blog about Multilayer Coextrusion Enhances TPUSWCNT Composite Conductivity

The growing demand for lightweight, highly conductive polymer composites in emerging fields like electronic skin and flexible sensors has driven researchers to explore innovative solutions. Carbon nanotubes (CNTs), with their exceptional conductivity, high aspect ratio, and lightweight properties, have emerged as ideal fillers for polymer-based composites. However, the challenge of achieving uniform CNT dispersion in polymer matrices while maintaining low percolation thresholds remains a critical research focus.

CNTs possess remarkable electrical properties, with intrinsic conductivity reaching approximately 10³ S/m. Incorporating CNTs into polymer matrices to create conductive materials has become a widely used technique, showing tremendous potential in applications ranging from sensors and wearable devices to shape-memory polymers, self-healing materials, and energy storage devices.

The electrical percolation threshold (ϕc) represents the critical CNT concentration where composite conductivity rapidly increases due to conductive network formation. Theoretical studies suggest that CNTs' high aspect ratio could enable achieving ϕc at extremely low loadings (as low as 0.1 wt.%). However, practical challenges including thermoplastic polymers' high viscosity, strong van der Waals forces between CNTs, and weak interfacial adhesion between CNTs and polymers have hindered achieving ideal ϕc at minimal loadings.

In thermoplastic matrix composites, ϕc typically falls between 0.2 to 15 wt.% CNT content. Common strategies to reduce ϕc include enhancing CNT solubility/reactivity through surface modification and purification, as well as using compatibilizers to improve dispersion. Processing method selection also proves crucial for achieving optimal filler distribution.

Various melt-processing techniques have successfully produced well-dispersed polymer/CNT composites, including co-rotating twin-screw extruders and intensive mixers. Less conventional approaches like layered structure assembly offer advantages through selective filler positioning and enhanced dispersion.

Forced assembly multilayer coextrusion provides a continuous, flexible melt-processing route that creates layered structures through repeated stretching, cutting, and stacking of melt flows based on baker's transformation. Typically, two separate polymer melts join in a conventional coextrusion feedblock to form an initial bilayer structure, then sequentially flow through layer-multiplying elements (LMEs) that split and recombine the melt to gradually increase layer count.

This polymer layer confinement has demonstrated enhanced mechanical, gas barrier, optical, dielectric, and shape-memory properties. Layer thickness primarily depends on each component's output and the number of formed layers. Research reports indicate maximum layer counts of 16,384 through multilayer coextrusion, with layer thicknesses ranging from microns to nanometers.

The study designed and fabricated a prototype device applying baker's transformation using small LMEs with DentIncx mixing channels. This approach offers simpler manufacturing requirements while maintaining effectiveness for melt extrusion processes.

The research selected industrial-grade thermoplastic polyurethane (TPU) for its flexibility, wear resistance, and chemical stability. Single-wall carbon nanotubes (SWCNTs) with high purity and uniform diameter distribution ensured optimal electrical properties. Polypropylene glycol (PPG) served as SWCNT pre-dispersant, offering good compatibility and low viscosity to facilitate CNT dispersion.

Researchers first pre-dispersed SWCNTs in PPG through ultrasonication to create homogeneous suspensions. They then mixed TPU with SWCNT/PPG suspensions in specific ratios using twin-screw extrusion at 180-200°C with screw speeds of 50-100 rpm. Static mixers installed at the extruder exit provided additional mixing and shear to enhance CNT dispersion.

The process fed molten TPU/SWCNT composites and pure TPU separately into multilayer coextrusion equipment containing a coextrusion feedblock and multiple LMEs. The initial bilayer structure formed in the feedblock underwent repeated layering, stretching, and recombination through LMEs, ultimately creating structures with hundreds or thousands of layers. Adjusting melt flow rates and LME quantities enabled precise control over layer thickness.

Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) revealed significantly improved SWCNT dispersion in TPU matrices after static mixing and multilayer coextrusion, with markedly reduced agglomeration. TEM observations further confirmed uniform SWCNT distribution and orientation within TPU layers.

Tensile testing demonstrated that TPU/SWCNT composites exhibited higher tensile strength and elastic modulus than pure TPU, though with slightly reduced elongation at break. Multilayer coextrusion produced composites with anisotropic mechanical properties, showing higher tensile strength along the extrusion direction compared to perpendicular orientations.

Four-point probe measurements revealed a conductivity threshold at 0.3 wt.% SWCNT content, indicating effective conductive network formation. Conductivity continued increasing with higher SWCNT loadings. Multilayer coextrusion produced composites with significantly higher conductivity than conventional melt-mixed counterparts, attributed to superior SWCNT dispersion and alignment.

The study demonstrates that multilayer coextrusion combined with SWCNT pre-dispersion and static mixing effectively enhances TPU/SWCNT composite conductivity. Pre-dispersion reduces SWCNT surface energy and agglomeration tendencies, while static mixing provides thorough melt homogenization and shear. Multilayer coextrusion optimizes SWCNT distribution through controlled layered structures, achieving exceptional conductivity at low CNT content.

The observed mechanical anisotropy correlates with SWCNT orientation within TPU layers. Along the extrusion direction, predominantly aligned SWCNTs increase tensile strength, while more random perpendicular orientations show lower strength.

This research successfully employed multilayer coextrusion to produce high-performance TPU/SWCNT composites. Through SWCNT pre-dispersion, static mixing, and multilayer coextrusion, the study achieved excellent SWCNT dispersion and alignment, yielding superior conductivity at low CNT content while maintaining flexibility.

Future research directions include:

• Optimizing multilayer coextrusion parameters for enhanced composite performance
• Investigating different CNT types' effects on composite properties
• Extending the technique to other polymer matrix composites
• Exploring applications in smart materials and biomedical composites

Multilayer coextrusion presents significant potential for advanced polymer composite development, promising to meet growing demands for high-performance, multifunctional materials across various industries.