Exploring the piezoelectric behavior of meltblown polymer nonwovens for the development of polymeric fibers in energy harvesting and sensing

This thesis investigates the piezoelectric behaviour and thermal stability of melt-blown polypropylene (PP) nonwoven ferroelectrets—a format not yet comprehensively characterised in the literature. As-received PP nonwovens and PP nonwovens produced from a blend containing 30 wt% of H_3 PO_4-treated pellets were studied through two Designs of Experiments (DoEs) and compared with existing results for other ferroelectret polymers. In DoE 1 (process-factor screening), a full factorial design (2 × 4 × 3, 72 tests) quantified the effect of phosphorus treatment (P), intermediate layers (N), and charging voltage (V) on d₃₃ through ANOVA. The phosphorus treatment is the dominant factor (43.7% of total variation, p < 0.001), followed by voltage (13.9%, p < 0.001), with a significant P × V interaction. In DoE 2, thermally stimulated charge decay (TSCD) and temperature-dependent d₃₃ measurements were carried out for the optimal configurations identified in DoE 1. The results show that as-received PP nonwovens exhibit a d_33 approximately 3–4 times higher than their H_3 PO_4-treated counterparts (~108–130 pC⁄N vs. ~30 pC⁄N at room temperature), while treated samples demonstrate improved relative thermal stability. Notably, H_3 PO_4 treatment has the opposite effect on d₃₃ in PP nonwovens (−70%) compared to LDPE (+50%), attributed to the pre-extrusion application altering fibre morphology rather than solely modifying surface trap properties. This work ranks in the middle range among ferroelectret polymers in the literature, with a remarkable d₃₃ for a non-optimised format but limited thermal stability. The sensitivity/stability trade-off, identified in the literature as "the central engineering challenge", is confirmed and quantified for the first time in the nonwoven format.