Gas diffusion expansion-increased thickness and enhanced electromechanical response of cellular polymer electret films

With a high-pressure gas treatment it is possible to force extra gas inside a cellular polymer film structure, expand the film and thus reduce the film stiffness. A permanent increase in the film thickness can be in the range of 100% when using ambient temperature of about 90/spl deg/C and gas pressure of about 20 bar during an 8-min long gas expansion process. High-pressure treatment at lower temperatures will increase the elastic nature of the film expansion and thus decrease the permanent increase in the film thickness. The results show that a clear increase in the electromechanical response up to 480 or 330 pCN/sup -1/ was achieved with 45 or 60 /spl mu/m thick polypropylene sample films, respectively. The corresponding gas contents were 48 and 39%, respectively.