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Sep 1st, 2004 Print this article High gas content improves microcellular BOPP filmsBy Robert Colvin Orientation system for thin cavitated films helps processors cut polymer costs while producing materials targeting new markets. Asmall engineering firm in Finland is looking for licensees of a newly developed technology to produce BOPP films and sheets with microcellular structures. Triaxcell, as it's called, produces films using half the density (less than .3 g/cu cm) of traditional extruded cavitated PP structures, says Markku Vilkki, managing director of Conenor Ltd. (Lahti, Finland). The resulting films have soft-touch properties but also exhibit electromechanical behavior suitable for actuators, sensors, and flat thin loudspeakers. Triaxcell film is comparable to thin (<5 µm) piezo elastic polyvinylidene chloride (PVdF) film used for such applications, says Vilkki. The microvoids and elasticity allow it to react as a 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, says Vilkki. 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 the high-pressure air or gas expanding the cavities. The degree of orientation, both in the machine and transverse directions, can be set by the processor. The company envisions cellular films as thin as 20 µm with 90% gas content. In initial trials, 30-µm film with a gas content of more than 75% was achieved. Vilkki says the equipment footprint is small, and total capital investment is reasonable compared to existing tenter-frame lines. 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, he says. Conenor is seeking a commercial-scale manufacturer. Besides traditional polyolefins, Vilkki sees the greatest potential with cyclic olefin copolymer (COC), which is suited for uses in electronics requiring stable performance at elevated temperatures. Emfitech Ltd. (Vaajakoski, Finland) 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. The films are also being tested in Finnish senior citizen homes for path control under carpets. Picking up walking movements in one direction, they register a lack of return movement as a warning to nurses in the building, who then check to see if a resident has fallen or gone astray. PanPhonics Ltd. (Espoo, Finland) 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. Vilkki also sees possible applications 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. Contact information
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