Preparation And Performance Study Of Polyethylene-based Composite Membrane For Moisture-permeable And Air-barrier

With the increasing concern for building energy consumption,the traditional way of regulating indoor temperature and humidity environment can no longer meet the requirements of energy saving and environmental protection,but also can guarantee indoor air quality.In order to solve this problem,a technology for waste heat recovery in building ventilation processes was developed.The air-to-air membrane-based total heat exchanger uses membrane material as a medium to achieve high efficiency energy recovery through simultaneous exchange of sensible and latent heat.In an air-to-air membrane-based total heat exchanger,the indoor exhaust airflow and the outdoor fresh airflow flow in separate channels separated by the membrane material,and when flowing through the membrane interface,temperature transfer and water vapor transmission were drived by the temperature difference and humidity difference between the exhaust airflow and the fresh airflow,thus achieving energy recovery from the exhaust airflow while blocking carbon dioxide（CO₂）from the exhaust airflow from entering the fresh air stream.Because of the quite high latent heat of water vapor in air,increasing the high selective permeability of membrane materials to water vapor as much as possible is the focus of current research in developing new total heat exchange membranes.Poly（N,N-dimethylacrylamide）（PDMAA）is a common hydrogel material and hygroscopic agent which is rich in amide groups and has a high affinity for water molecules.Based on this,PDMAA hydrogel was considered as the main water-absorbing and moisture-permeable component in this study.However,the single hydrogel membrane cannot be used as a humidity permeable and air barrier membrane alone owing to its drawbacks such as low strength,molding difficulty,and difficult to control the membrane thickness.In this study,we consider the preparation of PE/PDMAA composite membrane for moisture vapor transmission and gas barrier using porous polyethylene（PE）membrane as the support layer.To obtain PE/PDMAA composite membranes with high water vapor transmission rate and low CO₂transmission rate,the following works were carried out in this study:（1）An asymmetric composite membrane consisting of a thin layer of dense PDMAA gel and a porous PE membrane support layer was prepared by interfacial free radical polymerization.In the preparation process,DMAA monomer and crosslinker were dissolved in the water,while photoinitiator was dissolved in hexane,which is mutually insoluble with water.The PE membrane was placed between the two phases.When there is UV light source radiation,free radical polymerization and cross-linking reaction occurs at the interface between the monomer aqueous solution and PE membrane,forming a cross-linked PDMAA gel layer on the membrane surface.The thickness of PDMAA gel layer can be effectively regulated by adjusting the concentration of DMAA monomer solution and photoinitiation time.When the concentration of DMAA monomer was 20 wt%and the photoinitiation time was 3 s,the obtained PE/PDMAA composite membrane reached a water vapor transmission capacity of 1541 g/m²·24h and a CO₂ transmission rate of 4.5×10⁴ cm³/m²·24h·0.1MPa.（2）In order to solve the problems of PE/PDMAA asymmetric composite membrane with easily damaged selective separation layer and low service life,and to further improve the selective permeability of the composite membrane for moisture and gas.The PE/PDMAA symmetric composite membrane was prepared by in situ photoinitiated radical polymerization,whereien the porous PE membrane was pre-impregnated by the reaction solution consisting of DMAA monomer,crosslinker,and photoinitiator and then placed under a UV light source to initiate the polymerization and crosslinking of DMAA monomer in the pores.In the obtained PE/PDMAA symmetric composite membrane,a highly interlocked physical cross-linked structure was formed between PDMAA and PE,which solves the problem that the selective separation layer of PE/PDMAA asymmetric composite membrane is easily damaged.At the same time,the hydrophilic PDMAA gel filled within the PE membrane matrix provides a transport channel for the transfer of water molecules while ensuring the CO₂barrier performance of the composite membrane.At a monomer concentration of 80 wt%（M-80）,the prepared PE/PDMAA composite membrane has excellent water vapor transmission capacity（2420 g/m²·24 h）,while the CO₂ transmission rate is only1.19×10⁴ cm³/m²·24h·0.1MPa,which effectively balances the Trade-off effect between moisture permeability and gas barrier of the total heat exchange membrane.The sensible heat recovery efficiency of M-80 composite membrane is 94.51%,the latent heat recovery efficiency is 16.27%,and the total heat recovery efficiency is 38.51%.