authors of [17] in terms of their electrical properties. A special attention should be paid to the studies of Yokozeki and his team [18-20], where the authors described synthesis, mechanical and electrical properties as well as lightning damage resistance of CFRP composites with a matrix of PANI/epoxy composite with various dopants. As the previous and on-going research studies ICP-based composites show, this solution seems to be a promising direction for LSP applications, and the doped PANI seems to be the mostly studied conductive filler in such composites. The following study presents recent attempts of the group of authors on modeling, simulating, and then synthesis and characterization of PANI/epoxy composites which can be considered as a matrix of CFRP aircraft composites with increasing resistance to the lightning strike damaging. In this study the authors reported the concept of the developed material formulated basing on the results of numerical simulations, description of synthesis process as well as determination of fundamental physicochemical, mechanical, thermal and electrical properties of the developed material. The final studies cover high-voltage tests and characterization of damage sites caused by high-voltage electrical discharges.
2. Concept of electrically conducting polymeric composite Among the wide range of ICPs, only several can be applied commercially, i.e. polypyrrole (PPy), poly(3,4-ethylenedioxythiophene) (PTh) and PANI. Exhibiting high conductivities together with chemical and electrochemical stabilities makes them advantageous materials to be used in surface and coatings technology [21]. They could be also considered as a filler of a matrix of composite with LSP properties. The lack of mechanical stability, low processability and flexibility, however, are the main limitations for the use of conducting polymers and the main reasons for the development of composite materials.
In order to avoid the problem of mechanical instability of ICPs it is suitable to mix them with polymers with fine mechanical properties, typical for aircraft industry. The polymer used for manufacturing GFRP and CFRP interior fuselage elements is almost always the epoxy resin, which is a dielectric. Thus, it is necessary to reach a compromise between the properties of electrically conductive and dielectric polymers mixed together in order to obtain the polymer with sufficiently fine mechanical properties and electrical conductivity. Therefore, the optimal content of electrically conductive filler in the insulating matrix should be determined. This problem can be resolved by using the percolation theory. In order to determine the content of a conductive fillers of various types in the developed conducting polymer the numerical simulations were performed using the Monte Carlo simulation method and modified Dijkstra’s algorithm. Due to the specificity of the investigated problem the classical percolation conditions were modified in such a way that all of the sides of a domain should be connected by the critical percolation cluster, i.e. the resulting material should be fully electrically conductive regardless the selected sides of input and output of current. Naturally, this modification slightly increase the percolation threshold but ensures the representativeness of the considered domain, and allows for its generalization to higher dimensions due to the fractal properties of the critical percolation cluster (see e.g. [22]). This was also conducted in the numerical simulations performed for the PPy/epoxy mixture [23]. The conducting particles were modeled using the hard-core soft-shell model which allows obtaining more realistic conditions of the modeled phenomena. This model assumes that conductive particles (parametrized by their constant radius and spatiallocation parameters) were generated using the Poisson process ofthe constant concentration rate and the generated particles cannot interpenetrate each other, but their shells considered here as a hopping distance can. The phenomenon of hopping of electrons between conducting particles was considered in the model, which allows decreasing the percolation threshold significantly. The simulations were performed for three preliminarily selected ICPs: PANI, PPy and PTh. All the simulations were performed based on literature data and the obtained critical percolation thresholds were 35.7%, 0.1% and 38.3%, respectively. For more detailed description of the model and the obtained results see [24]. Due to the best mechanical stability and processing properties as well as availability, PANI was selected as a conductive filler in the further studies.
3. Synthesis and characterization of the conducting composite
3.1 Synthesis procedure For the synthesis of polyaninine (PANI), aniline (Acros, purity 99.8%), sulfuric acid (Acros, concentration 96%, purity 99%), ammonium persulfate (POCH, purity 99.9%), methanol (POCH, purity 99.9%) , ammonia (POCH, concentration 25%, purity 99.9%) and 10-camphor sulfonic acid (TCI, purity > 98%) were used as received. The epoxy resin was made from commercially available epoxide Epidian 6 (Ciech, MW < 700 g/mol) and amine hardener IDA (Ciech) with the volume content of PANI from 20 % to 70 %. The synthesis of polyaniline (PANI) consisted of the dissolution of 5.50 mol (51.1 g) of aniline monomer in 1500 ml of 1 M H2SO4. The mixture was pre-cooled to -3 oC in an ice-salt bath. Then, 0.24 mol (54.88 g) of 1.5 M (NH4)2S2O8 was dissolved in 160 ml of 1 M H2SO4 and added dropwise while stirring at -3 oC temperature. After 24 hours the resulting precipitate was collected on a Buchner funnel, repeatedly rinsed with deionized water until neutral pH and purified in the Soxhlet apparatus with CH3OH. PANI was then deprotonated in 1800 ml of 1.5 M NH3 aq solution. After 24 hours, PANI was filtrated and washed with deionized water until neutral pH. The resulting dark-blue precipitate of polymer was dried under vacuum for 24 hours. The overall reaction yield was equal to 63.6 %. The chemical structure of synthesized PANI was confirmed by means of Raman and IR spectroscopy [25]. Polyaniline was then doped with camphorsulfonic acid, CSA, by mixing PANI with CSA in a proportion by weight of 1:3 in the vibrational mill LMW (Testchem) operating for 5, 10 or 15 min. According to [26], mechanical mixing in the presence of CSA is supposed to increase the protonation degree of PANI. To determine the conductivity of resulting material, PANI/CSA mixture was placed on the surface of glass slides and dried in the oven. The conductivities of PANI/CSA films measured for mixtures blended for 5, 10 and 15 minutes showed no dependence on the time of mixing and were equal to 6.93 ± 1.38 S/cm (5 min), 7.61 ± 1.35 S/cm (10 min) and 6.14 ± 2.51 S/cm (15 min). CSA-protonated PANI synthesized in the previous step was used to prepare PANI/epoxy composites, containing different volume percentage of PANI (20%, 30%, 40%, 50%, 60%, 70%). With the increase in PANI content, the significant increase in the viscosity of mixture was observed. The volume concentration of PANI equal to 50% was found to be the highest amount of conductive filler to obtain liquid composition and enable the process of molding.
3.2 Physicochemical characterization The chemical structure of PANI and PANI/epoxy composites was characterized by means of Raman spectroscopy using Renishaw InVia confocal microRaman system equipped with laser operating at 830 nm, and a CCD detector. The morphology, thickness and elemental composition of materials were measured by means of scanning electron microscope Phenom ProX equipped with 3D Roughness Reconstruction software and energy dispersive X-ray spectroscope (EDS) analyzer. The dual mode bipotentiostat Autolab PGSTAT302N + BA was used to measure the conductivity of materials with the use of interdigitated electrode [27]. The measurements were performed in a chronoamperometric mode at a constant potential application (-0.5 V) for 60 s. The conductivity of samples was calculated according to [27] and [28]. After their synthesis, the PANI/epoxy composites were investigated by means of SEM. The analysis of the morphology of cross-sections of composite materials (Fig.1) reveals the presence of PANI agglomerates within the composite material, especially for composites with low PANI content (20-30%). When the amount of PANI is increased (50-70%), conducting filler is more uniformly distributed among epoxy resin. The conductivity measurements performed by means of four point probe indicated high resistance for the composites with lower volume percentage of PANI than 50%. The tendency to agglomerate and form clusters within the composite materials is responsible for the increase in the amount of conductive filler necessary to fulfill the percolation requirements. Two composite materials have been chosen for further research, i.e. PANI/epoxy with 30% and 50% content of conducting filler. These two materials have been selected due to the coincidence with a theoretical values in the first case, and due to the fact that 50% volume concentration was the highest amount of PANI enabling the process of molding. The determination of conductivity of selected samples was performed by means of bipotentiostatic approach. The conductivity of PANI/epoxy composite with 50% of conductive filler, calculated based on chronoamperometric curve (Fig.2) was equal to 0.440±0.330 S/cm, while the conductivity of composite with 30% content of PANI was below the limit of detection. It was substantially increased, however, when the surface of composite was polished with the abrasive paper. After this type of preconditioning, the conductivity of the sample increased to the value of 0.184 ± 0.132 S/cm. This indicates that the epoxy resin may form the outer shell of the composite isolating PANI from the surrounding.
Fig.1. SEM images of cross-sections of PANI/epoxy composites with different amount of conductive filler: 20% (a), 30% (b), 40% (c), 50% (d), 60% (e) and 70% (f). 100 µm
Fig.2. Chronoamperometric curve for PANI/epoxy composite (50 % PANI), recorded in bipotentiostatic mode for Eapplied = -0.5 V and Eoffset = 0.01 V.
To verify this hypothesis, Raman spectra of the surface and cross-sections of PANI/epoxy composites with 30 % and 50 % content of conductive filler were collected (Fig.3). In both spectra peaks and bands characteristic for PANI are present, i.e. a sharp peak around 1620 cm-1 which is assigned to the C–C stretching of the benzenoid ring [29-31] and a strong band observed around 1590 cm-1 linked to C=C stretching vibration in the quinonoid ring [2931]. The band at 1505 cm-1 may be assigned to an N–H deformation vibration associated with the semi-quinonoid structures [29,31]. The band at 1170 cm-1 corresponds to the C–H bending vibration of the semiquinonoid rings (cation-radical segments) [29,30] with a shoulder at 1180 cm-1 corresponding to C–H in-plane bending vibrations in benzenoid rings [29]. A broad structural band with local maxima at 870, 840, 780 and 745 cm−1 (corresponding to C–N–C wagging and benzene ring deformations) is present [29-31]. In both cases, only the Raman spectra of cross-sections exhibit a band at 1346 cm-1 which may be assigned to C–N+ vibration of delocalized polaronic structures [29,30] indicating the presence of PANI in its doped state. It shows that there is a substantial difference in chemical composition of surface and cross-section within one type of composite, confirming the hypothesis of the effect of insulating layer covering the composite material on its conductivity.
(a)
(b)
Fig.3. Raman spectra of the surface and cross-sections of epoxy/PANI composites with 30% (a) and 50% (b) PANI.
EDS elemental characterization of surface (Fig.4) and cross-section (Fig.5) of PANI/epoxy composites with 30% and 50% PANI shows the variation of the concentration of conductive filler across the samples.
Fig.4. SEM images and elemental composition of surface of PANI/epoxy composites with 30% (a,b) and 50% (c,d) PANI.
Fig.5. SEM images and elemental composition of the cross-sections of PANI/epoxy composites with 30% (a,b) and 50% (c,d) PANI.
The surface of 30%/PANI/epoxy is depleted with conducting filler; the atomic percentage of sulfur (present in CSA) is only 0.2%, while nitrogen (present in PANI) is not detected. When the content of conducting filler in PANI/epoxy resin is increased to 50%, both sulfur (1.0%) and nitrogen (0.9%) are present on the surface. These elements are also present in the crosssection of both types of composites, their atomic percentage equals 9.6 % nitrogen and 1.1% sulfur for 30%/PANI/epoxy, and 10.7% nitrogen and 1.8% sulfur for 50%/PANI/epoxy. Once again, the tendency of PANI to agglomerate is observed in the cross-sections of both composites.
3.3 Evaluation of mechanical properties The mechanical properties were determined in the tensile test using Zwick/Roell Z020 testing machine for the specimens of PANI/epoxy composites according to the standard PN-EN ISO 527-2:2012 with a gauge length of 40 mm and a speed of tension of 0.5 mm/min. The tensile test was repeated four times. The resulting stress-elongation plot and the picture of the fractured specimen is presented in Fig.6. The average ultimate tensile strength (UTS) was of 13.75 MPa and the average force at fracture was of 207.5 N. The UTS value of the PANI/epoxy composite is ca. 3 times lower than the typical epoxy used in the aircraft industry, however considering the reinforcement of PANI/epoxy by carbon fabric in the next studies, this decrease of UTS seems to be negligible. The character of stress-elongation curves suggests brittle fracture of the tested composite, typical for duroplasts and composites with duroplastic matrix. However, the non-linearity of these curves suggests its viscoelastic nature.
Fig.6. Stress-elongation curves from tensile tests and a fractured specimen after the test.
3.4 Thermoanalytics Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) of the synthesized composite was performed in order to evaluate its thermal resistance and its decomposition temperature as well as to identify the temperatures corresponded with thermal transitions. The analysis was performed in the Paint and Plastics Department of the Institute for Engineering of Polymer
Materials &
Daye’s (Gliwice,
Poland) using the
thermogravimetric analyzer TGA 851e manufactured by Mettler Toledo according to the standard PN-EN ISO 11358-1:2014. The parameters of the analysis were as follows: dynamic measurement, temperature range of 25÷1000°C, heating rate of 10°C/min, oxygen atmosphere with a flow rate of 60 ml/min. The resulting TGA and DTA curves are presented in Fig.7.
Fig.7. TGA curve of PANI/epoxy composite.
In Fig.7 one can observe four temperature regions of mass loss. The obtained results show that temperature of a total decomposition of PANI/epoxy composite (617.1°C) is higher than the decomposition temperature of the epoxy in a CFRP aircraft composite (equaled 500°C and obtained using TGA) [32], which means that the developed composite has better thermal resistance than pure epoxy. This leads to a conclusion that during the lightning strike it is expected that the damage extent in the proposed composite will be lower than in the case of CFRP with an epoxy matrix.
4. Simulation and testing in the conditions of lightning strike
4.1 Thermal response in lightning strike conditions Preliminary numerical simulations of a thermal response of the developed conducting composite during the lightning strike were performed in order to evaluate the influence of a presence of the PANI particles in the composite on resulting damage sites. The problem was defined as a transient heat transfer one with a source of heating resulting from a lightning. The calculations were performed for the 30%vol. content of PANI (this value is justified by the results of numerical simulations and physicochemical characterization) in the composite and with a presence of carbon fiber (as it is planned in the further studies). The thermophysical properties of the CFRP and CF/PANI/epoxy composites were determined using the rule of mixtures based on the properties of components obtained from the literature, and then the thermal response of both composites was compared. The obtained results show that due to electrical conductivity the heat flux values for the CF/PANI/epoxy composite was ca. three orders lower than for the CFRP. The numerical study and results were described in detail in [33]. The effectiveness of CF/PANI/epoxy composite with respect to CFRP is additionally confirmed by the higher decomposition temperature (see section 3.4).
4.2 High-voltage and high-current tests The high-voltage tests were performed on 30%PANI/epoxy specimens with dimensions of 100×100 mm and a thickness of 2 mm in order to determine the character of the electrical conductivity as well as to investigate the damage extent occurring during high voltage impulse. The PANI/epoxy specimens were subjected to the voltage impulse of 1.2/50 µs with a maximal voltage value of ca. 32 kV using the generator of lightning voltage impulses (see Fig.8). The specimen was located between a wire electrode and a flat electrode as presented in Fig.9a. The tests of striking the specimens by high-voltage impulse was repeated 11 times. At the constant charge voltage of the generator of 15.5 kV the obtained value of avalanche breakdown current averaged for 11 tests equaled 1.062 kA. These results indicated that the developed PANI/epoxy composite has an electrical conductivity as high as to conduct the current between electrodes through the material (i.e. not in the form of surface currents characteristic for dielectrics). The moment of the current strike is presented in Fig.9a. Due to the concentration of the electric current on the wire electrode, which significantly exceeds the average value, the degradation of the tested specimen can be observed (see Figs.9b and 9c). This phenomenon occurs due to the very high local electric current (around the wire
electrode) which, in consequence, causes the rapid increase of a temperature and burning out the material on the current path between the electrodes.
Fig.8. Experimental setup of the high-voltage testing.
(a) (b) (c) Fig.9. PANI/epoxy specimen during the high voltage tests: at the moment of the current strike (a), resulting damage from the top (b) and from the bottom (c) of the tested specimen.
In order to determine the avalanche breakdown current the tests were repeated on 17 specimens. The resulting average value of the avalanche breakdown current for the tested composite is of 0.857 kA, which is close to the previously obtained result. Two phases can be distinguished during the high-voltage discharge through the specimen. In the first phase the electrical field in the area of the contact of wire electrode with a specimen (with the much higher intensity than the average electrical field intensity of 19 kV/mm in the investigated case) causes the avalanche breakdown. Then, the second phase of this process occurred, namely, the absolute discharge through the specimen takes place. In this case, the generator of lightning voltage impulses becomes to be the generator of lightning current impulses, i.e. it was the source of the current. During the tests the current and voltage was registered in the function of time. The selected electrical characteristics captured during lightning voltage strikes are presented in Fig.10. The current was registered on the first channel (CH1) using the 200 A/V Pearson Electronic current monitor. The second channel (CH2) was used for the registration of the voltage drop. The duration of both pulses is about 3 µs.
Fig.10. Exemplary current (CH1) and voltage (CH2) characteristics during the lightning voltage impulse.
In order to evaluate the resistance of specimens to high-current impulse the generator of lightning current impulses was used. The system with the lightning current generator, an electrical capacity of 25 µF and resistivity of ca. 9 Ω was connected to the specimen through
the wire and flat electrodes (the specimen was placed in between). The experimental system is presented in Fig.11.
(a)
(b)
Fig.11. Experimental setup of the high-current testing: before discharge (a), and during discharge (b).
The performed tests show that at the constant charge voltage of the generator of 11.1 kV the avalanche breakdown occurred. The current conducted through the specimen had lightningtype aperiodic characteristic with the maximum value of 8.04 kA. The applied impulse after the avalanche breakdown of 12/50 µs differs from the standard impulse characteristics (e.g. 8/20 µs – see e.g. [34]), however the applied impulse was enough to evaluate how the dynamic lightning current influence on the tested composite. The exemplary current and voltage characteristics are presented in Fig.12. The performed tests show that the current impulse was conducted through the material, which additionally justified its effectiveness in conducting currents similar to those observed during lightning strikes. The tested specimens were destroyed during the current impulse due to the occurrence of the acoustic wave which acted mechanically on the tested specimen. However, this is an expected behaviour taking into consideration a fact that the tests were conducted on specimens without reinforcement. The further high-voltage and high-current experimental studies assume testing of CFreinforced specimens.
Fig.12. Exemplary current (CH1) and voltage (CH2) characteristics during the lightning current impulse.
4.3 Evaluation of resulting damage after high-voltage tests The comparison of SEM images and Raman spectra of the surface of 30%/PANI/epoxy composite before and after high-voltage tests (Fig.13) showed that as a result of the highvoltage tests the surface morphology and elemental composition of the composite material were evidently changed. Because of the presence of deep surface defects in the places where voltage was applied, the surface is no longer uniform and smooth. Raman spectra collected for the defects showed that the epoxy resin was burnt and carbonized. Two strong overlapping peaks observed at 1579 cm-1 and 1376 cm-1 are typical for carbonized materials and can be assigned to the stretching vibration mode with E2g symmetry in the aromatic layers of the graphite crystalline (G band at 1579 cm-1) and disordered graphite or glassy carbons (D band at 1376 cm-1) [35,36].
Fig.13. SEM images (a,b) and Raman spectra (c) of the surface of 30%/PANI/epoxy composite before (a) and after (b) high-voltage tests.
5. Conclusions In the following paper the results of theoretical and experimental studies focused on development of the new all-polymeric material with a possibility of conducting electrical current dedicated for matrices of aircraft composite structures were presented. Preliminary computations allow for evaluation of a content of the selected conducting polymer, polyaniline, in the resulting mixture of conducting and dielectric polymers in order to reach the compromise between conductivity, mechanical strength and chemical processability. The presented results show that this compromise was successfully reached. Further numerical and experimental studies were focused on thermal, electrical and mechanical resistance to the lightning strike. The performed numerical studies accompanied with thermogravimetric
analysis show that the developed material is more resistant to thermal shocks caused by lightning strikes than typical CFRP structures. Moreover, in order to confirm conductivity of the developed material in the conditions similar to those of lightning strike, the additional high-voltage and high-current tests were performed. The obtained results clearly show the ability of conducting high-voltage and high-current charges by the developed material. They were also show structural degradation of the material, which is typical during applying such high charges. The obtained results allow for determination the next directions of improvement of the developed material for lightning strike protection applications. Due to the inhomogeneous distribution of PANI particles and their agglomeration the resulting structure is anisotropic both in mechanical, thermal and electrical sense. Such anisotropy is not a desired property of the material, therefore it is necessary to synthesize particles with smaller dimensions and develop appropriate procedures to prepare homogeneous mixtures of conducting and dielectric polymers. This additionally allows for better theoretical prediction of its properties based on developed percolation models. The next directions of improvement will be focused on physical parameters of the resulting mixture, i.e. increasing of mechanical properties as well as thermal and electrical conductivity, which allow for better adaptation of the developed material to the operational conditions, in particular for lightning strike accidents. In the next studies the developed polymer will be used for manufacturing of CFRP-based composites and the mechanical, electrical and thermal tests will be performed for this composite. The results of these tests will give a possibility to evaluate the effectiveness of the developed composite in comparison with classical CFRP composite.
Acknowledgements The research is financed from the funds granted by the Foundation for Polish Science according the contract no. 128/UD/SKILLS/2015 on realization of award granted in the INTER contest, realized within the framework of SKILLS programme, co-financed from the European Social Fund.
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S0263-8223(16)31476-3 http://dx.doi.org/10.1016/j.compstruct.2016.10.028 COST 7854
To appear in:
Composite Structures
Received Date: Accepted Date:
8 August 2016 11 October 2016
Please cite this article as: Katunin, A., Krukiewicz, K., Turczyn, R., Sul, P., Łasica, A., Bilewicz, M., Synthesis and characterization of the electrically conductive polymeric composite for lightning strike protection of aircraft structures, Composite Structures (2016), doi: http://dx.doi.org/10.1016/j.compstruct.2016.10.028
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Synthesis and characterization of the electrically conductive polymeric composite for lightning strike protection of aircraft structures Andrzej Katunin1*, Katarzyna Krukiewicz2, Roman Turczyn2, Przemysław Sul3, Andrzej Łasica3, Marcin Bilewicz4 1
Institute of Fundamentals of Machinery Design, Faculty of Mechanical Engineering, Silesian
University of Technology, Konarskiego 18A, 44-100 Gliwice, Poland 2
Department of Physical Chemistry and Technology of Polymers, Faculty of Chemistry,
Silesian University of Technology, M. Strzody 9, 44-100 Gliwice, Poland 3
Institute of Theory of Electrical Engineering, Measurement and Information Systems,
Faculty of Electrical Engineering, Warsaw University of Technology, Koszykowa 75, 00-662 Warsaw, Poland 4
Institute of Engineering Materials and Biomaterials, Faculty of Mechanical Engineering,
Silesian University of Technology, Konarskiego 18A, 44-100 Gliwice, Poland
*
corresponding author
[email protected], tel. +48 32 237 2741
Abstract In this work, the development and testing of the new all-polymeric conductive composite material dedicated for the manufacturing of matrices of carbon fiber-reinforced composites for lightning strike protection applications was described. The synthesis procedure described in the paper allows obtaining the highly-conductive composition of PANI/epoxy mixture, where the content of particular polymers was evaluated basing on numerical simulations of electrical percolation between conductive particles in the dielectric matrix. The performed physicochemical, mechanical and thermal tests reveal acceptable stability of the developed material for applications as a matrix of structural composites dedicated to the aircraft applications. Finally, the numerical and experimental evaluation of behavior of the developed composite material in the conditions of the lightning strike indicate several advantages of application of the developed material in the aircraft industry with respect to the traditional CFRP structures.
Keywords Electrically conductive polymeric composite; lightning strike protection; aircraft structures; polyaniline
1. Introduction A still increasing use of polymeric composites in manufacturing of aircraft structures contributes to the decreasing of overall mass of vehicles, which leads to decreasing of fuel consumption and reduction of emissions to the environment, and simultaneously retaining or even increasing their mechanical properties. The use of polymeric composites, besides their numerous advantages, has a drawback: the polymers used in aircraft industry are electrically insulating, which causes that such structures are compliant to the lightning strikes and derivative effects like extremal heating in the area of plasma channel, which leads to the thermal decomposition of a polymeric matrix and pyrolytic processes in this area and surrounding and ablation of the reinforcement; generation of an acoustic shock waves, which influence mechanically on the stroked composite structure, etc. [1-3]. Therefore, the appropriate lightning strike protection (LSP) solutions are necessary to prevent or at least lower the damaging of aircraft composite structures during the lightning strike. Current LSP solutions used in commercial aircraft composite structures are based on impregnation of meshes or foils made of highly conducting metals and alloys. Such solutions reveal significant decrease of damaged area after the lightning strike, which is justified by numerous theoretical and experimental studies [4-9], however, they cause increasing of mass of structural elements and significantly complicate the manufacturing process. It should be mentioned that the solutions based on impregnation of metallic meshes and foils allow for decreasing the damage extent, but do not eliminate the problem. Thus, the new solutions that allow increasing resistance to atmospheric discharges and simplify the manufacturing process are highly desired. Several attempts in LSP solutions have been made during last decades. These solutions cover using metallized sprays and coatings, metallization of reinforcing fibers and dispersion of micro- and nanoscale conducting particles (see the review in [3]). However, in the case of using metallized sprays and coating the content of metallic particles is quite high, while the LSP effectiveness is low, therefore such solution found an application primarily in electromagnetic shielding of aircraft [1]. Covering of the reinforcement by metallic layers or dispersing metallic particles in the polymeric matrix of a composite increases the mass of a structure, since the content of particles should be high (in order to form critical percolation cluster) [10,11]. Moreover, the problems with adhesion between metallic particles and polymer may appear. The last group of above-presented solutions is based on dispersion of carbon nanostructures (CNS) in the dielectric matrix [12-14], which is an effective alternative to the impregnated metallic meshes, i.e. CNS increase significantly the electrical conductivity
of a resulting material, however, such solutions are still too expensive to apply them into industrial manufacturing of structural elements of aircraft. Another solutions which may solve the above-mentioned problems is a possibility of using the intrinsically conducting polymers (ICPs) as a conducting filler of composites. Their application allows increasing electrical conductivity significantly, while the mass of the resulting composite structure remains similar. Several studies on using ICPs as conductive fillers for LSP applications have been performed to date. Jeon et al. [15] proposed composite made of polycarbonate (PC) and polyaniline (PANI), Jia et al. [16] investigated composites of epoxy resin and PANI-dodecylbenzenesulphonic acid, similar composites were studied by the authors of [17] in terms of their electrical properties. A special attention should be paid to the studies of Yokozeki and his team [18-20], where the authors described synthesis, mechanical and electrical properties as well as lightning damage resistance of CFRP composites with a matrix of PANI/epoxy composite with various dopants. As the previous and on-going research studies ICP-based composites show, this solution seems to be a promising direction for LSP applications, and the doped PANI seems to be the mostly studied conductive filler in such composites. The following study presents recent attempts of the group of authors on modeling, simulating, and then synthesis and characterization of PANI/epoxy composites which can be considered as a matrix of CFRP aircraft composites with increasing resistance to the lightning strike damaging. In this study the authors reported the concept of the developed material formulated basing on the results of numerical simulations, description of synthesis process as well as determination of fundamental physicochemical, mechanical, thermal and electrical properties of the developed material. The final studies cover high-voltage tests and characterization of damage sites caused by high-voltage electrical discharges.
2. Concept of electrically conducting polymeric composite Among the wide range of ICPs, only several can be applied commercially, i.e. polypyrrole (PPy), poly(3,4-ethylenedioxythiophene) (PTh) and PANI. Exhibiting high conductivities together with chemical and electrochemical stabilities makes them advantageous materials to be used in surface and coatings technology [21]. They could be also considered as a filler of a matrix of composite with LSP properties. The lack of mechanical stability, low processability and flexibility, however, are the main limitations for the use of conducting polymers and the main reasons for the development of composite materials.
In order to avoid the problem of mechanical instability of ICPs it is suitable to mix them with polymers with fine mechanical properties, typical for aircraft industry. The polymer used for manufacturing GFRP and CFRP interior fuselage elements is almost always the epoxy resin, which is a dielectric. Thus, it is necessary to reach a compromise between the properties of electrically conductive and dielectric polymers mixed together in order to obtain the polymer with sufficiently fine mechanical properties and electrical conductivity. Therefore, the optimal content of electrically conductive filler in the insulating matrix should be determined. This problem can be resolved by using the percolation theory. In order to determine the content of a conductive fillers of various types in the developed conducting polymer the numerical simulations were performed using the Monte Carlo simulation method and modified Dijkstra’s algorithm. Due to the specificity of the investigated problem the classical percolation conditions were modified in such a way that all of the sides of a domain should be connected by the critical percolation cluster, i.e. the resulting material should be fully electrically conductive regardless the selected sides of input and output of current. Naturally, this modification slightly increase the percolation threshold but ensures the representativeness of the considered domain, and allows for its generalization to higher dimensions due to the fractal properties of the critical percolation cluster (see e.g. [22]). This was also conducted in the numerical simulations performed for the PPy/epoxy mixture [23]. The conducting particles were modeled using the hard-core soft-shell model which allows obtaining more realistic conditions of the modeled phenomena. This model assumes that conductive particles (parametrized by their constant radius and spatiallocation parameters) were generated using the Poisson process ofthe constant concentration rate and the generated particles cannot interpenetrate each other, but their shells considered here as a hopping distance can. The phenomenon of hopping of electrons between conducting particles was considered in the model, which allows decreasing the percolation threshold significantly. The simulations were performed for three preliminarily selected ICPs: PANI, PPy and PTh. All the simulations were performed based on literature data and the obtained critical percolation thresholds were 35.7%, 0.1% and 38.3%, respectively. For more detailed description of the model and the obtained results see [24]. Due to the best mechanical stability and processing properties as well as availability, PANI was selected as a conductive filler in the further studies.
3. Synthesis and characterization of the conducting composite
3.1 Synthesis procedure For the synthesis of polyaninine (PANI), aniline (Acros, purity 99.8%), sulfuric acid (Acros, concentration 96%, purity 99%), ammonium persulfate (POCH, purity 99.9%), methanol (POCH, purity 99.9%) , ammonia (POCH, concentration 25%, purity 99.9%) and 10-camphor sulfonic acid (TCI, purity > 98%) were used as received. The epoxy resin was made from commercially available epoxide Epidian 6 (Ciech, MW < 700 g/mol) and amine hardener IDA (Ciech) with the volume content of PANI from 20 % to 70 %. The synthesis of polyaniline (PANI) consisted of the dissolution of 5.50 mol (51.1 g) of aniline monomer in 1500 ml of 1 M H2SO4. The mixture was pre-cooled to -3 oC in an ice-salt bath. Then, 0.24 mol (54.88 g) of 1.5 M (NH4)2S2O8 was dissolved in 160 ml of 1 M H2SO4 and added dropwise while stirring at -3 oC temperature. After 24 hours the resulting precipitate was collected on a Buchner funnel, repeatedly rinsed with deionized water until neutral pH and purified in the Soxhlet apparatus with CH3OH. PANI was then deprotonated in 1800 ml of 1.5 M NH3 aq solution. After 24 hours, PANI was filtrated and washed with deionized water until neutral pH. The resulting dark-blue precipitate of polymer was dried under vacuum for 24 hours. The overall reaction yield was equal to 63.6 %. The chemical structure of synthesized PANI was confirmed by means of Raman and IR spectroscopy [25]. Polyaniline was then doped with camphorsulfonic acid, CSA, by mixing PANI with CSA in a proportion by weight of 1:3 in the vibrational mill LMW (Testchem) operating for 5, 10 or 15 min. According to [26], mechanical mixing in the presence of CSA is supposed to increase the protonation degree of PANI. To determine the conductivity of resulting material, PANI/CSA mixture was placed on the surface of glass slides and dried in the oven. The conductivities of PANI/CSA films measured for mixtures blended for 5, 10 and 15 minutes showed no dependence on the time of mixing and were equal to 6.93 ± 1.38 S/cm (5 min), 7.61 ± 1.35 S/cm (10 min) and 6.14 ± 2.51 S/cm (15 min). CSA-protonated PANI synthesized in the previous step was used to prepare PANI/epoxy composites, containing different volume percentage of PANI (20%, 30%, 40%, 50%, 60%, 70%). With the increase in PANI content, the significant increase in the viscosity of mixture was observed. The volume concentration of PANI equal to 50% was found to be the highest amount of conductive filler to obtain liquid composition and enable the process of molding.
3.2 Physicochemical characterization The chemical structure of PANI and PANI/epoxy composites was characterized by means of Raman spectroscopy using Renishaw InVia confocal microRaman system equipped with laser operating at 830 nm, and a CCD detector. The morphology, thickness and elemental composition of materials were measured by means of scanning electron microscope Phenom ProX equipped with 3D Roughness Reconstruction software and energy dispersive X-ray spectroscope (EDS) analyzer. The dual mode bipotentiostat Autolab PGSTAT302N + BA was used to measure the conductivity of materials with the use of interdigitated electrode [27]. The measurements were performed in a chronoamperometric mode at a constant potential application (-0.5 V) for 60 s. The conductivity of samples was calculated according to [27] and [28]. After their synthesis, the PANI/epoxy composites were investigated by means of SEM. The analysis of the morphology of cross-sections of composite materials (Fig.1) reveals the presence of PANI agglomerates within the composite material, especially for composites with low PANI content (20-30%). When the amount of PANI is increased (50-70%), conducting filler is more uniformly distributed among epoxy resin. The conductivity measurements performed by means of four point probe indicated high resistance for the composites with lower volume percentage of PANI than 50%. The tendency to agglomerate and form clusters within the composite materials is responsible for the increase in the amount of conductive filler necessary to fulfill the percolation requirements. Two composite materials have been chosen for further research, i.e. PANI/epoxy with 30% and 50% content of conducting filler. These two materials have been selected due to the coincidence with a theoretical values in the first case, and due to the fact that 50% volume concentration was the highest amount of PANI enabling the process of molding. The determination of conductivity of selected samples was performed by means of bipotentiostatic approach. The conductivity of PANI/epoxy composite with 50% of conductive filler, calculated based on chronoamperometric curve (Fig.2) was equal to 0.440±0.330 S/cm, while the conductivity of composite with 30% content of PANI was below the limit of detection. It was substantially increased, however, when the surface of composite was polished with the abrasive paper. After this type of preconditioning, the conductivity of the sample increased to the value of 0.184 ± 0.132 S/cm. This indicates that the epoxy resin may form the outer shell of the composite isolating PANI from the surrounding.
Fig.1. SEM images of cross-sections of PANI/epoxy composites with different amount of conductive filler: 20% (a), 30% (b), 40% (c), 50% (d), 60% (e) and 70% (f). 100 µm
Fig.2. Chronoamperometric curve for PANI/epoxy composite (50 % PANI), recorded in bipotentiostatic mode for Eapplied = -0.5 V and Eoffset = 0.01 V.
To verify this hypothesis, Raman spectra of the surface and cross-sections of PANI/epoxy composites with 30 % and 50 % content of conductive filler were collected (Fig.3). In both spectra peaks and bands characteristic for PANI are present, i.e. a sharp peak around 1620 cm-1 which is assigned to the C–C stretching of the benzenoid ring [29-31] and a strong band observed around 1590 cm-1 linked to C=C stretching vibration in the quinonoid ring [2931]. The band at 1505 cm-1 may be assigned to an N–H deformation vibration associated with the semi-quinonoid structures [29,31]. The band at 1170 cm-1 corresponds to the C–H bending vibration of the semiquinonoid rings (cation-radical segments) [29,30] with a shoulder at 1180 cm-1 corresponding to C–H in-plane bending vibrations in benzenoid rings [29]. A broad structural band with local maxima at 870, 840, 780 and 745 cm−1 (corresponding to C–N–C wagging and benzene ring deformations) is present [29-31]. In both cases, only the Raman spectra of cross-sections exhibit a band at 1346 cm-1 which may be assigned to C–N+ vibration of delocalized polaronic structures [29,30] indicating the presence of PANI in its doped state. It shows that there is a substantial difference in chemical composition of surface and cross-section within one type of composite, confirming the hypothesis of the effect of insulating layer covering the composite material on its conductivity.
(a)
(b)
Fig.3. Raman spectra of the surface and cross-sections of epoxy/PANI composites with 30% (a) and 50% (b) PANI.
EDS elemental characterization of surface (Fig.4) and cross-section (Fig.5) of PANI/epoxy composites with 30% and 50% PANI shows the variation of the concentration of conductive filler across the samples.
Fig.4. SEM images and elemental composition of surface of PANI/epoxy composites with 30% (a,b) and 50% (c,d) PANI.
Fig.5. SEM images and elemental composition of the cross-sections of PANI/epoxy composites with 30% (a,b) and 50% (c,d) PANI.
The surface of 30%/PANI/epoxy is depleted with conducting filler; the atomic percentage of sulfur (present in CSA) is only 0.2%, while nitrogen (present in PANI) is not detected. When the content of conducting filler in PANI/epoxy resin is increased to 50%, both sulfur (1.0%) and nitrogen (0.9%) are present on the surface. These elements are also present in the crosssection of both types of composites, their atomic percentage equals 9.6 % nitrogen and 1.1% sulfur for 30%/PANI/epoxy, and 10.7% nitrogen and 1.8% sulfur for 50%/PANI/epoxy. Once again, the tendency of PANI to agglomerate is observed in the cross-sections of both composites.
3.3 Evaluation of mechanical properties The mechanical properties were determined in the tensile test using Zwick/Roell Z020 testing machine for the specimens of PANI/epoxy composites according to the standard PN-EN ISO 527-2:2012 with a gauge length of 40 mm and a speed of tension of 0.5 mm/min. The tensile test was repeated four times. The resulting stress-elongation plot and the picture of the fractured specimen is presented in Fig.6. The average ultimate tensile strength (UTS) was of 13.75 MPa and the average force at fracture was of 207.5 N. The UTS value of the PANI/epoxy composite is ca. 3 times lower than the typical epoxy used in the aircraft industry, however considering the reinforcement of PANI/epoxy by carbon fabric in the next studies, this decrease of UTS seems to be negligible. The character of stress-elongation curves suggests brittle fracture of the tested composite, typical for duroplasts and composites with duroplastic matrix. However, the non-linearity of these curves suggests its viscoelastic nature.
Fig.6. Stress-elongation curves from tensile tests and a fractured specimen after the test.
3.4 Thermoanalytics Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) of the synthesized composite was performed in order to evaluate its thermal resistance and its decomposition temperature as well as to identify the temperatures corresponded with thermal transitions. The analysis was performed in the Paint and Plastics Department of the Institute for Engineering of Polymer
Materials &
Daye’s (Gliwice,
Poland) using the
thermogravimetric analyzer TGA 851e manufactured by Mettler Toledo according to the standard PN-EN ISO 11358-1:2014. The parameters of the analysis were as follows: dynamic measurement, temperature range of 25÷1000°C, heating rate of 10°C/min, oxygen atmosphere with a flow rate of 60 ml/min. The resulting TGA and DTA curves are presented in Fig.7.
Fig.7. TGA curve of PANI/epoxy composite.
In Fig.7 one can observe four temperature regions of mass loss. The obtained results show that temperature of a total decomposition of PANI/epoxy composite (617.1°C) is higher than the decomposition temperature of the epoxy in a CFRP aircraft composite (equaled 500°C and obtained using TGA) [32], which means that the developed composite has better thermal resistance than pure epoxy. This leads to a conclusion that during the lightning strike it is expected that the damage extent in the proposed composite will be lower than in the case of CFRP with an epoxy matrix.
4. Simulation and testing in the conditions of lightning strike
4.1 Thermal response in lightning strike conditions Preliminary numerical simulations of a thermal response of the developed conducting composite during the lightning strike were performed in order to evaluate the influence of a presence of the PANI particles in the composite on resulting damage sites. The problem was defined as a transient heat transfer one with a source of heating resulting from a lightning. The calculations were performed for the 30%vol. content of PANI (this value is justified by the results of numerical simulations and physicochemical characterization) in the composite and with a presence of carbon fiber (as it is planned in the further studies). The thermophysical properties of the CFRP and CF/PANI/epoxy composites were determined using the rule of mixtures based on the properties of components obtained from the literature, and then the thermal response of both composites was compared. The obtained results show that due to electrical conductivity the heat flux values for the CF/PANI/epoxy composite was ca. three orders lower than for the CFRP. The numerical study and results were described in detail in [33]. The effectiveness of CF/PANI/epoxy composite with respect to CFRP is additionally confirmed by the higher decomposition temperature (see section 3.4).
4.2 High-voltage and high-current tests The high-voltage tests were performed on 30%PANI/epoxy specimens with dimensions of 100×100 mm and a thickness of 2 mm in order to determine the character of the electrical conductivity as well as to investigate the damage extent occurring during high voltage impulse. The PANI/epoxy specimens were subjected to the voltage impulse of 1.2/50 µs with a maximal voltage value of ca. 32 kV using the generator of lightning voltage impulses (see Fig.8). The specimen was located between a wire electrode and a flat electrode as presented in Fig.9a. The tests of striking the specimens by high-voltage impulse was repeated 11 times. At the constant charge voltage of the generator of 15.5 kV the obtained value of avalanche breakdown current averaged for 11 tests equaled 1.062 kA. These results indicated that the developed PANI/epoxy composite has an electrical conductivity as high as to conduct the current between electrodes through the material (i.e. not in the form of surface currents characteristic for dielectrics). The moment of the current strike is presented in Fig.9a. Due to the concentration of the electric current on the wire electrode, which significantly exceeds the average value, the degradation of the tested specimen can be observed (see Figs.9b and 9c). This phenomenon occurs due to the very high local electric current (around the wire
electrode) which, in consequence, causes the rapid increase of a temperature and burning out the material on the current path between the electrodes.
Fig.8. Experimental setup of the high-voltage testing.
(a) (b) (c) Fig.9. PANI/epoxy specimen during the high voltage tests: at the moment of the current strike (a), resulting damage from the top (b) and from the bottom (c) of the tested specimen.
In order to determine the avalanche breakdown current the tests were repeated on 17 specimens. The resulting average value of the avalanche breakdown current for the tested composite is of 0.857 kA, which is close to the previously obtained result. Two phases can be distinguished during the high-voltage discharge through the specimen. In the first phase the electrical field in the area of the contact of wire electrode with a specimen (with the much higher intensity than the average electrical field intensity of 19 kV/mm in the investigated case) causes the avalanche breakdown. Then, the second phase of this process occurred, namely, the absolute discharge through the specimen takes place. In this case, the generator of lightning voltage impulses becomes to be the generator of lightning current impulses, i.e. it was the source of the current. During the tests the current and voltage was registered in the function of time. The selected electrical characteristics captured during lightning voltage strikes are presented in Fig.10. The current was registered on the first channel (CH1) using the 200 A/V Pearson Electronic current monitor. The second channel (CH2) was used for the registration of the voltage drop. The duration of both pulses is about 3 µs.
Fig.10. Exemplary current (CH1) and voltage (CH2) characteristics during the lightning voltage impulse.
In order to evaluate the resistance of specimens to high-current impulse the generator of lightning current impulses was used. The system with the lightning current generator, an electrical capacity of 25 µF and resistivity of ca. 9 Ω was connected to the specimen through
the wire and flat electrodes (the specimen was placed in between). The experimental system is presented in Fig.11.
(a)
(b)
Fig.11. Experimental setup of the high-current testing: before discharge (a), and during discharge (b).
The performed tests show that at the constant charge voltage of the generator of 11.1 kV the avalanche breakdown occurred. The current conducted through the specimen had lightningtype aperiodic characteristic with the maximum value of 8.04 kA. The applied impulse after the avalanche breakdown of 12/50 µs differs from the standard impulse characteristics (e.g. 8/20 µs – see e.g. [34]), however the applied impulse was enough to evaluate how the dynamic lightning current influence on the tested composite. The exemplary current and voltage characteristics are presented in Fig.12. The performed tests show that the current impulse was conducted through the material, which additionally justified its effectiveness in conducting currents similar to those observed during lightning strikes. The tested specimens were destroyed during the current impulse due to the occurrence of the acoustic wave which acted mechanically on the tested specimen. However, this is an expected behaviour taking into consideration a fact that the tests were conducted on specimens without reinforcement. The further high-voltage and high-current experimental studies assume testing of CFreinforced specimens.
Fig.12. Exemplary current (CH1) and voltage (CH2) characteristics during the lightning current impulse.
4.3 Evaluation of resulting damage after high-voltage tests The comparison of SEM images and Raman spectra of the surface of 30%/PANI/epoxy composite before and after high-voltage tests (Fig.13) showed that as a result of the highvoltage tests the surface morphology and elemental composition of the composite material were evidently changed. Because of the presence of deep surface defects in the places where voltage was applied, the surface is no longer uniform and smooth. Raman spectra collected for the defects showed that the epoxy resin was burnt and carbonized. Two strong overlapping peaks observed at 1579 cm-1 and 1376 cm-1 are typical for carbonized materials and can be assigned to the stretching vibration mode with E2g symmetry in the aromatic layers of the graphite crystalline (G band at 1579 cm-1) and disordered graphite or glassy carbons (D band at 1376 cm-1) [35,36].
Fig.13. SEM images (a,b) and Raman spectra (c) of the surface of 30%/PANI/epoxy composite before (a) and after (b) high-voltage tests.
5. Conclusions In the following paper the results of theoretical and experimental studies focused on development of the new all-polymeric material with a possibility of conducting electrical current dedicated for matrices of aircraft composite structures were presented. Preliminary computations allow for evaluation of a content of the selected conducting polymer, polyaniline, in the resulting mixture of conducting and dielectric polymers in order to reach the compromise between conductivity, mechanical strength and chemical processability. The presented results show that this compromise was successfully reached. Further numerical and experimental studies were focused on thermal, electrical and mechanical resistance to the lightning strike. The performed numerical studies accompanied with thermogravimetric
analysis show that the developed material is more resistant to thermal shocks caused by lightning strikes than typical CFRP structures. Moreover, in order to confirm conductivity of the developed material in the conditions similar to those of lightning strike, the additional high-voltage and high-current tests were performed. The obtained results clearly show the ability of conducting high-voltage and high-current charges by the developed material. They were also show structural degradation of the material, which is typical during applying such high charges. The obtained results allow for determination the next directions of improvement of the developed material for lightning strike protection applications. Due to the inhomogeneous distribution of PANI particles and their agglomeration the resulting structure is anisotropic both in mechanical, thermal and electrical sense. Such anisotropy is not a desired property of the material, therefore it is necessary to synthesize particles with smaller dimensions and develop appropriate procedures to prepare homogeneous mixtures of conducting and dielectric polymers. This additionally allows for better theoretical prediction of its properties based on developed percolation models. The next directions of improvement will be focused on physical parameters of the resulting mixture, i.e. increasing of mechanical properties as well as thermal and electrical conductivity, which allow for better adaptation of the developed material to the operational conditions, in particular for lightning strike accidents. In the next studies the developed polymer will be used for manufacturing of CFRP-based composites and the mechanical, electrical and thermal tests will be performed for this composite. The results of these tests will give a possibility to evaluate the effectiveness of the developed composite in comparison with classical CFRP composite.
Acknowledgements The research is financed from the funds granted by the Foundation for Polish Science according the contract no. 128/UD/SKILLS/2015 on realization of award granted in the INTER contest, realized within the framework of SKILLS programme, co-financed from the European Social Fund.
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