On the Synergistic Effect of Multi-Walled Carbon Nanotubes and Graphene Nanoplatelets to Enhance the Functional Properties of SLS 3D-Printed Elastomeric Structures

At a Glance

Metadata	Details
Publication Date	2020-08-17
Journal	Polymers
Authors	Gennaro Rollo, Alfredo Ronca, Pierfrancesco Cerruti, Xin Gan, Guoxia Fei
Institutions	Institute of Polymers, Composites and Biomaterials, Vilnius University
Citations	37
Analysis	Full AI Review Included

Technical Analysis: Synergistic Carbon Fillers in SLS-Printed Elastomers

Executive Summary

This research successfully demonstrates the fabrication of multifunctional Thermoplastic Polyurethane (TPU) elastomeric structures via Selective Laser Sintering (SLS) using a synergistic mixture of Multi-Walled Carbon Nanotubes (MWCNTs) and Graphene Nanoplatelets (GE).

Synergistic Enhancement: The combination of MWCNTs and GE (70/30 wt/wt) at a low 1 wt.% loading minimizes particle coalescence during sintering, leading to a highly effective segregated conductive network and improved electrical properties compared to single-filler systems.
Robust Piezoresistivity: All porous structures exhibit a robust negative piezoresistive behavior, achieving high absolute Gauge Factor (GF) values of approximately -13 at 8% strain. GF values reached up to -70 in the sensitive 1% strain range, confirming suitability for high-sensitivity strain sensing.
Mechanical and Thermal Stability: The carbonaceous fillers significantly improve the thermal stability of the TPU matrix (degradation onset shifted to 310 °C). Gyroid (G) geometries demonstrated superior mechanical properties and better percolating networks than Diamond (D) geometries.
Broadband EM Absorption: The structures exhibit outstanding Electromagnetic Interference (EMI) shielding properties, primarily through absorption, across a wide frequency spectrum.
High-Frequency Performance: Microwave absorption coefficients range from 0.70 to 0.91 in the Ku-band (12-18 GHz). THz absorption is near-perfect (close to 1) between 300 GHz and 1 THz.
Tailored Porosity: The resulting materials possess a multi-level porosity structure (cellular SLS lattice pores and nanoscale defects in the skeleton), allowing for the tailoring of EM absorption characteristics across different frequency slots.

Technical Specifications

Parameter	Value	Unit	Context
Matrix Material	Thermoplastic Polyurethane (TPU)	N/A	Mophene3D T90A
Filler Type	MWCNTs and GE	N/A	MWCNTs/GE ratio: 70/30 wt/wt
Total Filler Loading	1	wt.%	Used for all composite powders
Porosity Range (Designed)	20 to 60	%	Gyroid (G) and Diamond (D) unit cells
Max Elastic Modulus (D20)	15	MPa	TPU/MWCNTs system
Min Elastic Modulus (D60)	1.5	MPa	TPU/MWCNTs system
Max Absolute Gauge Factor (GF)	-13	N/A	Measured at 8% strain for all systems
High-Sensitivity GF Range	-70 to -20	N/A	Observed over 1% to 5% strain range
DC Conductivity (G20, MWCNTs-GE)	~0.02	S/m	Low frequency (0.02 kHz-1 kHz)
Thermal Degradation Onset	310	°C	First degradation step for TPU/(MWCNTs-GE)
Microwave Absorption Coefficient (G20)	0.70 to 0.91	N/A	Ku-band (12-18 GHz), 10.6 mm thickness
THz Absorption Coefficient	Close to 1	N/A	300 GHz-1 THz, 2 mm thickness
G60 Trabeculae Thickness	1.360 ± 0.001	mm	30% thicker than D60 trabeculae
SLS Laser Power	14	W	Set at 40% of maximum energy
Part Bed Temperature	85	°C	Optimized sintering parameter
Layer Thickness	100	µm	SLS printing resolution

Key Methodologies

The fabrication process involved two main stages: nanocomposite powder preparation and Selective Laser Sintering (SLS) 3D printing.

Filler Dispersion: MWCNTs and GE (70/30 wt/wt) were pre-dispersed using wet ball milling (1 h at 300 rpm) followed by sonication (40 W for 1 h) in anhydrous ethanol to ensure a stable dispersion.
TPU Powder Coating: TPU powder was added to the dispersion to achieve a final 1 wt.% filler content and mechanically stirred for 2 h.
Powder Processing: The resulting mixture was filtered, vacuum dried at 70 °C for 24 h, and sieved to remove particles larger than 150 µm. Silica powder was added to optimize powder flowability for SLS.
Structure Design: Triply Periodic Minimal Surfaces (TPMS) equations (Gyroid and Diamond) were used to generate CAD models for cubic structures (10 x 10 x 10 mm³) with porosities of 20%, 40%, and 60%.
SLS Fabrication: Structures were printed using a lab-scale SLS machine with optimized parameters: 14 W laser power, 85 °C bed temperature, and 100 µm layer thickness.
Morphological Analysis: SEM and TEM confirmed the formation of a segregated conductive network where fillers were confined between sintered TPU particle boundaries (thread thickness 200 to 500 nm).
Piezoresistive Testing: Samples were subjected to cyclic compression (up to 8% strain) while monitoring electrical resistance (2-probe method) to calculate the Gauge Factor (GF).
Electromagnetic Characterization:
- Low-frequency conductivity (DC-like) was measured using an LCR-meter (100 kHz-1 MHz).
- Microwave response (12-18 GHz, Ku-band) was measured using a vector analyzer and rectangular waveguide.
- THz response (0.2-1 THz) was measured using time-domain spectroscopy to determine absorption coefficients.

Commercial Applications

The resulting multifunctional elastomeric structures are highly relevant for applications requiring flexibility, high strain sensitivity, and effective electromagnetic management.

Application Area	Relevant Property	Specific Use Cases
Flexible Electronics & Sensing	High Negative Piezoresistivity (GF up to -70)	Wearable strain sensors, electronic skin, flexible pressure mapping arrays, soft robotics feedback systems.
EMI Shielding & Absorption	Broadband EM Absorption (Ku-band & THz)	Lightweight, high-efficiency radar absorbers for defense and aerospace platforms; shielding enclosures for sensitive high-frequency electronics.
Additive Manufacturing (SLS)	Tailorable Architecture (TPMS)	Customized orthopedic implants, flexible seals, and gaskets with integrated sensing or thermal management capabilities.
Smart Actuators	Electrical Conductivity in Elastomer	Electro-active polymers (EAPs), soft actuators, and components requiring integrated heating elements or electrical pathways under high strain.
Thermal Management	Enhanced Thermal Stability	Components exposed to elevated operating temperatures where TPU stability is critical, such as automotive or industrial seals.

View Original Abstract

Elastomer-based porous structures realized by selective laser sintering (SLS) are emerging as a new class of attractive multifunctional materials. Herein, a thermoplastic polyurethane (TPU) powder for SLS was modified by 1 wt.% multi-walled carbon nanotube (MWCNTs) or a mixture of MWCNTs and graphene (GE) nanoparticles (70/30 wt/wt) in order to investigate on both the synergistic effect provided by the two conductive nanostructured carbonaceous fillers and the correlation between formulation, morphology, and final properties of SLS printed porous structures. In detail, porous structures with a porosity ranging from 20% to 60% were designed using Diamond (D) and Gyroid (G) unit cells. Results showed that the carbonaceous fillers improve the thermal stability of the elastomeric matrix. Furthermore, the TPU/1 wt.% MWCNTs-GE-based porous structures exhibit excellent electrical conductivity and mechanical strength. In particular, all porous structures exhibit a robust negative piezoresistive behavior, as demonstrated from the gauge factor (GF) values that reach values of about −13 at 8% strain. Furthermore, the G20 porous structures (20% of porosity) exhibit microwave absorption coefficients ranging from 0.70 to 0.91 in the 12-18 GHz region and close to 1 at THz frequencies (300 GHz-1 THz). Results show that the simultaneous presence of MWCNTs and GE brings a significant enhancement of specific functional properties of the porous structures, which are proposed as potential actuators with relevant electro-magnetic interference (EMI) shielding properties.

Tech Support

Original Source

DOI: https://doi.org/10.3390/polym12081841

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