A Finnish company has recently developed cellular BOPP film with density as low as 0.3gm/cm3 compared to the 0.9gm/cm3 density of conventional BOPP film.

The resulting films have soft-touch properties and also exhibit electromechanical behaviour suitable for actuators, sensors and flat thin loudspeakers. This cellular BOPP film known as Triaxcell is comparable to thin (<5 µm) piezo elastic polyvinylidene chloride (PVdF) film used for such applications. The microvoids and elasticity allow it to react as an electret material, which is one that is permanently polarized dielectrically. Thickness differences induced through voltage changes within the film, generate the output signal.

The cavitation is created by mineral fillers, such as calcium carbonate, in the melt; these cause the nuclei to rupture when the film passes through the machine-direction orientation (MDO) unit immediately downstream from the extruder. The film then enters a hot-gas diffusion chamber kept at about 10 bar, where it serpentines over a series of rollers located at the top and bottom of the chamber. The inert gas in the chamber, generally air (to cut costs due to inevitable minor leakage), diffuses into the cavities, causing an overpressure. Immediately after exiting the chamber, the film is oriented in the transverse (TDO) and the thickness directions in another temperature-controlled unit, with high-pressure air or gas expanding the cavities. The degree of orientation, both in the machine and transverse directions can be controlled.

According to the company, the equipment required for this type of film is less capital intensive compared to the existing tenter frame line. The TDO unit and gas diffusion chamber are the only new elements. Processors making unoriented cast PP film could retrofit their existing lines with both units.

Besides traditional polyolefins, the greatest potential exists with cyclic olefin copolymer (COC), which is suited for uses in electronics requiring stable performance at elevated temperatures. Emfitech Ltd. is using a 65- to 70-µm cavitated film for its EMFiT sensors to reverberate sound from the walls of acoustic guitars. The film was chosen because of its strong electromechanical response when charged, said to be 10 times higher than non-foamed and oriented piezoelectric films made of PVdF.

PanPhonics Ltd. has produced an electrostatic panel transducer from the oriented film for use in loudspeakers. The company says the sound field is even, the directionality is controllable to produce narrow sound fields and the frequency response is good with low distortion.

There is also a possibility of using such a film in container labels to reduce polymer use and scrap as well as in food packaging, where the oriented voids could provide insulation and improve barrier properties for products such as ice cream.
(developed by a Conenor Ltd., Lahti, Finland)