Improvement of the electro-elastic properties for piezoelectric energy harvesting smart polyurethane composite coating by integration of a conducting additive: Finite element analysis-based prediction and experimental validation

 
Piezoelectric energy harvesting smart composite coating formulations are mostly based on a non-piezoelectric polymer matrix phase (acrylic, epoxy, alkyd, and polyurethane chemistry) having a piezo-active ceramic as the primary inclusion phase. In our work, we have focused on solvent-borne polyurethane paint systems mainly composed of four primary ingredients. The matrix phase is castor-oil-derived flexible polyurethane (COPU) polymer matrix, piezoelectrically active BaTiO3 (BT) as the primary pigment, compatible acetone solvent, and multiwalled carbon nanotubes (MWCNTs) as an additive. The primary objective of our study is to explore the enhancing effect of the conductive MWCNTs as an additive on the electro-elastic properties of the composite coating. Castor-oil-derived COPU/BT/MWCNTs-based three-phase (CPUBT/CNT) smart coating films were synthesized having 0.03, 0.12, and 0.25 piezo-active BT inclusion volume fractions (vf ). The MWCNTs concentration was maintained to be constant at vf = 0.0037 with respect to the COPU matrix phase. The energy harvesting open circuit voltage (VOC) output, and short-circuit current (ISC) study was done by applying a dynamic compressive load of 25 N, applied at variable impact frequencies (0.5 Hz, 1.0 Hz, 2.0 Hz, and 5.0 Hz) equivalent to human walking to the running frequency range. A maximum ISC output of ~27.7 nA and VOC output of ~6.5 V is obtained for the three-phase composite having 0.25 vf BT, at 25 N dynamic load applied at 5.0 Hz impact frequency. With the incorporation of MWCNTs, output ISC and VOC values increase by ~1.22 and ~1.28 times respectively compared to two-phase (COPU/BT) composite coating films with 0.25 vf BT filler particles. Microstructural analysis (scanning electron microscopy) confirmed a reasonably uniform distribution of the barium titanate particles within the polyurethane matrix phase. Finite element models based on the microstructural characterization for three-phase piezoelectric composites also generated effective electro-elastic properties, which closely matched the material properties derived from experiments. The experimental piezoelectric strain coefficient (d33), relative dielectric constant (k/k0), and elastic modulus (E) of the composite coating films were compared with finite element analyses (FEA) based prediction data. FEA prediction methodology is based on kinematic uniform boundary conditions (KUBC), effectively studied for four variable representative volume element (RVE) models for every filler volume fraction (having different BT particle distributions within the COPU matrix phase). The predictive k/k0 value evaluated for the three-phase composite having 0.25 vf BT is 15.37 ± 0.47, comparable to the experimental data of 11.15 ± 0.05. The experimental elastic modulus values for the same three-phase composite were evaluated to be 11.63 ± 0.04 MPa (relatively overestimating compared to the predictive elastic modulus of 6.89 ± 0.54 MPa). The FEA-based predictive d33 value for three-phase CPUBT/CNT having 0.25 vf BT filler is 2.2 ± 0.04 pC/N, a close match to the experimental d33 value of 1.8 ± 0.1 pC/N. With improved electro-elastic output properties, these coatings can be applied to monitor the structural health by identifying stress-induced cracks on vertical walls of big constructions. However, a post-coating environmental exposure study is critical for the identification and reliability of the energy-harvesting performance of these smart coatings.