Study On Forming Mechanism And Pore Control Of Polyolefin Multilayer Microporous Membrane

Polyolefin multilayer microporous membranes are widely used in medical,construction,water treatment,battery separators,food packaging and other fields with their unique structure and performance.With the continuous improvement of battery energy density in the field of new energy,the demand for high-performance polyolefin composite diaphragms with safety characteristics is increasing.Due to the complex preparation process of the composite diaphragms and the problems of poor composite effect and easy peeling between layers,the production and control of the composite diaphragm is extremely difficult.The polyolefin composite microporous membranes are prepared by coextrusion-casting and dry stretching,so that the multilayer melts are compounded in the molten state,and the interlayer adhesion performance can be improved.In this paper,co-extrusion and dry-process mechanism of polyolefin multilayer composite microporous membrane were systematically studied,and the influence mechanism of process factors on the pore-forming performance of the composite microporous membrane was explored which provided important guidance for the preparation of high-performance composite microporous membranes.First of all,the co-extrusion process equipment of the composite precursor membrane was designed according to the characteristics of the co-extrusion casting process and the flow analysis was carried out based on the finite element method.Aiming at the influence of the flow ratio,composite temperature and draw ratio on the composite precursor membrane,the crystallinity and mechanical properties were studied by DSC.2D-XRD analysis and other analysis methods.The influence mechanism of process parameters on the degree of crystallinity,the degree of lamella orientation,the long period and the mechanical properties were analyzed.The film forming properties of composite substrate films with different viscosity relations and the evolution of the microscopic crystal structure with the change of process parameters were studied.Aiming at the annealing and stretching process that affect the pore structure of the multilayer composite microporous membrane,the effect of annealing process parameters on the crystal structure of composite precursor membrane was investigated by 2D-SAXS analysis and other analysis methods.The properties and morphology analysis were combined to study the mechanical behavior and micropore structure evolution of the composite precursor membrane during cold stretching and hot stretching.The deformation mechanism and pore structure performance of the different stages of cold and hot stretching were analyzed.The pore-forming mechanism of multilayer composite microporous membrane was described.And the effect of compound temperature on pore structure and properties was discussed.On the basis of the above mechanism research,the forming process of the multilayer composite microporous membrane was systematically optimized.Taking draw ratio,flow ratio,annealing temperature,annealing time,cold draw ratio,and hot draw ratio as experimental factors.Taking the porosity as optimization objective,the response surface function model of process parameters and porosity was established.By variance analysis and response surface analysis,the independent and interaction influence of process parameters on porosity were analyzed,and the optimal combination of process parameters was determined.