Constrained Foaming Behavior Of Polypropylene Film Assisted With Supercritical Carbon Dioxide And Its Application In Piezoelectret

Polypropylene(PP)has excellent performances.However,it is a semi-crystalline polymer.Due to the limitation of the crystalline region in the polymer matrix,the temperature operation window of foaming process is narrow and the foam product quality is poor.It is very difficult to prepare PP foam with ideal cell structure.And the performance of the foam material is closely related to the morphology of the cell.The oriented cell structure can enhance the performance of the foam.Therefore,improving the foaming performance and controlling the cell structure of PP foam is a necessary way to broaden the application of PP foam.This paper focuses on the factors affecting the formation of oriented cell structure.According to the crystallization characteristics of PP under CO2 atmosphere,a new foaming strategy is designed to improve the performance of PP during foaming.In the CO2 atmosphere,the non-isothermal crystallization induced perfect crystallization with a high melting point.The perfect crystallization can support PP melt strength,which improves the foaming performance of PP and makes PP foam have oriented cell structure with high orientation in the constrained foaming process.PP piezoelectret was prepared by using the oriented PP foam,and the factors affecting piezoelectricity and charge stability in PP piezoelectret were explored.Compared the constrained foaming behavior with linear polypropylene(iPP),long branched polypropylene(LCBPP)with small thickness(h=0.1 mm,0.3 mm)was difficult to form excellent quality foam by regulating the temperature and pressure conditions due to gas diffusion.LCBPP with larger thickness(h=0.5 mm,1.0mm)could form uniform oriented cell structure by changing the conditions.However,iPP couldn’t get a uniform oriented cell structure as LCBPP.In the constrained foaming process,the form of the oriented cell structure is related to the expansion ratio and the melt strength of the sample.When the system viscosity is high,it is not benificial for bubbles to grow,the expansion ratio is low,the bubble is difficult to deform.When the system viscosity is conducive to bubble growth,the expansion ratio is high.The surface tension has a positive correlation with the melt strength.The higher viscoelasticity of the LCBPP makes the surface tension higher,which supports the deformation of the bubble.Therefor,it is easier for LCBPP to form oriented cell structure.iPP foam with uniform oriented cell structure was successfully prepared by the new foaming strategy.The orientation of the cell could be increased to 6.6 by increasing the crystallinity of the iPP system through non-isothermal crystallization in the CO2 atmosphere.The choice of the initial temperature was very important.If there was residual crystal after saturation,it could be mucleation point which greatly promoted the improvement of crystallization.Likewise,the new foaming strategy also effectively improved the foaming properties of LCBPP,making LCBPP easier to obtain highly oriented cell structure.The iPP and LCBPP foam samples with oriented cell structure under the new foaming strategy were selected to be piezoelectrets.The cells had an oriented structure with a cell length of 100-200 μm and a height of 20-30 μm.The piezoelectric coefficient(d33)of iPP and LCBPP was up to 300-400 pC/N.By adjusting the height of samples through hot press process,the piezoelectric activity of the PP piezoelectret could be improved.Compared with iPP,LCBPP had better charge stability.LCBPP has long branched chain structure,and the long chain can be entangled with each other.These entangled microstructures provide deeper traps for charge storage,which increase the charge stability.