Effect of the areal weight of CNT-doped veils on CFRP electrical properties

Nonwoven veils fabrication

Thermoplastic copolyamide (coPA), which belongs to the group of hot melt adhesives, was used as a polymer selected to nonwovens fabrication. This polymer possesses melting range adjusted to CFRP fabrication conditions. It was used in the form of masterbatch doped with 10 wt.% of multi-wall carbon nanotubes (NC7000, Nanocyl, Belgium) with 9.5 nm diameter, 1.5 µm length and 90% purity, which are presented in Figure 1. The masterbatch, which was also supplied by Nanocyl, was subjected to the microstructure observations in order to evaluate the dispersion of the nanotubes in the polymer matrix. Figure 2(a) shows the image form scanning transmission electron microscopy (STEM) and Figure 2(b) from light optical microscopy. Both images confirm the presence of nanotubes agglomerates in the polymer matrix. OPEN IN VIEWER

OPEN IN VIEWER

Figure 2.

The image of coPA + 10 wt%MWCNT pellets (a) from STEM and (b) from light optical microscope; (c) extruded nanocomposite fiber with marked diameters.

Thermoplastic nonwovens were produced in a two-step process: in the first step, the vacuum-dried masterbatch was processed on a laboratory twin-screw extruder, resulting in nanocomposite fibres. Then, the extruded fibers were cut into 70 mm-long pieces that were randomly deposited on a special carrier and pressed on both sides using a thermomechanical press. The whole process was described in PL 221848. ²² The diameter of fabricated fibers was examined using scanning electron microscopy and its average value was 372.1 ± 32.01 µm. Figure 2(c) shows an example of extruded nanocomposite fiber.

Three types of nonwoven veils were made with different areal weight (GSM): 15, 30 and 45 GSM, respectively. The thickness of each veil was measured using a micrometer screw with an accuracy of 0.001 mm and was equal to 113 ± 10.1 µm, 124 ± 11.3 µm and 136 ± 14.1 µm for 15 GSM, 30 GSM and 45 GSM, respectively. The obtained thickness of nonwovens is the result of double-sided pressing of nonwovens at temperature – 170 ± 3℃ and pressure – 0.7 bar. As a result, the fibers in the nonwoven fabric have an elliptical cross-section, so that the thickness of the veils is less than the average of a single nanocomposite fibre. Figure 3(a) to (c) shows the obtained nonwovens which were collected using digital camera and binarized in a graphic program. OPEN IN VIEWER

Laminates fabrication

Four types of laminate panels were fabricated using commercial carbon-epoxy unidirectional prepreg with 12 k of carbon fibers, 33% weight fraction of epoxy resin and the areal weight of 145 g/m². The first one – reference laminate – was only made of 14 layers of prepreg whose arrangement was [0]₁₄. In the case of other laminates, the thermoplastic nonwovens were placed between each prepreg layer. As for the reference laminate, the modified laminates consisted of 14 layers of prepreg and 13 layers of nonwovens. The thickness of the manufactured laminates and the weight fraction of carbon fibers are presented in Table 1. All composites were manufactured using out of autoclave method cured according to manufacturer’s recommendations which is presented in Figure 4. OPEN IN VIEWER

OPEN IN VIEWER

Table 1.

Characteristics of manufactured laminate panels.

Material	Laminate designation	Average thickness (mm)	Carbon fibers weight fraction (%)
Reference laminate	Lref	2.19	67.56
Laminate with veil coPA + MWCNT (15 GSM)	L1	2.32	63.91
Laminate with veil coPA + MWCNT (30 GSM)	L2	2.56	59.89
Laminate with veil coPA + MWCNT (45 GSM)	L3	2.75	56.85

After curing, laminates were cut to the samples with the dimensions according to the selected standards using the cutting-off machine from Dewalt.

Methods

To investigate the effect of thermoplastic nonwovens of different areal weight on CFRP electrical conductivity, volume and surface electrical conductivity were examined. In the case of electrical volume conductivity, it was measured in three directions: along the carbon fibers (X-direction), perpendicular to the carbon fibers (Y-direction) and through the laminate thickness (Z-direction). The Keithley 6221/2182A device equipped with a measuring cell with copper electrodes (Figure 5) was used to test samples with the following dimensions: 80 mm × 10 mm (for measurements in X and Y directions) and 15 mm × 15 mm (for measurements in Z direction), where the distance between the electrodes was equal to the thickness of the samples. In each case, five samples were tested, performing eight measurements on each of them. Measurements were made using the four-point method, in the delta mode, which ensures the elimination of thermoelectric forces during the measurement of resistance. To improve the contact between the sample and the electrodes, a silver conductive paste (CW7100, Chemtronics) was used. In addition, each sample before the test was cleaned using acetone to remove dust and dirt. OPEN IN VIEWER

The 6517B Electrometer/High Resistance Meter equipped with an 8009 Test Fixture, which was presented in Figure 6, was used to measure electrical surface conductivity, according to ASTM D-257. Five samples from different laminate area with dimensions 70 mm × 70 mm were placed between stainless-steel concentric ring electrodes with a conductive rubber pad. This solution allows a very good contact between the sample and the electrodes, and the use of a conductive silver paste or carbon tape is unnecessary. The test parameters were as follows: voltage applied was 1 V, number of readings was adjusted to 5 and the test lasted 15 s. OPEN IN VIEWER

The microstructure observations of manufactured laminates were performed using a scanning electron microscope (SEM, TM 3000 Hitachi, Japan). Samples with dimensions of 10 mm × 10 mm were polished using papers of three types of graininess: 240, 800 and 1200 and then coated with layer of gold and palladium and the sputtering parameters were as follows: voltage – 1.5 kV, current – 8–10 mA, time – 80 s. Observations of the prepared samples were made at an acceleration voltage of 15 kV. SEM was also used to analyze microstructure of laminates after mechanical tests with the same applied voltage.

The interlaminar shear strength (ILSS) was examined using MTS Q/Test 10 testing machine. Eight samples were tested according to ASTM D2344 and they were 25 mm in length and 6.4 mm in width. In addition, the test was carried out at room temperature and the speed was 1 mm/min. Samples for all tests were cut from different sections of the composite panels.

Electrical conductivity

The obtained results of electrical volume conductivity in all directions are presented in Figure 7(a) to (c). The largest increase of electrical conductivity was obtained for the nonwoven of 30 GSM. Along the carbon fibers direction, it was 2623 ± 305 S/m, perpendicular to the carbon fibers – 8.7 ± 0.83 S/m and through the laminate thickness was 3.16 ± 0.403 S/m, while the reference laminate was 2066 ± 126 S/m, 7.3 ± 0.46 S/m and 0.69 ± 0.082 S/m, respectively. In the case of nonwoven fabrics with an areal weight of 15 GSM, there was a decrease of about 18,6% in the electrical conductivity along the carbon fibers and increase in the Y and Z directions of about 10.1% and 45.5%, respectively in comparison to reference one. The lower values compared to 30 GSM could be caused by the fact that for 15 GSM too small number of fibers was present per surface area. A similarity as for nonwovens with 15 GSM was observed for 45 GSM. The decrease in electrical conductivity along the carbon fibres and the increase in other directions were found. In this case, the lower values of conductivity obtained than in the case of 30 GSM can be explained by too high amount of thermoplastics between carbon fiber layers as well as the highest veil thickness what caused the increase of distance between carbon fibres layers and increase of epoxy resin amount. It also resulted in decrease of carbon fibres weight fraction in composite. The increase in electrical conductivity in Z-direction of all modified laminates was caused by the creation of conductive paths by carbon nanotubes in thermoplastic nonwovens. ²³ In addition, the observed agglomerates of carbon nanotubes in the structure of nonwovens have an impact on the standard deviation of electrical conductivity results. OPEN IN VIEWER

The same situation was observed in the case of surface electrical conductivity (Figure 7(d)). The surface electrical conductivity of the reference laminate was 2.79E-05 ± 1.79E-06 S and the highest increase in conductivity was obtained when using a nonwoven veil with an areal weight of 30 GSM. In this case, the increase of electrical conductivity was 96.8% compared to the reference laminate, while in other cases 38.7% and 88.9% for 15 GSM and 45 GSM, respectively. The increase in surface conductivity is caused by the fact that modification using CNT-doped veils allowed to improve conductivity twice in comparison to reference one, which is due to the fact that the current does not flow only on the surface, but also on the upper part (volume) of the material where the nonwovens were introduced. ²⁴

Microstructure observations

The microstructure images of manufactured CFRP panels are shown in Figure 8(a) to (d). Single layers of prepreg are visible in the reference laminate, and in the case of modified laminates - also layers of thermoplastic nonwovens. The nonwovens were melted during CFRP manufacturing process which re-shaped the CNT-doped thermoplastic fibres. Due to coPA sticky properties, they adhere to carbon fibres of the reinforcement well. The porosity of the laminates was determined using the ImageJ program. The images from SEM were binarized and then the pores, which were marked with black small dots, were counted. All laminates were characterized by very low porosity, marked as: L_ref – 0.11 ± 0.02%, L₁ – 0.08 ± 0.04%, L₂ – 0.48 ± 0.15 % and L₃ – 0.33 ± 0.11%, which proves the high quality of produced laminates. It was possible to obtain this by applying the appropriate temperature profile and pressure in the crosslinking process. Furthermore, due to the significant thickness of nanocomposite fibres, the presence of the epoxy resin was observed at the ends of the nonwoven fabric which was a disadvantageous effect because the polymeric matrix is an insulator and it can limit the increase of electrical conductivity. OPEN IN VIEWER

Mechanical properties

A short-beam strength test was carried out to determine the interlaminar shear strength (ILSS) of the manufactured laminates. The results are presented in Figure 9. The highest strength value was obtained for the reference laminate and in the case of modified laminates, a decrease of interlaminar shear strength was observed. The highest ILSS for laminates with thermoplastic nonwovens doped with CNTs was obtained for L₂. It was equal to 87.1 ± 5.99 MPa, compared to the reference laminate L_ref, where the ILSS was 119.2 ± 0.88 MPa. Decrease of ILSS could be explained by reducing the weight fraction of carbon fibres, as presented in Table 1. OPEN IN VIEWER

The implementation of thermoplastic nonwovens into the composite structures leads to different type of cracking of the samples during the mechanical loading. In the case of modified laminates, after reaching the maximum force, CFRP deformed plastically, when the reference laminate broke fragile after reaching the maximum force. However, the modified laminates were characterized by higher deformation. To achieve higher strength, nonwovens should be as thin as possible. In addition, microstructure observations after the strength tests were carried out, as shown in Figure 10(a) to (d). In the case of modified laminates, much more cracks were observed which caused lower strength. It could be also concluded that the thickness of the nonwovens influenced on the capability of cracks formation in the area of the epoxy resin, which was accumulated at the end of thermoplastic fibres. OPEN IN VIEWER