MWCNT/Rubber Composites for Pressure Sensing Application

The development of various types of sensors to make our living much more comfortable has become a forefront research topic in the modern world. Pressure sensor is one such development. Even though there are different pressure sensors, developing an easy handle, and low-power sensor specifically to measure in-situ pressure change of passenger and industrial tires in motion remains a challenge. Developing a sensing material with flexibility and stretchability to withstand the harsh conditions of tires while being compatible with the tire material is one of the main challenges faced during the production of such pressure sensors. The use of piezoresistive sensing materials is quite a significant approach for this matter. Therefore, in this work, rubber-based piezoresistive materials, which could embed to the tire, were developed. Since rubber materials are electrically insulative, highly conductive Multi-Walled Carbon Nanotubes (MWCNTs) were incorporated to the rubber matrix to obtain the piezoresistive effect. π orbitals of MWCNTs being more delocalized to the outer area of their cylindrical structure and having σ bonds slightly out of the plane, σ-π rehybridization occurs making them mechanically stronger, electrically and thermally more conductive. Therefore, it was assumed that the use of MWCNTs to develop piezoresistive rubber compounds, which could withstand the high loads and harsh conditions as a more promising solution. Since rubber is flexible and stretchable, once a certain amount of pressure is applied on the material, it tends to compress. The rubber vulcanizate obtained after milling MWCNTs with rubber would have evenly spread MWCNTs across the rubber matrix. Therefore, when the rubber matrix gets compressed under certain pressure, the electron tunneling distances between the MWCNTs would reduce. Since MWCNTs are nanoscale 1D cylindrical tubes with delocalized π-electron cloud, under the applied pressure they would form more conductive networks across the rubber composite resulting in low resistance compared to its original form. It was identified that the types of rubber and composition of MWCNTs of the composite would affect the piezoresistive behavior of the composite. Therefore, in this study, 20 phr MWCNT incorporated natural rubber (NR), ethylene propylene diene monomer rubber, nitrile rubber, butadiene rubber, and styrene-butadiene rubber composites were tested for their piezoresistive behavior to identify the most suitable rubber matrix. MWCNT/NR composite showed the highest resistance change of more than 1000 folds of its original resistance when a load of 200 kN was applied and removed in a certain time interval. Therefore, it became evident that NR shows better flexibility and relaxation compared to other synthetic rubber lattices. Then, MWCNT/NR composites with varying MWCNT compositions of 10, 20, 30, and 40 phr were developed to observe their piezoresistive behavioral change under a range of loads. All these composites showed a similar pattern in exponential decrease of their resistance with increasing pressure. However, when applying high-pressure values in the range of 2000 – 12000 kNm^-2 their logarithmic resistance (log R) showed a linear reduction with increasing of pressure. At low-pressure values, log R reduced exponentially and move to a plateau around 500 kNm^-2. However, a linear relationship is expected even at low-pressure values, if the MWCNT composition is comparable with the percolation threshold value. Since the resistance values depend on the thickness of rubber composites, all these data were collected under a constant thickness. Currently, the development of a digital prototype of a piezoresistive pressure sensor to measure the real-time, in-situ pressure changes based on the obtained data is in progress. Here the analog signal is converted to a voltage signal via a potential divider under a low power supply and obtaining the real-time pressure reading to a display via Bluetooth.