Modification Of Polyolefin Membranes By Sol-gel Technique (SiO₂ Gel)

Polyolefin fibers have many advantages such as resistance to alkali and acid, good mechanical behavior and antioxidant performance. At the same time, disadvantages are obvious when they are used in the field of battery separators such as weak liquid absorption and strong thermal contraction.Many factors that affect the stability of sol have been analyzed by orthogonal test, it was found that factors in order of importance were temperature>ethanol>water> H+ and DMF, ethanol>water>temperature when the factors were optimized. The longest storage time of SiO2 sol was 35 days when the best factors were chose. It was found by particle size analysis that the size of sol particles and their distribution situation were the key factors to the stability of sol. The porosity of its gel was 89.33% after calculation and porous materials could be proved for it.At the basis of scanning electron microscope and fourier transform infrared spectroscopy, four kinds of theories about combination of SiO2 gel and polyolefin fibers were put forward and they were theory of mechanical joint, theory of chemical bonds, theory of diffusion and theory of weak bonds connection.There were some common indicators tested for the polyolefin membranes modified by gel layer. It was found that the average breaking strength was increased from 103.3N to 106.3N, and the distribution of them became more centralized; the average elongation at break was decreased from 23.6% to 21.2%, its distribution had the same tendency, which indicated the flexibility of the fiber surface reduced. The thickness of membranes tested was decreased from 0.189mm to 0.185mm after gel coated modification. It was also known that the number of large pores of polyolefin membranes modified by gel layer increased while small pores of them decreased using transmission rate and porosity of experiments. It was proved that the modulus of fiber surface was increased from the force-distance curves, which was consistent with the test results of elongation at break. The intensity of crystallization peaks was declined significantly and a little right skewness after XRD test, which meant the thermal contraction of membrane was decreased with the crystallization.The effects of thermal properties by gel coating on polyolefin membrane were research priority in this paper. Differences in combustion phenomena led to gel-coated fiber combined with the characterization studies, as was shown from the scanning electron microscopy (SEM) images, which reflected a layer of thin film or granular layer was formed on the pits or smooth surface of fibers, some gel block was full among fibers; atomic force microscope (AFM) images showed the stacking gel particles on the fiber surface, the dents on the fiber surface played a role filled so that the surface morphology of different fibers line; the infrared absorption peaks about the polyethylene and polypropylene fibers could be found from the infrared spectroscopy at the same time, which also contained the infrared absorption peaks of silicon oxide, which confirmed the chemical combination of silicon oxide and polyolefin membranes; it also could be confirmed the existence of PP fibers and PE fibers in the polyolefin membranes from differential thermal scanning calorie tests (XRD) because of two endothermic peaks near at 125℃and 160℃, and the value of Delta H of gel-coated polyolefin membranes were increased by 171.8%, 155% and 142.4% respectively, which indicated that the membranes with the addition of gel could absorb more heat and raise the thermal safety performance of membranes.The velocity of liquid absorption and liquid content of gel-coated polyolefin membranes were both improved compared with pure polyolefin membranes; the number of charge-discharge cycle of gel-coated polyolefin membranes was increased by 2 times in 120 hours, the efficiency of charge-discharge capacity was also improved for the gel layer, and more concentrated discharge capacity was relatively stable, which proved the mechanical properties of gel-coated polyolefin membranes were improved, at the same time, the electrochemical performance was also optimized.