Piezoelectric property improvement of polyethylene ferroelectrets using postprocessing thermal‐pressure treatment

Abstract

In this work, biaxially stretched polymer foams with well‐defined cellular structures were prepared from polyethylene via blown‐film extrusion and subjected to corona charging to produce a piezoelectric response. The charging parameters were first optimized in terms of charging voltage and needle distance, as well as the gas type and pressure to investigate their effect on the piezoelectric coefficient (d33). The results show that samples charged under nitrogen (N2) at 100 kPa had better d33 coefficient than those charged under ambient air or N2 at 20 kPa. Moreover, 2 different thermal pressure treatments were imposed to obtain an optimized eye‐like cellular structure with different cell aspect ratios (AR). The results showed that when the cells were elongated in both the longitudinal and transverse directions (higher AR), higher d33 coefficients were achieved. From all the samples produced, the best results were obtained for a longitudinal aspect ratio (AR‐L) of 7.1, a transversal aspect ratio (AR‐T) of 4.6, and a relative foam density of 0.52 leading to a d33 coefficient of 935 pC/N. This coefficient was further increased using reverse charging and multilayered films, reaching a maximum of 2550 pC/N. This value is much higher than typical ones reported so far for any polyethylene and polypropylene ferroelectrets. These results could increase the use of polyethylene in piezoelectric applications as these materials are very attractive for the large‐scale production of electret‐based sensors and transducers due to their low cost and easy processing.

Time and thermal stability improvement of polyethylene ferroelectrets

“…As described in our previous work, two charging techniques can be combined to improve the piezoelectric coefficient. First, the films were charged two times by reversing their position (flip‐over) under the corona discharge (reverse charging method), and then three‐layer films of the reversely charged sample were superposed by attaching the positive and negative charges to the surface with the same charge .…”

“…The material used in this study was a biaxially stretched foamed PE film with an eye‐like cellular morphology . A blend of linear low density polyethylene (LLDPE 8555 from Exxon Mobil Chemical, Irving, TX) and low density polyethylene (LDPE LF‐0219‐A from NOVA Chemicals, Calgary, AB, Canada) was used as the matrix polymeric system.…”

“…The samples were charged under nitrogen (N 2 ) at a pressure of 100 kPa with a needle–sample distance of 4 cm, which was shown to give the best piezoelectric properties. More details on the film production and morphological analysis can be found in our previous work . The optimized film had a good piezoelectric coefficient d 33 reaching 935 pC/N for film S1 (previously coded 2n‐6.4‐TPT1) .…”

“…More details on the film production and morphological analysis can be found in our previous work . The optimized film had a good piezoelectric coefficient d 33 reaching 935 pC/N for film S1 (previously coded 2n‐6.4‐TPT1) . This sample has a density of 490 kg/m 3 , a longitudinal cell aspect ratio (AR‐L) of 7.18, a transversal cell aspect ratio (AR‐T) of 4.67, and a cell density of 4.83 × 10 6 cell/cm 3 .…”

“…In our previous work, cellular films based on PE with high piezoelectric properties ( d 33 = 935 pC/N) were successfully produced by controlling the cellular morphology in terms of the cells aspect ratios in both the longitudinal and transversal directions, as well as the cell density and the film density. The d 33 values obtained were well above typical values for PP (around 800 pC/N) considered as the workhorse of piezoelectric polymers.…”