Synthesis Of Functional Monomers And Study On Membrane Formation Process For Anovel Polyimide-Urethane Composite Ro Membrane Material

Reverse osmosis as the water purification technology of the 21st century is becoming more important because of the increasing shortage of global water resources. Nowdays the aromatic polyamide composite (PA) RO membranes are the leading products due to their excellent separation performance. However, such membranes are easily fouled and susceptible to chlorine attck, which directly causes a limitation of their further applications in water purification. In this paper, a novel composite membrane material is proposed to improve the chlorine resistance performance of RO membrane.N,N'-dimethyl-m-phenylenediamine(DMMPD) and N,N'-diethyl-m- phenylenediamine(DEMPD) are two key functional monomers designed according to the chlorine degradation mechanism of PA membrane. The novel polyimide-urethane composite membranes made from DMMPD, DEMPD and 5-choroformyloxyisophaloyl chloride(CFIC) through interfacial polymerization have outstanding chemical stability to chlorine.The m-phenylenediamine as substrate was first reacted with the acylating agent of formic acid or acetic anhydride to get the intermediate N, N'-diformyl-m-phenylenediamine or N, N'-diacetyl-m-phenylene diamine, then the resulting intermediate was reduced by NaBH4-I2 to obtain the final product . The synthetic technology was further optimized. The total yield of DMMPD could reach 76.6%, and the total yield of DEMPD could achieve 80.2%. This current method for preparing the DMMPD and DEMPD has several advantages, including the accessible substrate, mild reaction condition, relatively low cost and high yield. DMMPD-CFIC, DEMPD-CFIC and MPD-TMC composite ROmembranes were prepared through interfacial polymerization technique. Scanning electron microscope(SEM), Fourier attenuated total reflectance infrared (FTIR-ATR) and X-ray photoelectronic spectroscopy (XPS) were used to analyse surface properties, element composition and chemical structure of three RO membranes. According to the FTIR-ATR and XPS results, the active layers of three membranes were composed of crosslinked polymer and linear polymer with pedant. Both DMMPD-CFIC and DEMPD-CFIC membranes contain urethane functional bond(-OCONR-), imide functional bond (-CONR-), carboxyl acid functional group (-COOH) and hydroxyl functional group(-OH), while the MPD-TMC membrane contains amide functional bond (-CONH-) and -COOH group. Correspondingly the imide functional group contributed to the excellent stability to chlorine attack of DMMPD-CFIC and DEMPD-CFIC membranes. Furthermore, the effects of preparing technology on the performance of RO composite membrane were discussed, such as concentration of monomers, solvent, pH value, reacting temperature, reacting time and cure temperature and so on. Finally, the reasons for relatively low salt rejection of resulting DMMPD-CFIC and DEMPD-CFIC membranes were analysed.The chemical stabilities to chlorine of three membranes were evaluated by immersing membrane into different concentration NaClO solutions, and FTIR-ATR was used to analyse the changes of chemical structure of membrane active layer before and after immersing. The results showed that the resulting DMMPD-CFIC and DEMPD-CFIC membranes had outstanding chlorine resistance performance.