Drying polymer membranes for preservation: a review

Preservation is essential to safeguard the performance and structural integrity of membranes post-preparation i.e., during module assembly, transport, and storage in industrial-scale membrane manufacturing. This review synthesises insights from scientific literature, commercial datasheets and patents to identify the most promising preservation procedures for industrial membrane production. First, membrane drying is highlighted as a more efficient route over wet membrane preservation due to easier membrane handling and transport, avoiding the need for biocides while ensuring optimal membrane-adhesive compatibility. However, pore collapse and membrane embrittlement occur during membrane drying, deteriorating respectively filtration performance and membrane integrity. Second, a fundamental understanding of membrane drying is established in which the pore collapse model is revisited to correct the definition of capillary force and account for the surface tension force. Third, three methods for dry preservation are critically reviewed i.e., solvent exchange, use of pore preserving agents (PPAs), and freeze-drying. PPAs combine the most attractive properties as they best preserve the membrane performance, offset brittleness, enable the most straightforward application, are the greenest option, and impose the lowest cost, hence justifying their position as state-of-the-art preservation technique. Finally, potential shortcomings of current preservation procedures are identified through a survey among a selection of membrane industry professionals, revealing long-term preservation stability and waste reduction as important areas of interest which should be addressed in future research.