By Edward C. Gregor
Over the past 40 years, significant growth has been the hallmark for membranes of every variety in high-efficiency liquid filtration applications. Microporous membranes are most commonly found in single-use pleated cartridges in process industry systems and medical devices, whereas RO and UF membranes appear in spiral wound elements and to a lesser degree as hollow fibers with both used in ‘cross-flow’ mode and systems. This article focuses on reverse osmosis (RO), microfiltration (MF), nanofiltration (NF) and ultrafiltration (UF) flat stock spiral wound membranes and their support fabric substrates. Whether in an undersink RO or a coastline desalination plant, these fabrics are the substrate.
Membrane construction and process
RO, MF, NF and UF membranes are all constructed from highly engineered compounds using wet cast polymeric materials including PVDF, cellulose acetate and polyethersulfone. During manufacture, the polymer compound is cast (or more aptly, ‘coated’) onto one side of a fabric support structure on which a membrane film forms. This thin polymeric membrane fabric substrate or support consists of a woven, nonwoven or wetlaid nonwoven fabric. Its purpose is to provide the coating surface, dimensional stability, strength, tear resistance and durability, thereby allowing for processing into spiral wound modules.
Anatomy of membrane support fabric
RO, MF, NF and UF membrane manufacturers primarily purchase 42/44 inch wide fabrics, although there are circumstances for 62/64 inch wide fabric. Although the majority of all spiral wound modules are ultimately trimmed to the width necessary for a particular application, extra base fabric width allows for material processing and working loss during the membrane casting process and final production of the modules.
The majority of membrane elements utilize a polyester fabric as the base substrate, whereas polypropylene fabric substrates are used when chemical resistance and/or inertness is essential, especially when aggressive cleaning agents are employed. The typical base support is constructed as a wetlaid nonwoven fabric, whether polyester or polypropylene. These wetlaid fabrics are supplied by the producer to the membrane manufacturer in large bulk rolls ready for direct membrane casting. Spunbond nonwovens and woven fabrics were used for many years as the most popular base substrate, but that has declined in recent years in favor of the wetlaid nonwovens, which have proven to be more consistent, although not an ideal construction.
Membrane support fabrics require a high degree of consistency and an imperfection-free surface for coating. The surface must be exceptionally flat and very smooth without loose or standing fibers. Standing fibers are the single biggest on-going headache for membrane manufacturers. When individual or groups of fibers are loose or stand up above the plane of the substrate, it is impossible for the polymer to form an uninterrupted imperfection-free surface during the casting process. These surface imperfections typically cause defects in the membrane, such as pinholes or larger voids referred to as ‘holidays’ in addition to lacking surface integrity at the point of imperfection and constitute a defective module.
The inherent downside of a wetlaid nonwoven fabric substrate support construction is the wetlaid papermaking process, which requires millions of short cut fibers (typically three to six mm long) per square meter. When fibers protrude vertically or randomly upward from the horizontal fabric coating surface plane, problems begin. Unless these fibers are flattened onto the web by post-calendaring, flame-singeing or other means, they cause the liquid polymer to flow around or migrate away from the fiber. Pinholes and defects form as the polymer begins to solidify during the casting process.
Other membrane processing issues arise with membrane substrates, including the consistency of the substrates thickness. Problems occur when thickness variations occur across the width of the fabric. Any side-to-side thickness inconsistency of the web creates difficultly when casting the membrane, such as thin fabric areas causing the polymer to puddle in the valleys or low areas. High spots across the width result in a thin polymer coating and become a risk for pinholes. When thickness varies across the width of the fabric, it becomes difficult or next to impossible to roll the coated membrane fabric into a spiral with even tension and firmness. Embedded foreign matter or clumps of fibers can easily result in lumps, depressions or even holes in the fabric, causing considerable difficulty during the membrane casting process, as well as module failure. Fabric imperfections are major quality issues and the predominant cause of membrane rejection and/or subsequent field failure if undetected. Often, flaws do not reveal themselves until a module is subjected to initial or subsequent cleaning processes.
To date, wetlaid nonwoven substrates are favored over spunbond and woven fabrics for membrane support; however, the process of using millions of discrete fibers per square meter creates inordinate risks of defects and uneven surfaces. This presents a significant need and an important opportunity for a new innovative support. The ideal substrate would be thin, adequately strong, dimensionally stable, porous with even thickness across the width and, above all, would eliminate standing fibers and foreign matter by the nature of its makeup. After almost half a century of living with membrane support fabric deficiencies, the industry is ready to evaluate new creative concepts, an alternative material process and construction for use as a membrane support.
About the author
Edward C. Gregor & Associates, LLC specializes in creating growth for companies having new or under-exploited technologies, specialty materials and products in the filtration, fiber and nonwoven fabric industries. Edward C. Gregor can be reached at 704-442-1940 or by visiting www.egregor.com.