The prediction of the service life of flexible membrane liners (FMLs) when exposed to chemicals has usually been by way of testing for physical or mechanical property changes after periodic exposure times. This paper presents an alternate approach by evaluating the chemical properties via five different mass transport related measurements of the exposed FML. These tests are water vapor transmission (WVT), radioactive tracer transmission (RT), water absorption (WA), water vapor absorption (WVA), and benzene absorption (BA).
This approach was tested for exposures of up to 15 months. The FMLs used were polyvinyl chloride (PVC), ethylene-propylene-diene monomer (EPDM), chlorinated polyethylene (CPE), and high density polyethylene (HDPE). The chemicals used were 10% sodium hydroxide (NaOH) (in water), 10% sulfuric acid (H2SO4) (in water), 10% phenol (in water), 100% xylene, and water (as a control). The WVT and RT were found to be quite reliable test methods, whereas the WA and WVA techniques experienced serious problems in regard to obeying simple one-dimensional diffusion theory. The BA method worked well for HDPE. The various transport coefficients showed all the expected types of behavior with chemical exposure: (1) constancy with exposure (acids and bases on most all FMLs and all chemicals on HDPE), (2) decrease with exposure (plasticizers leaching from FMLs), and (3) increase with exposure (Phenol-treated CPE). The WVT and RT results were generally complimentary to one another. The transport approach was quite successful in predicting the instability of CPE exposed to phenol. This instability was further verified by differential scanning calorimetry measurements.
The work reported herein lends credence to the use of mass transport measurements to determine structural change in FMLs. Thus the monitoring of transport properties, over a reasonable exposure time to particular chemicals, could serve as the basis for a predictive method for FML lifetime.