Pressure-sensitive Composites Research Articles

Multiwalled Carbon Nanotube-Coated Poly-Methyl Methacrylate Dispersed Thermoplastic Polyurethane Composites for Pressure-Sensitive Applications

Thermoplastic polyurethane (TPU) is a widely used polymer for a variety of pressure sensing applications because of its softness and shape memory. This work reports the synthesis of novel TPU-based three-dimensional structured (3D) pressure-sensitive composites via the melt mixing method. Poly-methyl methacrylate (PMMA) microbeads of different sizes (5, 10, and 20 µm) were first coated with multi-walled carbon nanotubes (MWCNT) and then incorporated into the TPU matrix for achieving an early electro conductive percolation threshold compared to without PMMA beads. The addition of MWCNT coated PMMA beads reduced the TPU insulated areas by creating a 3D conductive network that finally reflected the early percolation threshold during external pressure. Raman microscopy and XRD results confirmed the MWCNT coated nicely on the surface of PMMA beads. The pressure sensitivity results also confirmed the decrease in resistance of the composites with the increase in the applied external pressure. Composites with 10 µm PMMA bead size showed the most linear responses to the decrease in resistance with increasing pressure and showed a higher strain gauge factor value (3.15) as compared to other composites, which had values of 2.78 and 2.42 for 20 and 5 µm, respectively. Microstructure analysis of the composites by SEM, capacitance, permeability, and thermal conductivity measurements was also investigated to support the above evidence. The results support the suitability of this novel composite as a potential candidate for pressure sensing applications.

External field alignment of nickel-coated carbon fiber/PDMS composite for biological monitoring with high sensitivity

Abstract Flexible, pressure-sensitive composites can be prepared through the inclusion of electrically conductive particles as functional fillers into an elastomeric polymer matrix, which have been used for the applications of wearable devices for health monitoring and electronic skins. A key challenge associated with these composites is developing anisotropic pressure sensitivity while retaining their flexibility (or low filler content). Herein, we demonstrate a simple and scalable method for aligning anisotropic nickel-coated carbon fibers (NiCF) along with the thickness direction of a polymer matrix by applying a magnetic field. The aligning mechanisms and kinetics of NiCF in the polydimethylsiloxane (PDMS) precursor are revealed by in situ optical microscopy images while a magnetic field is applied. The aligned nickel-coated carbon fibers in the polymer effectively endow the composite films excellent pressure-sensitive performance. The pressure sensitivity of NiCF/PDMS composite films has been systematically studied and can be used for biological monitoring. We believe that this magnetic field assisted processing strategy provides a promising material solution for manufacturing fiber embedded polymer composites with enhanced pressure sensitivity, which is essential for future wearable health monitoring electronics and electronic skin.

Behavior of Pressure-Sensitive Adhesives Filled with Metallized Inorganic Particles

The electrically conductive pressure-sensitive composite adhesives based on acrylic polymer and silver-coated inorganic particles have been investigated in this article. The electrical conductivity of the pressure-sensitive adhesives containing silver coated spherical inorganic particles is lower at the same concentration of the filler in comparison with silver coated inorganic fibers, the strength of adhesive joint to aluminum being higher in the case of the pressure-sensitive adhesive containing silver-coated inorganic fibers. After the thermal treatment the strength of adhesive joint to aluminum of electrically conductive pressure sensitive adhesives increases significantly. The suitable concentration proposition of the silver-coated inorganic filler in the pressure-sensitive adhesive requires a compromise solution of the problem taking into account the ultimate adhesive and electrical properties of the investigated pressure-sensitive adhesives composites.

Fabrication process and electrical behavior of novel pressure-sensitive composites

Abstract A novel route was developed to fabricate a new pressure-sensitive composite by dispersing homogeneously conductive carbon particles in an insulating silicone rubber matrix. The composites showed a gradual change in electrical resistivity with applied pressure within percolation threshold region at a constant temperature. This type of gradual fall of resistivity with applied pressure is very important to fabricate pressure sensors. Various amounts of carbon particles were dispersed in a rubber matrix to understand the effect of volume fraction of conductive filler with applying external pressure on resistivity. A quantitative general effective media (GEM) theory was used to understand the resistivity of carbon–rubber composites system over a large range of volume fraction of carbon with applied pressure. The use of two different sizes of silicon rubber particles showed a significant effect in gradual fall of resistivity with applied pressure in the narrow range of percolation threshold. However, a large variation in resistivity from 1st measuring to 10th measuring was observed. A significant improvement in successive measuring of resistivity variation from 1st measuring to 10th measuring was observed when composites were fabricated in hexane solvent media. Finally, nano-sized Al2O3 was dispersed to control the resistivity variation upon successive measurement and to improve the mechanical properties of the composites. The material was suggested to use as unique materials as pressure sensors in practical applications mainly for robots.

Piezoresistivity in Polymer‐Ceramic Composites

Composite materials composed of randomly dispersed semiconducting ceramic particles in an insulating polymer matrix show a pronounced change in resistivity with pressure. Different amounts of iron oxide (Fe3O4) powder and antimony‐doped tin oxide (SnO2:Sb) powder were dispersed in an epoxy polymer matrix to form pressure‐sensitive composites. In each family of materials, an insulator‐to‐semiconductor transition is observed in agreement with percolation theory. Composites within a certain range of filler content showed substantial piezoresistive effect under both uniaxial and hydrostatic pressure in which sensitivity is controlled by the choice of filler material and the volume fraction. The effect of temperature on the piezoresistance effect was also examined. Piezoresistors made from Fe3O4 composites showed larger temperature changes than those filled with Sb‐doped SnO2.