Synergistic Electrical Enhancement in Epoxy Composites with Carbon Nanotubes and Graphite Nanoplatelets

Abstract

We investigated how electrical conductivity in epoxy‑based composites evolves with varying concentrations of graphite nanoplatelets (GNPs), multi‑wall carbon nanotubes (MWCNTs), and their hybrid combination. The hybrid filler was added at 0.24 vol % while MWCNT loading ranged from 0.03 to 4 vol %. Prior to mixing, GNPs underwent 20 min of UV‑ozone treatment to improve surface activity. In the low‑viscosity resin system, two distinct percolation thresholds appeared: a primary transition at 0.13 vol % for MWCNT‑only composites and a secondary transition at 0.42 vol % for the hybrid blends. Above the percolation point, the hybrid composites displayed a synergistic conductivity increase—up to twenty‑fold compared with the single‑filler composites. Modeling of conductive chain networks and contact resistance explained this behavior: the hybrid architecture reduces polymer inter‑particle layers, lowering inter‑particle resistance and amplifying the conductive network.

Background

Using multiple fillers simultaneously is now a proven strategy for tailoring composite properties. When conductive nanomaterials such as carbon fibers, carbon black, or graphene interact within a polymer matrix, they can form interconnected networks that dramatically improve electrical and thermal conductivity, as well as mechanical performance. The phenomenon of “double percolation”—where two distinct conductive pathways form—has been observed in systems combining fibrous and particulate fillers, leading to a pronounced synergistic effect.

Classical percolation theory predicts a single sharp transition from insulating to conductive behavior. However, many experimental studies report more complex, multi‑step transitions, often attributed to particle geometry, dispersion quality, and interfacial effects. Understanding these mechanisms is crucial for designing high‑performance composites.

Recent work on hybrid nanofillers—especially combinations of carbon nanotubes (CNTs) and graphite nanoplatelets (GNPs)—has shown that the unique aspect ratios and morphologies of each filler can be leveraged to create more efficient conductive networks. In our study, we aimed to quantify how a small amount of GNPs influences the percolation behavior of MWCNT‑filled epoxy composites.

Methods

Materials

Multi‑wall carbon nanotubes (MWCNTs, purity ≥90 %) and thermally expanded graphite (TEG) were used to prepare GNPs. The GNPs were exfoliated in water by 30 h of ultrasonic dispersion, yielding particles with lateral dimensions of 0.2–5 µm and thicknesses ranging from 5 to 55 nm (maximum at 28 nm). Aspect ratios varied from ~40 to 900, favoring high electrical conductivity.

Epoxy Larit 285 (Lange Ritter GmbH) served as the polymer matrix (viscosity 600–900 mPa s). The hardener H285 (viscosity 50–100 mPa s) was added at a stoichiometric ratio to cure the resin.

Preparation of Composites

Two composite families were fabricated:

• Mono‑Composite Materials (MCMs): epoxy resin filled with either GNPs or MWCNTs (0.03–4 vol %).
• Hybrid‑Composite Materials (HCMs): epoxy resin filled with MWCNTs (0.03–4 vol %) plus a fixed 0.24 vol % of GNPs.

For each batch, the filler was dispersed in acetone, ultrasonically mixed with the epoxy resin (30 min for GNPs, 60 min for MWCNTs), and then the hardener was added. The mixture was poured into molds and cured at room temperature for 48–72 h. Test specimens (3.5 × 3.5 × 10 mm³) were cut for conductivity measurements.

Electrical Testing

Conductivity was measured over a range of 10⁻¹² to 10 S m⁻¹ using a four‑probe method for samples with resistance ≤10⁴ Ω and a teraohmmeter for higher resistances. Measurements were performed across temperatures 6–300 K. Three specimens per composition were tested to ensure reproducibility.

Results and Discussion

Percolation Behavior

Figure 2 shows that both MCMs and HCMs exhibit two percolation transitions. For MWCNT‑only composites, the first transition occurs at 0.13 vol % (σ > 10⁻⁶ S m⁻¹). GNP‑only composites reach a single threshold at 1.7 vol %. Hybrid composites with 0.24 vol % GNPs display a critical concentration of 0.42 vol %, falling between the two single‑filler values.

The initial quasi‑dynamic transition is attributed to mobile, isolated CNTs forming conductive chains before the resin hardens. The subsequent static transition follows classical percolation theory, described by σ ∝ (ϕ – ϕ_cr)ᵗ, where t is the critical exponent.

Synergistic Conductivity Enhancement

Above the percolation threshold, HCMs exhibit a dramatic increase in conductivity—up to twenty‑fold at 2 vol % and ten‑fold at 4 vol % relative to the corresponding MCMs. This synergy arises from the reduced polymer layer thickness between filler particles, lowering contact resistance (R_k) and enabling more conductive chains.

Our effective‑resistance model, based on particle geometry and packing factors, quantitatively captures these effects. Calculations show that hybrid composites achieve the lowest R_k (10⁶–10⁷ Ω) and a substantial number of uninterrupted chains (N*_chain) compared to single‑filler systems.

Simulations of tunneling resistance confirm that the thinner inter‑particle polymer layers (δ ≈ 0.8–1.1 nm for hybrids) significantly diminish R_k, whereas larger δ in GNP‑only composites leads to higher resistance.

Conclusions

Two percolation thresholds were identified in low‑viscosity epoxy nanocomposites: a quasi‑dynamic transition driven by mobile CNTs and a static transition governed by classical percolation. Above the critical point, hybrid composites (MWCNT + GNP) exhibit a pronounced synergistic conductivity enhancement. The effect is attributed to reduced contact resistance stemming from thinner polymer layers and an increased number of conductive pathways. These findings provide a clear pathway for engineering high‑conductivity epoxy composites through strategic filler combinations.