Preparation And Properties Of TPX Microporous Membranes

Poly (4-methyl-1 pentene) (TPX) is a kind of polyolefin with excellent performance which can be used to prepare high performance membrane. Melt-stretching method has many advantages such as simple process, safe operation and no organic solvent used during the fabrication. In this paper, the initial cast film structure under different melt draw ratios and crystalline structure under different annealing process were studied. Also, the influence of initial crystalline structure, annealing process and stretching parameters on final microporous membrane were investigated. Electronic testing machine, differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and scanning electron microscopy (SEM) were employed to characterize the samples.The experimental results showed that:1. The influence of different melt-draw ratios on the structure and properties of TPX cast film was investigated. In the stress-strain curve, with increasing draw ratios, the yield strength and elongation of the cast film gradually reduced. But, the plastic region became shorter and elastic recovery increased with increasing melt-draw ratios. At low melt-draw ratio, many spherulites were formed. With the increase of melt-draw ratios, the crystalline morphology transformed from spherulites to parallel lamellae perpendicular to extrusion direction. Only form I type crystal existed when the melt-draw ratio was higher than 164. Meanwhile, with the melt-draw ratio increasing, the crystalline orientation degree increased. A high melt-draw ratio of 164 was one of the essential requirements to prepare TPX cast film with parallel lamellae crystalline structure, which was suitable for preparing microporous membrane.2, Within our experiment die temperature, only form Ⅰ crystal existed in the cast film, indicating that the die temperature had no effect on the crystals transformation. With increase of the die temperature, the yield stress and energy consumption during stretching process of the extrusion cast films were decreased. In two cycle loading experiment, we notice that the stress relaxation significantly reduced when die temperature was increased. With increase of die temperature from 255℃ to 270℃, the lamellar structure became better and uniform, but little influence on the crystalline orientation degree. The die temperature showed mainly influence on the lamellar structure. Under die temperature of 270℃, the obtained cast film showed better lamellar structure and perfect mechanical properties.3、Annealing process had significant effect on the structure and properties of cast film. With the increase of annealing temperature from 120℃ to 140℃, the elastic recovery, crystallinity and the crystalline orientation degree increased. But the structure became worse and the crystalline orientation degree decreased when the annealing temperature was further increased. Some spherulites appeared under the annealing temperature of 180℃. Meanwhile, the elastic recovery, crystallinity and the crystalline orientation degree increased with increase of time. The results showed that cast film annealed at 140℃ for 4 hour had high crystallinity and crystalline orientation degree.4、When the stretching ratio reached 60%, microporous membrane with better microstructure could be obtained. With stretching ratio further increasing, bridges were broken down and lamellae slipped, some pores on final microporous membrane were closed. A higher lamellae separation and uniform pore distribution would be obtained with the stretching rate increasing from 50 to 300 mm/min. When the stretching rate exceeded 300 mm/min, the structures of final microporous membrane turned worse.5、The cast film with regular lamellae structure could be obtained at extrude temperature of 270℃, and the better microporous structure was obtained after cold and hot strerching. The final microporous membrane showed better microstructure when annealed at 140℃ for 4h. Further improving annealing temperature, the pore structure turned worse. Meanwhile, the annealing time had same effect on the structure. The better microstructure could be obtained with the increase of annealing time from 1 to 4h. When the annealing time exceeded 4h, the structures of final microporous membrane turn worse.