Electrical properties of extruded milled carbon fibre and polypropylene

Abstract

A milled carbon fibre and polypropylene polymer composite at high filler loading was developed to produce conductive polymer composites for high conductive applications. Current research of conductive polymer composite material has reported about in-plane conductivity that was often higher than through-plane conductivity, which contradicted with the target of applications that required higher electrical conductivity in the through-plane direction. Therefore, electrical conductivity in parallel and transverse to extrusion directions were investigated. The general-effective media and modified fibre contact model were adapted to predict the electrical conductivity of the composite material. The experimental conductivity data of polypropylene/milled carbon fibre composites for transverse and parallel directions were not correlated with the general-effective media model with 2.009 and 0.663 S/cm, respectively, at the highest filler loading of 80 wt.%. This disagreement was due to various critical exponential, t values (2–3.25) that were obtained in this study. However, the modified fibre contact model seemed to have good agreement with the experimental data in the parallel to extrusion direction. This model was unable to predict electrical conductivity in the transverse direction due to lack of orientation occurring in that direction. The electrical conductivity increased as the filler loading increased as explained in percolation theory. Predicting the electrical conductivity of conductive polymer composites material is still in the preliminary stages where the researcher often obtains fluctuating agreement with the experimental values. Thus, contact between filler and orientation is considered as the main factor that influences the electrical conductivity and mechanical strength of the conductive polymer composites material.

Keywords

Carbon fibre electrical properties computational modelling mechanical testing extrusion

Introduction

Conductive polymer composites (CPCs) are obtained by mixing an insulating polymer matrix with conductive fillers based on the various needs of various applications.^1–4 CPCs materials consist of several interesting features such as good mechanical and chemical treatments, low density, good corrosion resistance and good electrical conductivity.^1,2,5 CPCs materials are currently applied in the military, automotive industries and utilities. However, extensive research needs to be conducted in order to balance the electrical and mechanical properties of the CPCs materials based on the target application.^4–6 Thus, to balance overall performance based on mechanical properties and electrical conductivity, various conductive composite materials such as powder, particles, fibres and manufacturing methods are utilised in the development of the CPCs.^7,8

High filler loading is generally needed to meet high electrical conductivity requirements since in high filler loading the electrical conductivity would approach filler electrical conductivity.⁹ However, this leads to a major drawback in reducing both the mechanical strength of the CPCs materials and the difficulties in the manufacturing process. Thus, CPCs materials consists of polymer resin such as thermoplastic materials including polypropylene (PP) and highly conductive filler such as carbon black, carbon nanotube (CNT) and carbon fibre were developed in order to produce highly CPCs materials. PP is naturally insulating material in the thermoplastic group with several advantages including shorter moulding times, the ability to re-melted and having lower cost compared to thermosetting polymers.⁸ Developments of suitable materials for high conductive application are important to maintain high electrical conductivity, better mechanical performance and low manufacturing cost. Hence, replacing the metallic-based materials with conductive fillers seems most effective way in producing highly CPC due to better corrosion resistance, light weight and ease of manufacturing.^7,10 Meanwhile, carbon fibre is a type of conductive filler which aids in improving the filler dispersion and distribution of a matrix as it offers a high aspect ratio. Fillers with a high aspect ratio are able to produce high conductivity as they generally orientate during the dispersion in the polymer matrix.¹¹ Carbon fibre tents to disperse, orientate and make contact in many ways, which may affect the entire conductivity of the polymer matrix.¹²

In order to predict the conductivity of CPCs materials for high filler loadings, many theoretical models have been proposed. For instance, percolation theory is derived from observing the conductivity at certain filler concentrations.¹³ The electrical conductivity of the composite materials is nearly similar to the insulating material at low filler content. As the filler concentration is greater than the critical content which is the percolation threshold, the electrical conductivity will rapidly increase until remains as a plateau. This percolation model is able to predict the percolation threshold of the composite materials and verify the material environment in both insulated and conductive conditions. Yet, no model has been created that is able to account for further influence using different materials and processing methods as the model is usually reliant on individual material. Hence, there is no good correlation between experimental and theoretical aspects when applying specific models.^13,14 McLachlan introduced a model, the general-effective media (GEM) equation, to predict the conductivity of a binary mixture with an anisotropic grain structure based on percolation theory. The model described conductivity in terms of volume fractions and critical component factors in the simplest way.¹⁵ Previous researchers have reported that the GEM model is able to predict the electrical conductivity of polymer composites consisting of either single or multiple fillers.^14–17

In this article, we studied the effect of carbon fibre as a primary filler in the CPCs due to the fact that it has high electrical conductivity and is able to replace the costly and low durability traditional graphite using the twin-screw extruder method as the main processing technique.¹⁸ The GEM equation is adapted to predict the electrical conductivity of the composite polymer with different filler compositions. This model is extended to the in-plane conductivity of the composites to predict the electrical conductivity of the CPCs. The GEM model of two component systems is shown in equation (1)^14,15:

\frac{(1 - Ø_{CF}) (σ_{PP}^{\frac{1}{t}} - σ_{m}^{\frac{1}{t}})}{σ_{PP}^{\frac{1}{t}} + A σ_{m}^{\frac{1}{t}}} + \frac{Ø_{CF} (σ_{CF}^{\frac{1}{t}} - σ_{m}^{\frac{1}{t}})}{σ_{CF}^{\frac{1}{t}} + A σ_{m}^{\frac{1}{t}}} = 0

(1)

where the electrical conductivity, σ is a function of filler volume fraction ∅, t is critical exponential and A is related to the percolation threshold of the component matrix.^14,15 Nevertheless, the GEM model has limitations in predicting electrical conductivity for the filler at extreme geometry. Differing from the GEM model, the modified fibre contact model (FCM) considers additional factors including orientation, filler contact and length, which give a different predicted electrical conductivity. The modified FCM can be defined as follows:¹²

σ_{c} = σ_{m} + [\frac{4}{π} (\frac{d_{c}}{d}) (\frac{l}{d}) (\cos 2 φ) (v_{p} σ_{f}) X]

(2)

where

σ_{c}

is conductivity of the composite,

σ_{m}

is conductivity of the matrix,

σ_{f}

is conductivity of the filler, diameter of circle of contact d_c, diameter of filler d, average filler length l, filler orientation angle φ, volume fraction v_p and factor dependent on filler contact number X.¹²

By predicting the electrical conductivity of the composite material, this will aid in tailoring the certain materials used based on applications needs such as in bipolar plate used in proton exchange membrane (PEM) fuel cells. Researchers have generally reported that electrical conductivity is studied in the in-plane direction while neglecting the through-plane direction due to the fact that the through-plane direction is often half of the electrical conductivity of the in-plane direction.^6,15,19,20 Thus, in this article, electrical conductivity in parallel and transverse to extrusion directions is studied. This is because electrical conductivity in transverse to extrusion directions is important in order to determine the electrical conductivity of the through-plane direction.

Experimental

Milled carbon fibre (MCF) used in this present experiment was obtained from ShenZhenYataida High-Tech. Co., Ltd with a density of 1.75 g/cm³, a diameter 9 µm, an aspect ratio of 43 and an average length of 300 µm as reported by the manufacturer. This MCF was chosen as it has the ability to increase electrical conductivity at low loading and disperse evenly in the polymer matrix through the extrusion process.¹⁶ PP powder grade Titan (600) with a density of 0.91 g/m³, a maximum micron size of 90 µm and melt index of 10 g/10 min at 160℃ was supplied by Goonvean Fibers Ltd. PP was chosen due to its unique properties such as good gas impermeability, as well as having good mechanical strength, low density and electrical resistance.⁷ Details of the materials specifications are shown in Table 1. The polymer composites of the PP and the MCF were physically pre-mixed in a small container at room temperature with a mechanical mixer, model RM 20-KIKA-WERK at 1200 rpm for 60 s before being melt compounded in a Thermo Haake TSE extruder. For the extrusion process, the twin screw extruder of Thermo Haake TSE was set at a temperature of 210℃ with the rotational speed at 50 rpm for 30 min of extrusion time.

Table 1.

Material specifications of MCF and PP.

	Milled carbon fibre (MCF)	Polypropylene (PP)
Density (g/cm³)	1.75	0.91
Diameter (µm)	9	–
Aspect ratio	43	–
Fibre length (µm)	300	90
Tensile strength (GPa)	3.5	–
Young modulus (GPa)	230	–

The electrical conductivity of the composite in-plane direction was measured with a four-point probe technique using a Jandel four-point probe and an RM3 test unit.^5,6,14 Besides that, the structure of the composite was observed using the optical microscope, Optical Olympus Leica DLMM and SEM Quanta FEI 400 F. The density of the composites was measured according to the ASTM D792 standard by the Archimedes method, while the Shore hardness (scale D) was measured by using a Digital Shore Hardness tester based on ASTM D2240.^5,6,20,21

Results and discussion

Figure 1 shows the microstructure image of MCF and PP in cut plane parallel and transverse to extrusion directions. The MCF has a cylindrical, rod-like shape surrounded by the PP.¹⁴ The observations were made at both transverse and parallel to extrusion directions to study the effect of the addition of MCF to the mechanical and electrical properties. This high aspect ratio of the MCF had the potential to create such motions, which made most of the long rod-like shape fillers align with the flow field and form a longitudinal orientation during the extrusion due to shear force. Nevertheless, there were fillers that did not align in the flow direction due to shear deformation near the wall and die surface.^22,23 Furthermore, low viscosity and shear rate also contributed to filler alignment as low shear stresses were used to transfer onto the filler in order to align.^22,24,25 However, the Brownian motion could be neglected since the composite materials used in this study was in bulk scale.^26,27 Nonetheless, only the maximum amount of 80 wt.% of the MCF could be extruded into a test specimen as the viscosity of the polymer composite started to increase beyond it.^{8,14,20,24,25}

Figure 1.

Microstructure image of 80 wt.% of MCF in (a) parallel and (b) transverse to extrusion direction. Arrow indicates the flow direction.

Effect of milled carbon fibre

The effects of MCF addition in the in-plane electrical conductivity of the composite materials in parallel and transverse to extrusion directions is shown in Figure 2. Electrical conductivity was measured in both parallel and transverse to extrusion direction to observe the effects of filler orientations as the composite materials being extruded. There were increases in electrical conductivity with the addition of the MCF for both directions in the in-plane conductivity. The in-plane conductivity at 80 wt.% of the MCF for both parallel and transverse direction is 0.67 and 2 S/cm, respectively. This low conductivity obtained at the parallel direction was due to the lack of continuity to end the filler contacts and consequently, lowered the net conductivity network. The higher conductivity in the transverse direction was due to the formation of a dense carbon fibre during extrusion as shown in Figure 1.^7,12,28 This also clearly demonstrated in Figure 3 where there were porosity exits as the addition of the filler increased. The porosity however started to increase drastically from a pinhole as the filler increased from 50 to 80 wt.% of the MCF. This phenomenon occurred as the fillers started to entangle and agglomerate at which the fillers were no longer being wetted by the polymers thus affecting filler distribution and consequently, lowering the electrical conductivity of the polymer composite. Moreover, porosity occurred as there was insufficient polymer to bind the composite matrix.^7,9,19,20 Nevertheless, the shear deformation that occurred near the walls had caused the filler to align to the flow direction, hence enabling the filler to be oriented parallel to the flow direction.²³ Besides that, shear flow started to generate as the fillers flowed through the die gaps. This scattered and un-uniformed MCF explained the tumbling effect created due to the existence of the shear force.²⁷ The MCF that tend to agglomerate and cluster was attributed to the effect of low shear rates.²⁵

Figure 2.

Effect of milled carbon fibre on in-plane conductivity.

Figure 3.

Microstructure image of (a) 50 wt.% and (b) 80 wt% of MCF in parallel to extrusion direction. Arrow indicates the flow direction.

Effect of extrusion temperature

The electrical conductivity and mechanical performance at different temperatures (190, 210, 230 and 250℃) of the extrusion process were investigated. The parameter of the twin screw extruder was set at 50 rpm. Figure 4 shows the electrical conductivity at 80 wt.% of the MCF for both the transverse and parallel to extrusion directions. It can be clearly observed that the electrical conductivity in the parallel direction was higher than the transverse. At 250℃, the electrical conductivity suddenly dropped due to the fact that the PP had deteriorated at this extreme temperature. However, the melting point of the polymer matrix remained constant even when the amount of conductive filler varied.²⁹ This increasing pattern in electrical conductivity with increasing temperature indicated that a conductive network was formed as the porosity decreased due to the fact that the MCF tends to become denser. In addition, the cylindrical rod-like shape MCF formed a continuous end to end conducting network, which made the electrical conductivity in the parallel direction higher than in the transverse direction.¹² Conclusively, based on the experimental results, the best condition was determined at the temperature of 230℃, which had given higher electrical conductivity with good mechanical performance.

Figure 4.

Electrical conductivity of 80 wt.% MCF at different temperature.

Theoretically, the mechanical performance of the composite material will deteriorate as the electrical conductivity increases.⁷ In the extrusion process, a simple shear flow is able to orientate fillers to be aligned with the flow field. One of the important factors is that a filler with higher aspect ratio tends to orientate more and more easily.^12,22 However, near the extrusion walls, the filler starts to orientate parallel with the flow giving the longitudinal orientation in the tube as the shear deformation is predominant.²³ Figure 5 illustrates that the shear rate increased as the temperature increased, which suggested that more fillers intended to align parallel to the direction. As the filler experienced low viscosity and shear rate during the extrusion process, it would cause a randomly orientated filler in the polymer composite matrix.^24,25 Besides, the shear rate is an efficient way to minimise filler agglomeration and enhance filler dispersion.²⁶ Interestingly, shear flow inducing filler orientation has been reported by other researchers as well especially when discussing mechanical performance.²⁷ However, none of the studies have studied its effect on the electrical conductivity. As the temperature increases, the viscosity of the polymer composite increases and enhances its shear rate.^24,25 However, at a higher shear rate, the filler is prone to break into a smaller size, which causes electrical conductivity to drop at 250℃.

Figure 5.

Shear rate of 80 wt.% MCF.

Mechanical performance

Hardness is one of main mechanical properties in CPC materials. As shown in Figure 6, the shore hardness of the composite decreased as the filler loading increased, which can be observed clearly in both of the extrusion directions. However, the hardness value in the transverse to extrusion direction is higher than in the parallel to extrusion direction at 80 wt.% of the MCF with 40.78 D and 23.86 D, respectively. This is because the filler is denser and more compact in the transverse to extrusion direction than that of in the parallel to extrusion direction. Moreover, in the parallel direction, there is evidence of pinholes and porosity, which decreased the hardness of the material especially at 80 wt.% of the MCF. These uniformed, dense and aligned distributions of the carbon fibre minimized penetration at the surface of the composite and consequently enhanced the hardness of the composite materials.²⁸ The carbon fibre shape having ribbon or glassy carbon structures also contributed to the hardness of the composite materials.¹⁰ Figure 7 highlights that the hardness of the MCF in transverse and parallel directions increased as the temperature increased. At 230℃, the hardness of the MCF in the transverse and parallel directions were 44.82 D and 28.66 D, respectively. This phenomenon can be related to the ability of the polymer to bind the filler closely resulting from reduced porosity and void at high temperature. Hence, the material structure was expected to be more rigid and dense.

Figure 6.

Effect of milled carbon fibre on hardness.

Figure 7.

Effect of temperature on hardness.

The bulk density results of the MCF are shown in Figure 8. It can be noted that the maximum bulk density at 80 wt.% of the MCF is 1.293 g/cm³. This result indicated that the addition of the filler increased the bulk density of the material. This high density might produce a structure with increased conductive networks and consequently, increased the electrical conductivity of the material. However, the bulk density of the material varied with temperature as indicated in Figure 8. It is clearly observed that at temperatures above 230℃, the bulk density of the composite started to deteriorate because of the characteristic of the PP, which was unable to withstand the extreme temperature.

Figure 8.

Effect of MCF addition and extrusion temperature on bulk density.

Table 2 shows the mechanical properties of extruded MCF/PP composite at various of mixing temperature. The highest mechanical properties obtained were at 230℃ as the composite are well mixed at that temperature. Meanwhile, the tensile strength and flexural strength of the composite started to deteriorate as the temperature increase above 230℃.

Table 2.

Mechanical properties of extruded MCF/PP at 80 wt.% of MCF.

Temperature (℃)	Flexural strength (MPa)	Maximum tensile stress (MPa)	Modulus Young (MPa)
190	10.6	4.8	480
210	20.6	5.12	520
230	27.8	7.7	1221
250	26.0	4.48	727

Modelling of electrical conductivity

In this study, the GEM model and modified FCM were adapted to predict the electrical conductivity of polymer composites. In agreement with the findings of other researchers, the GEM model was able to predict the electrical conductivity of the polymer composite with single and multiple fillers.^8,15–17 This study showed that the GEM model was able to predict both through-plane and in-plane conductivity when using phenol formaldehyde resin with natural graphite, carbon black and carbon fibre. However, this model was unable to predict precisely the electrical conductivity of the polymer composite having fillers with a high aspect ratio and extreme geometry. This was because the parameters in the model needed to be optimised individually in order to produce a results, which were comparable with the experimental data when using epoxy as the resin with synthetic graphite and MCF.^14,15 This limitation in the GEM model was caused by the fact that the model was derived based on material composition without considering other factors such as the shape factor and orientation. Thus, the modified FCM has a better agreement with the experimental data as the model considered the parameters obtained from experimental data. However, previous studied have highlighted the difficulty in measuring the degree of orientation in the modified FCM in order to predict electrical conductivity, which currently can be solved using the SEM method.³⁰

Figure 9 displays the difference in conductivity obtained from the predicted model and experimental data for both transverse and parallel to extrusion directions. The conductivity obtained from the predicted GEM model did not seem to be in agreement with the experimental data. However, the conductivity obtained from the predicted modified FCM was in agreement and had the same trend with the experimental data. Moreover, the model showed better agreement as the electrical conductivity increased with the addition of the MCF in both directions. In this study, the t value varied from 2 to 3.25, which was acceptable for the extreme and complex filler geometry within a composite structure.^{14,16,31–33} The modified FCM was in good agreement with the experimental data at parallel direction while only having an increase trend at the transverse to extrusion direction. These results were due to the fact that the modified FCM was unable to predict electrical conductivity as there was no orientation existing in the transverse direction. The 5° of the orientation was measured in the MCF at parallel direction while the contact area between the filler was assumed to be at a single point of 1 nm size. However, this modified FCM had a few limitations as it needed to measure orientation visually and was unable to predict electrical conductivity based on filler intensity.

Figure 9.

Electrical conductivity for parallel and transverse direction and predicting GEM and modified FCM.

Conclusions

The electrical conductivity of the MCF/PP in parallel and transverse to extrusion directions were investigated where the electrical conductivity in the transverse direction was found to be higher than in the parallel direction. First, the CPC was developed using the extrusion process to melt compound with the MCF as the conductive filler and PP as its polymer matrix. Next, the prepared polymer composite was used to investigate electrical conductivity and mechanical properties in both parallel and transverse to extrusion directions. The electrical conductivity was measured in terms of in-plane conductivity. The highest conductivity was measured at 80 wt.% of the MCF for both parallel and transverse directions with 0.67 and 2 S/cm, respectively. The electrical conductivity for both transverse and parallel directions increased to 3.657 and 3.434 S/cm, respectively, at 230℃. The mechanical properties of the composite polymer decreased with an increase in the MCF. However, contradictory results were obtained when the temperature started to increase, whereby the hardness of the composite polymer increased to 45.78 D and 31.12 D for the transverse and parallel directions, respectively. This study also found that the GEM model was unable to predict electrical conductivity accurately for materials with difference in parameters as it only predicted based on material composition. In contrast, the modified FCM seemed to have good agreement and trend with the experimental data as it considered a few factors including orientation and contact between fillers. However, the contact between fillers and the ability of the model to predict the electrical conductivity accurately need further improvement.

Footnotes

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: Ministry of Higher Education Malaysia (MOHE), grant number FRGS/1/2013/TK04/UKM/01/2, AP-2013-010, DIP-2014-006, and MyBrain15 programme.

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