Preparation of a polyimide nanofiltration membrane for lubricant solvent recovery

Polyimide (PI) membrane has been proven to be an efficient approach for solvent recovery. However, the inherent fragility of the PI membrane limits the range of separation conditions and process economics. In this study, copolyimides were synthesized from 3,3′,4,4′‐benzophenone–tetracarboxylic dianhydride (BTDA) and 4,4′‐biamino‐3,3′‐dimethyldiphenyl–methane (DMMDA) by chemical imidization in a two‐step procedure. Then, a PI nanofiltration (NF) membrane was prepared through a phase‐inversion process for solvent recovery from lube oil filtrates. The results indicated that the immersion of the PI (BTDA–DMMDA) NF membrane in a 1,6‐diaminohexane/ethanol crosslinking agent solution carried on the chemical crosslinking modification, which could effectively improve the solvent resistance of the NF membrane. Moreover, the addition of inorganic salt in the polymer solution further enhanced the solvent resistance and pressure resistance of the membrane, which was favorable for the solvent recovery. The lubricant rejection was above 93%, and the solvent flux was about 30 L m^?2 h^?1 with the NF membrane prepared in optimum conditions, and this membrane showed great potential for future development in the application of solvent recovery from lube oil filtrates. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40338.