Determination of carbon nanotubes penetration level into epoxy-woven glass fibre composite laminates manufactured with vacuum infusion process via electrical conductivity measurement

Abstract

This study investigates the carbon nanotube (CNT) penetration phenomenon in vacuum infusion (VI) process by means of electrical conductivity measurement. Effects of resin temperature, CNT content, and the processing technique on the overall composite laminate quality were investigated by monitoring electrical properties of composite sheets. Epoxy/woven glass fibre composites loaded with various CNT contents were also manufactured using compression moulding and vacuum bagging techniques for comparison purposes. VI was achieved at two different temperatures (25°C and 50°C) to study the effect of variation in viscosity on CNT penetration during infusion. The results showed that electrical conductivity measurement was a very effective tool to assess the CNT penetration state of composite laminates in VI method. Both the resistive and capacitive behaviour in composite laminates due to filtration of CNTs at higher CNT contents was observed. However, the CNT penetration was improved by increasing the temperature of epoxy/CNT mixture before the infusion.

Keywords

Carbon nanotubes nanoparticles filtration penetration vacuum infusion electrical conductivity measurement fibre reinforced composites epoxy

Introduction

Fibre reinforced polymer (FRP) composites are highly desirable as primary and secondary structures in aerospace, automotive, marine, energy, infrastructure, armour, and biomedical applications due to their high specific stiffness and strength, high degree of dimensional and thermal stability, and good resistance to corrosion [1].

It has been reported that the use of nanoparticles as secondary reinforcement in FRP composites results in further improvements in electrical [2-4], thermal [4-6], and mechanical [7-9] properties leading to manufacturing new multifunctional composite materials. Among the nano reinforcements, carbon nanotubes (CNTs) are commonly used as nano fillers in FRPs due to their extraordinary mechanical [10, 11], electrical [12, 13], and thermal properties [14]. Thus, enormous efforts have been attempted to produce CNT/polymer composites [15-21] and CNT/FRP composites [22-25] for functional applications. These functionalities include enhanced interlaminar fracture behaviour [22], self-sensing of strain and damage monitoring [3, 25], and electromagnetic shielding [26] when CNTs are added to FRP composites.

The incorporation of CNTs in FRP composites can be generally achieved either by grafting CNTs onto fibres [27] or dispersing CNTs into polymer resin [28]. The addition of CNTs onto reinforcing fabrics leads to improved interfacial strength at the fibre-matrix interface [7]. Although this type of modification on the fibre surfaces leads to localised functionality at the fibre-matrix interface, the modification and functionality of epoxy resin throughout the FRP composite cannot be achieved by growing CNTs onto fibres. On the other hand, dispersion of CNTs into a polymer matrix is simple and allows industrial-scale manufacturing approach for multiscale functional FRP composites.

Incorporation of CNTs as modifier in polymer matrix or FRP composite structures requires homogenous distribution of CNTs throughout polymer resin to achieve the desired properties for advanced functionality. However, this is a challenging task because agglomerations due to electrostatic interaction and van der Waals forces might lead to limited dispersion of nano fillers in polymer matrix. Furthermore, the large surface area results in large interphase area between the filler and polymer matrix preventing the fillers to uniformly distributed throughout polymer matrix. Since CNTs have a high aspect ratio (>1000) with an extremely large surface area, the dispersion of CNTs is rather difficult compared to other modifiers such as carbon black and graphite which have a lower surface area [29-33]. Therefore, great efforts have been attempted to produce CNT/polymer composites either by using various dispersion methods [15-17, 34-38] or by applying surface modifications (functionalisation) to CNT walls [29-33] for functional and structural applications. Through functionalisation of CNTs, enhancement in the solubility, processibility, and ease of dispersion within a polymer matrix can be succeeded [29].

There are several manufacturing methods of FRP composites including autoclave moulding, compression moulding (CM), vacuum bagging (VB), and vacuum resin infusion moulding (VI) [1]. Among them, VI enables one to produce large, complex, and high-performance FRP structures at low cost. In this process, reinforcing dry fabrics are impregnated with polymer resin via liquid resin infusion under the vacuum at controlled temperature [39]. However, the penetration of CNT-filled polymer resin into the reinforcing fabrics during impregnation process might present some difficulties such as non-uniform CNT distribution throughout the fabric due to filtration of CNTs by fibre tows, especially at higher nano filler contents [24, 39-41]. Zhang et al. [42] successfully investigated the filtration effect of graphene nanoplatelets (GNPs) during resin infusion process by evaluating the electrical conductivity of composite through the infusion direction. Costa et al. [40] studied the CNT filtration in the resin transfer moulding process of carbon and glass fibre reinforced epoxy composites. Surface-modified and unmodified carbon nanotubes (CNTs) were incorporated into a commercial epoxy resin. Results showed that the scattering in conductivity values is higher in the unmodified system due to non-uniform distribution of CNTs caused by filtration. Louis et al. [43] carried out research to characterise the filtration of alumina and silica nanoparticles in the resin transfer moulding process of aramid epoxy composites. The alumina nanoparticles were found to have higher tendency of filtration than that of the silica nanoparticles due to forming microscale agglomerates. Costo and Skortos [44] developed analytical and numerical models of liquid moulding of nanoparticle-loaded thermoset composites. In their model, variables such as porosity, permeability, and viscosity as a function of nanoparticle loading were considered. The proposed models showed accurate approximation in the thickness direction. In most dispersion scenarios, the CNTs are present in both entangled form and non-dispersed aggregates within epoxy resin. During infusion process in VI method, both the entangled CNTs and non-dispersed CNT bundles can re-agglomerate due to filtration effect. This difficulty is more challenging phenomenon especially as the CNTs are used at high concentrations in the matrix due to an increase in viscosity. As a result of filtration, CNTs are more likely to be found abundant in the infusion inlet location and tend to gradually decrease in concentration through the infusion outlet. Since the CNT filtration causes inhomogeneous spatial distribution of CNTs throughout the composite structure, the required electrical, thermal, and mechanical properties throughout the composite structure cannot uniformly be obtained. Hence, it is essential to determine the appropriate infusion parameters such as CNT concentration and resin temperature prior to the infusion for manufacturing functional FRP composite structures.

In this study, the influence of multiwalled carbon nanotube (MWCNT) modified epoxy resin temperature on the penetration phenomenon during infusion in VI process for various CNT contents was reported. Woven glass fibre/epoxy composites reinforced with various CNT contents were manufactured using CM and VB processing techniques for comparison purpose. The effect of resin temperature, CNT content, and the processing technique on the overall composite laminate quality were investigated by electrical conductivity measurements. The electrical percolation threshold and the viscosity of unreinforced CNT/epoxy nanocomposites were determined prior to fibre reinforced composite fabrication.

Experimental

Materials

2D woven glass fibres with an areal density of 200 g m⁻² (METYX) were used as reinforcement material in nanocomposite laminates. An epoxy resin as matrix material was purchased from Duratek™, Turkey. This matrix material was composed of two components: DTE 1000 resin and DTS 1105 hardener. The mixing ratio of resin to hardener was 100:35 by weight. Multi-wall carbon nanotubes (MCNTs) were employed as carbon nanofillers in the epoxy composites. MWCNTs were kindly donated by Nanografi Co. Ltd. Turkey. Some physical properties of the polymer and fillers are listed in Table 1.

Table 1

Some physical properties of the materials used in the study.

Property	Epoxy	CNT
Commercial name	DTE1000	NG01MW0314
Density (g cm⁻³)	1.2	0.25
Viscosity (Pa.s)	150
T_g (^oC)	60
Surface area (m²g⁻¹)		250–300

Laminate fabrication

The probe ultrasonication was employed for the dispersion purpose of CNTs within epoxy. The ultrasonication procedure can be found in the study previously published by authors [45]. Fifteen plies of woven glass/epoxy composite laminates filled with 0%, 0.2%, 0.4%, and 0.8% wt. MWCNTs were manufactured using CM, VB, and VI methods. Figures 1 and 2 show the schematic representation of manufacturing methods and experimental steps of VB and VI methods to fabricate composite laminates, respectively. VI was achieved at two different temperatures (25°C and 50°C) to study the effect of viscosity on CNT filtration during infusion for the CNT content of 0.4 and 0.8 wt-%. Specimen codes based on manufacturing method and CNT content implemented within composites are shown in Table 2. Woven carbon fibres with the length of 30 mm at both edges were placed on top and bottom of each laminate to enhance lower contact resistance during electrical measurement. Curing of composite laminates was achieved with the following procedure: (i) one hour at 100°C and (ii) one week at 25°C under the constant pressure of 0.1 MPa. After curing, composite plates were cut into smaller pieces for electrical characterisation.

Table 2

Specimen codes used in the study.

Specimen code	Manufacturing method	CNT content (wt-%)
CM02	Compression moulding	0.2
CM04	Compression moulding	0.4
CM08	Compression moulding	0.8
VB02	Vacuum bagging	0.2
VB04	Vacuum bagging	0.4
VB08	Vacuum bagging	0.8
VI02-25	Vacuum infusion at 25 ⁰C	0.2
VI04-25	Vacuum infusion at 25 ⁰C	0.4
VI08-25	Vacuum infusion at 25 ⁰C	0.8
VI04-50	Vacuum infusion at 50 ⁰C	0.4
VI08-50	Vacuum infusion at 50 ⁰C	0.8

Electrical conductivity measurement

Rectangular specimens with the size of 70 × 10 × 3 [45] and 140 × 13 × 3 mm were employed for electrical conductivity measurement of unreinforced CNT/epoxy composites and glass-reinforced CNT/epoxy composites, respectively. Figure 3 shows the electrical resistance measurement set-up for glass fibre reinforced specimens. Carbon fibres were applied to both ends of the specimens to achieve good contact with the filler network within the specimen. Four-probe AC impedance spectroscopy was performed using a LCR meter (GW-INSTEK LCR 8110G) with a voltage amplitude of 2 V over a frequency range from 20 to 10⁶Hz with 10-points average measurement. All measurements were repeated three times for consistency.

Results and discussion

The percolation threshold for electrical conductivity was found as 0.1 wt-% [45] which is a compatible result with literature [46]. Since the percolation threshold was found as 0.1 wt-%, CNTs were used at higher contents (0.2, 0.4, and 0.8 wt-%) than percolation threshold for woven glass reinforced epoxy composites. Figure 4 shows the log-log plot of specific admittance for 0.2 wt-% CNT filled glass/epoxy composites manufactured by CM, VB, and VI (at 25°C) methods. It is seen that specimens exhibit resistive behaviours for all manufacturing methods. This result indicates that CNTs are evenly distributed and percolated throughout the composite plates. The average conductivities obtained at 20 Hz are almost the same for specimens manufactured by CM and VB while specimens manufactured by VI at 25°C show the highest conductivity (Figure 4(d)). The impregnation of woven fibres with CNT/epoxy blend with roller in CM and VB process might slightly reduce CNT content in laminates by trapping some amount of CNTs whereas impregnation of woven fibres in VI method is succeeded by direct infusion of CNT/epoxy blend without any external interaction that might cause CNT entrapment.

Figure 5 shows the log-log plot of specific admittance as a function of frequency for 0.4 wt-% CNT-filled glass/epoxy composites manufactured by (a) CM, (b) VB, (c) VI (25°C), and (d) VI (50°C) method, and (e) their specific conductivities at 20 Hz with standard deviations. Resistive behaviour is seen in all 0.4 wt-% CNT-filled specimens manufactured by CM and VB. However, the first four specimen from inlet shows the resistive behaviour while the fifth specimen exhibit capacitive behaviour in VI-25 method (Figure 5(c)) which is not the case when 0.2 wt-% CNT is used (Figure 4(c)). This result is due to the fact that increment in viscosity because of higher CNT content leads to CNT filtration at small gaps of woven fabrics. Increase in viscosity reduces the flow rate of 0.4 wt-% CNT-filled epoxy resin during the infusion process causing CNTs to accumulate at small gaps between woven glass fibres. However, the viscosity of 0.2 wt-% CNT-filled epoxy is not high enough to cause any CNT filtration in VI method at 25°C. When the infusion process of 0.4 wt-% CNT filled epoxy resin is done at the resin temperature of 50°C, filtration effect which is seen at 25°C does not appear in the last specimen (Figure 5(d)). This result suggests that a decrease in viscosity due to higher temperature leads to higher flow rate of CNT/epoxy blend preventing CNTs from stopping and accumulating at small gaps in woven fibres.

Owing to CNT filtration, the average conductivity of 0.4 wt-% filled composites manufactured by VI method at 25°C is lower than the composites manufactured at 50°C (VI-50) Figure 5(e). CNT filtration at 25°C also causes a high level of discrepancy in the conductivity values resulting in high level of standard deviation (SD).

As shown in Figure 6(a,b), all specimens manufactured by CM and VB show resistive behaviour when 0.8 wt-% CNTs is used as in lower CNT contents. However, the effect of CNT filtration at this CNT content is more pronounced due to higher increment of viscosity in VI process at the resin temperature of both 25 and 50°C (Figure 6(c,d)). It is seen from Figure 6(c) that the first two specimens from resin inlet exhibit resistive behavior whereas the next three specimens show capacitive behaviour due to high-level CNT filtration. Although increasing resin temperature to 50°C reduces the effect of CNT filtration on electrical conductivity (Figure 6(d)), this enhancement is not effective enough to provide resistive conductivity in the last specimen for 0.8 wt-% CNT-filled composites. Therefore, infusion should be achieved at resin temperature higher than 50°C to obtain better distribution of CNTs throughout composite plates. However, the resin temperature and the infusion duration should be well determined to guarantee that epoxy resin must remain uncured during infusion process, especially for large composite structures. Herein, it should also be noted that the SD of average conductivities for VI08-25 and VI08-50 is higher than that of VI04-25 since high level of CNT filtration at higher CNT content leads larger discrepancy in conductivity values of different zones in composite plates. Figure 7 shows the in-plane penetration level of CNTs from the inlet to the outlet through the composite plates manufactured for different temperatures and CNT concentrations. The arrows indicate infusion direction from the inlet region to the outlet region so that the filtration level is clearly seen in the outlet region in this incremental order: VI08-25 > VI08-50 > VI04-25 > VI04-50. This macroscale observation is compatible with the quantitative results of electrical characterisation of the composite specimens.

Figure 8 shows the DC conductivity of composite specimens manufactured by VI08-25 method with the representative of CNT distribution that might occur within specimens. The textile woven fabrics, peel ply, and resin distribution media that are successively employed in VI method as shown in Figure 2 can act as micro-sized mesh membranes causing filtration of CNTs in the regions close to the resin inlet during infusion. Moreover, the accumulation of CNTs leads to bigger CNT aggregates are getting bigger due to CNT accumulation at gaps in woven fibres during infusion, resulting in no conductivity as a result of lacking CNT network in the region of resin outlet.

Figure 9 shows the effect of CNT content on the electrical conductivity for all manufacturing methods. Electrical conductivity increases with increasing CNT content. However, the increment rate in conductivity slows down for the CNT content above 0.2 wt-% and this reduction in conductivity becomes more pronounced when the CNT content goes up to 0.4 wt-%.

The effect of CNT content on the mechanical properties of woven/epoxy composites under three-point bending test was investigated. For this purpose, specimens were manufactured by CM method because it yielded the best CNT penetration. Figure 10 shows the effect of CNT content on the flexural strength and flexural modulus of woven/epoxy composites manufactured by the CM method. It was obtained that the composites filled with the CNT content of 0.2, 0.4, and 0.8 wt-% showed improvements in flexural strength by 15.2%, 25.7%, and 11.4% and 6.3%, 37.5%, and 31.2% in flexural modulus, respectively. Enhancement in mechanical properties was seen for all CNT contents whereas improvement in mechanical properties was low for the CNT content of 0.8 wt-% due to the formation of bigger agglomerations and disentangled bundles.

Conclusion

In summary, CNT penetration quality into woven glass fibre/epoxy composites during VI process was successfully determined by monitoring electrical resistance of composite plates. CNT penetration rate significantly reduces with the increasing CNT content due to the increase in viscosity of CNT/epoxy mixture. It was found that CNT filtration can be prevented by increasing the temperature of CNT/epoxy resin mixture before the infusion operation as a result of reduction in viscosity. Increase in the flow rate of nanoparticle-filled resin due to lower viscosity at higher temperatures increases the nanoparticle mobility resulting in the reduction of CNT filtration. This was evident from the result that the average conductivity of both 0.4 and 0.8 wt-% filled composites was enhanced at 50°C. In the case of 0.8 wt-% filled composites, the infusion at 50°C was not enough to prevent the filtration of CNTs in the region close to the resin outlet. Therefore, higher resin temperature is necessary for the complete penetration of nanoparticles. However, depending on the CNT content, operation temperature and infusion time should be well arranged to prevent unexpected resin curing during infusion and to provide uniform CNT distribution in VI process. On the other hand, uniform CNT distribution was observed throughout the composite plates manufactured by CM and VB methods. It was found that the electrical conductivity increased with an increase in CNT content. Improvement in flexural strength and flexural modulus values of composites was observed for all CNT loadings although reinforcing effect of CNTs diminished at 0.8 wt-% due to high level of agglomerations. Consequently, this study suggests that optimised CNT content should be employed in composite application to obtain a material with a preferred balance between electrical and mechanical properties.

Footnotes

Disclosure statement

No potential conflict of interest was reported by the author(s).

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