Mechanical and Chemical Stabilities of Polymeric Membranes

摘要

Low-pressure membranes (microfiltration and ultrafiltration) are increasingly replacingconventional water treatment processes as the result of more stringent regulation andgreatly improved competitive pricing. One important but less certain factor indetermination of the total costs of low-pressure membrane plants is membranereplacement costs. Low-pressure membranes have to experience mechanical andchemical stresses during their service life. There are many factors contributing to themembrane life. Among them, mechanical and chemical stabilities of membranes playimportant roles.Mechanical behavior of polymeric membranes is more complex than elastic solids. Thefactors contributing to mechanical stability include medium composition, architecture,morphology, and membrane construction, as well as the nature of stress. Membraneswith high mechanical stability have high mechanical strength while being sufficientlyelastic to allow the stress to disperse. Composite membranes experience both tangentialand shearing stress on the interface of different media that may lead to delaminating.Chemical stability of polymeric membranes strongly relates to the molecular weight andcrystallinity of the membrane medium. High crystallinity would limit the diffusion ofmolecules of the attaching chemicals within the medium, thus retarding chemicaldegradation.Fatigue tests of polymeric membranes are the useful tool for assessing both mechanicaland chemical stabilities. For mechanical fatigue test, it needs to emulate the conditionsthat are applied in the actual backwash operations with an increased frequency to shortenthe length of the test. Significant increase in elasticity modulus indicates a tendency offracture. For chemical fatigue test, a simpler approach would be to design the test onconservative conditions to ensure the actual exposure of membranes during service lifewithin the regime covered by testing conditions.Membrane plants for drinking water filtration are typically designed for 20 years of lifewith one or more membrane replacements. The membrane replacement can be a verysignificant factor in determining the economics of membrane plants. There are manyfactors contributing to the membrane life. Among them, mechanical and chemicalstability of membranes play important roles. Adequately assessing mechanical andchemical stabilities is critical to ensure the service life of membranes meetingexpectation.