Why re-extrusion and regrind change bulk resistivity in GNP-filled anti-static thermoplastics

Introduction

Re-extrusion or regrind changes bulk resistivity primarily because mechanical shear and thermal cycling during reprocessing alter GNP network morphology, interflake contacts, and interfacial adhesion, which modifies the percolation pathways that enable static dissipation. Mechanistically, repeated melt shear and heat exposure reduce effective platelet aspect ratio (via flake fragmentation and edge damage), promote re-aggregation or restacking, and can degrade polymer–GNP interfacial chemistry so contact resistance and tunneling gaps increase. These effects are boundary-limited: they become significant when GNP loadings are near the electrical percolation threshold (commonly in the sub- to few‑wt% range depending on aspect ratio and dispersion quality) and when processing imposes high cumulative shear or temperatures approaching matrix degradation. As a result, resistivity shifts are most pronounced for formulations with marginal network redundancy, poor initial dispersion, or when regrind fraction exceeds the design allowance for virgin/resin ratio. Measurement context matters: bulk resistivity and surface resistance can diverge depending on test geometry, so reported shifts depend on the chosen method (volume vs. surface/ASTM test). Unknowns include precise fragmentation kinetics for specific GNP grades and the exact per-grind fraction tolerance for a given polymer matrix; therefore this explanation is scoped to mechanistic drivers rather than precise numeric predictions.

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Common Failure Modes

Primary Failure Modes

• Observed: Sheet resistivity rises after one or more re-extrusion cycles. Mechanism mismatch: network damage from flake fragmentation and increased interflake contact resistance because mechanical shear reduces lateral platelet size and increases the number of insulating gaps. See also: Why carbon black causes resistivity overshoot in ESD plastics — role of contact-limited conduction and thermal effects (comparison to GNP/FLG)..
• Observed: Batch-to-batch variability in ESD performance following use of regrind. Mechanism mismatch: inconsistent regrind particle size and heterogeneous GNP redistribution because grinding creates polymer-rich and filler-rich zones that break continuous conductive paths. See also: Why Carbon Black Migrates Or Blooms In Polyolefin Anti Static Compounds.
• Observed: Initial low resistivity recovers partially after thermal anneal but not fully after repeated reprocessing. Mechanism mismatch: irreversible interfacial debonding and restacking because heat plus shear promotes hydrophobic reaggregation and loss of functional interfacial chemistry that previously aided percolation.

Secondary Failure Modes

• Observed: Increased sample-to-sample anisotropy in conductivity after reprocessing. Mechanism mismatch: shear-induced reorientation and selective fragmentation because re-extrusion flow aligns surviving platelets unevenly and breaks poorly oriented ones, changing directional percolation.
• Observed: Rapid resistivity drift during humidity cycling in regrind-containing parts. Mechanism mismatch: changed microstructure and higher porosity because grinding introduces microvoids and exposes fresh edges that alter water uptake pathways and surface conduction behavior.

Conditions That Change the Outcome

Primary Drivers

• Variable: GNP lateral size and layer count. Why it matters: larger, few-layer platelets create lower contact resistance and require fewer contacts to percolate, therefore fragmentation reduces network connectivity faster because each break removes more conductive area.
• Variable: GNP loading relative to percolation threshold. Why it matters: near-threshold systems are more sensitive because small increases in tunneling distance or contact resistance push the network from percolating to non-percolating; above-threshold, redundancy partly buffers damage.
• Variable: Polymer matrix rheology and melt viscosity. Why it matters: higher viscosity increases shear stresses during extrusion, accelerating platelet breakage and limiting re-dispersion because mobility of platelets in the melt governs reformation of contacts.

Secondary Drivers

• Variable: Re-extrusion shear rate, temperature, and number of cycles. Why it matters: higher shear rates and temperatures increase mechanical fragmentation and thermal aging of the interface, therefore cumulative processing energy determines extent of network degradation.
• Variable: Regrind particle size and fraction in feed. Why it matters: coarse regrind yields non-uniform filler distribution and trapped agglomerates while high regrind fraction increases probability of previously damaged network elements dominating final microstructure.
• Variable: Surface chemistry and compatibilizer presence. Why it matters: functional groups or coupling agents reduce interflake restacking and strengthen polymer–GNP contact so their loss or absence increases contact resistance and susceptibility to re-aggregation under shear.

How This Differs From Other Approaches

• Mechanism class: Network fragmentation vs. network dilution — fragmentation removes conductive continuity by breaking platelets and increasing contact resistance, whereas dilution reduces the probability of nearest-neighbor contacts by lowering filler concentration; both change percolation but via different causal steps.
• Mechanism class: Interfacial debonding vs. interflake restacking — interfacial debonding increases tunneling gaps at polymer–filler interfaces because adhesion weakens, while restacking increases van der Waals-bound aggregates that reduce accessible surface area and effective contact geometry.
• Mechanism class: Shear-induced reorientation vs. shear-induced fragmentation — reorientation changes anisotropic connectivity because platelets align with flow, altering direction-dependent percolation, while fragmentation reduces platelet size and thereby increases the number of insulating gaps.
• Mechanism class: Thermal oxidation/chemical modification vs. purely mechanical damage — chemical modification changes intrinsic conductivity at edges and contact chemistry because oxidation increases defect density, whereas mechanical damage primarily alters geometry and contact topology.

Scope and Limitations

• Applies to: thermoplastic systems and melt-processing routes (extrusion, injection molding, melt re-extrusion) where GNPs are present as dispersed platelets and entire percolative networks are formed in the bulk because melt shear and thermal exposure are the dominant processing inputs.
• Does not apply to: solvent-cast films, room-temperature cured thermosets, or vapor-phase deposited graphene networks where no melt shear or regrind occurs because the primary damage mechanisms (melt fragmentation, grinding redistribution) are absent.
• May not transfer when: GNPs are chemically grafted with robust covalent bonds to the matrix or embedded within continuous metal coatings because covalent anchoring and encapsulation prevent contact loss and fragmentation effects from changing resistivity in the same way.
• Physical/chemical pathway (separated): Absorption — GNPs are not absorbing energy here; they provide conductive pathways. Energy conversion — during re-extrusion mechanical work converts to localized stress at platelets producing fracture and increased edge defects. Material response — platelet fragmentation reduces lateral size and increases contact resistance; restacking and interfacial debonding increase tunneling distances and decrease effective coordination number, therefore percolation probability drops and bulk resistivity rises.
• Because the dominant causal chain is mechanical energy -> flake geometry change -> contact topology change -> altered percolation, therefore resistivity shifts follow processing history; unknowns remain in exact fragmentation rates per GNP grade and in threshold regrind fractions for specific polymer/GNP systems.

Related Links

Application page: ESD & Anti-Static Plastics

Failure Modes

• Why carbon black causes resistivity overshoot in ESD plastics — role of contact-limited conduction and thermal effects (comparison to GNP/FLG).
• Why Carbon Black Migrates Or Blooms In Polyolefin Anti Static Compounds
• Why Cnts Overshoot Conductivity Targets In Static Dissipative Plastics

Mechanism

• How Gnp Percolation Threshold Emerges In Polymer Compounds

Key Takeaways

• Re-extrusion or regrind changes bulk resistivity primarily.
• Observed: Sheet resistivity rises after one or more re-extrusion cycles.
• Variable: GNP lateral size and layer count.

Engineer Questions

Q: What processing variable most quickly increases resistivity during re-extrusion?

A: High shear rate (screw speed, narrow die) combined with elevated melt temperature; these increase mechanical stresses on platelets and accelerate fragmentation and interfacial degradation, raising contact resistance and breaking conductive paths.

Q: At what point does regrind fraction become risky for ESD properties?

A: Exact numeric thresholds are formulation-specific and depend on GNP size/loading and matrix rheology; risk increases sharply when regrind fraction produces local concentrations of damaged filler or when the effective GNP loading approaches the percolation threshold because network redundancy is lost.

Q: Can a post-process thermal anneal restore resistivity after re-extrusion?

A: Partial restoration is possible if chain mobility allows reformation of contacts and relaxation of residual stresses, but irreversible fragmentation and interfacial chemistry loss cannot be fully reversed because lateral size and broken bonds do not reconstitute under typical anneal conditions.

Q: How does GNP lateral size affect sensitivity to regrind?

A: Larger lateral size provides fewer required contacts to percolate, so fragmentation of large platelets removes proportionally more conductive area and makes networks more sensitive to shear-induced breakage compared with already small platelets.

Q: Which analytical checks detect regrind-induced network damage?

A: Combine electrical resistivity mapping, Raman spectroscopy for increased defect ID/IG ratio, and microscopy (SEM/TEM) for platelet size distribution and agglomeration; together these reveal increased defects, reduced lateral size, and contact topology changes.

Q: What formulation controls reduce resistivity drift from re-extrusion?

A: Use of appropriate compatibilizers, control of processing shear via melt viscosity and screw/die design, higher designed redundancy in filler loading above percolation, and limiting regrind fraction because these reduce fragmentation, maintain interfacial contact, and prevent dominance of damaged filler in final parts.