Novel structured non-woven composite materials for broadband acoustic damping in automotive environments

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Abstract

The electrospinning technique allowed for the formation of piezoelectric composite fibres utilising an electroactive polymer in PVDF and an inorganic salt in KCl and piezoelectric, dielectric and thermally conductive ceramic materials including NaNbO3, KNbO3, CeNbO4, SiC, and Si3N4. These fillers were all added to provide control over the average fibre diameters of the PVDF fibres and induce the nucleation of the piezoelectric β polymorph in the PVDF for broadband sound damping applications in the automotive industry.

The addition of the inorganic salt had been demonstrated to assist in the complete removal of beads within the PVDF fibres producing average fibre diameters ranging between 191m – 239 nm. It was also demonstrated that the KCl salt could influence a reduction in the fibre diameter of bead-free fibres with a larger diameter. These fibre diameters were 560 nm – 810 nm. The fibres containing KCl also displayed the highest content of the β polymorph where a value of 99.7 % was obtained with a concentration of 0.8 wt% KCl for the thinner fibres and 86.0 % obtained with a concentration of 0.6 wt% KCl for the thicker fibres. The decrease in fibre diameters for the PVDF / KCl fibres agreed with the increase in the content of the β polymorph within the crystalline phase of the polymer. These results demonstrate that the addition of an inorganic salt can influence the nucleation of the β polymorph and be near phase pure.

The fibre composite mats involving the PVDF, and five ceramic components demonstrated different levels of control over the average fibre diameters of the PVDF. Fibre diameters ranged between 500 nm – 1100 nm for all the fibre mats with the KNbO3 displaying the highest influence on the average fibre diameter. The fibre morphology was dependent on the electrical properties and particle size of the ceramic fillers. Nucleation of the β polymorph was achieved with the addition of all five ceramics with values ranging between 60 % – 74 % with KNbO3 overall assisting to influence the higher values in the content of the β polymorph. The thermal stability of the composite fibre varied with the ceramic type with the niobium oxides decreasing thermal stability and silicon-based ceramics increasing the stability.

In most cases, the PVDF fibre composites enhanced the sound absorption properties of existing sound damping materials with each ceramic type displaying different levels of sound absorption coefficients and resonance peaks. Resonance peaks and overall sound absorption were heavily influenced by the changes in the foamed backing of polyurethane and polyethylene terephthalate with the polyurethane foam-backed setups demonstrating a higher sound absorption coefficient across the entire frequency range. Overall, the PVDF / KNbO3 composite fibres backed with polyurethane foams demonstrated the highest noise absorption coefficient of 0.52 and a shift in the first resonance peak to lower frequencies of 1250 Hz with an absorption coefficient of 0.90.

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