Issue link: https://www.e-digitaleditions.com/i/1067339
28 January 2019 Tablets & Capsules Bonnets present their own challenge due to their geometry and the fact that the open-mesh fiber density is low, making them prone to damage from abrasion at the site of a crease or fold. When storing bags and bonnets, ensure that the fabric doesn't always fold at the same place to prevent fatigue and other damaging effects. Creases can also encourage the concentration of static charge. This is especially true in nonconductive fabrics but can also occur in conductive (anti-static) fabrics where the bag has not been constructed to fully ground a static charge through the hangers and skirt. Static charge buildup in the filter fabric can cause the particles in the fluid bed to adhere to surfaces and each other, offset the attractive forces of sprayed binders for agglomeration, cause bed channeling, or otherwise disrupt the gas veloc- ity profile across the fluid bed. Discharge of static buildup may cause small burn holes in the fabric from sparks, which can create residue that may ultimately con- taminate the product. Washing and drying Most often, companies clean production fluid-bed dryer bags with commercial washing and drying equip- ment, similar to that used in commercial laundries. Technical fabrics woven from polyester and nylon fibers are strong and resilient and usually hold up well after many wash cycles using standard detergents. To increase filter bag longevity, minimize the amount of time the filters are exposed to heat and mechanical agitation during the washing process. In many cases, pre- soaking the filter bags using a mild cleaning solution (slightly acidic for polyester; slightly alkaline for polyam- ides) or even just fresh water can help lift coarse material from the bag surface and dissolve stubborn residue. Cleaning agents are selected based on the process materials and the residues they will leave on the fabric after the filtration process. Ideally, the cleaning solution and rinse water should be as close to pH neutral as possi- ble. You may be able to use a single cleaning agent for both polyester and nylon bags but should avoid chlori- nated agents for either type. Purified water is ideal. Don't use softened water, because the dissolved solids in the softening chemicals remain on the fabric after washing. Try to wash bags promptly after removing them from the fluid-bed unit to prevent compounds from setting on the fabric surface and to ensure consistent washing results. Wash water temperatures between 105° and 110°F are common for conductive fabrics, but higher temperatures (not exceeding 130°F) may be used for nonconductive fabrics. For machine drying, a temperature of 140°F is common. In general, use the lowest possible wash and dry temperature to get the job done. It's a good idea to use chart or electronic records to provide a basis for validation and ensure that the machines aren't using excessive temperatures. When using commercial laundry services, confirm that their drying equipment uses some form of indirect gas heating applications with substrates such as beads and large gran- ules. For the purposes of this article, I'll use the term "bags" to refer to both fluid-bed filter bags and bonnets. Environmental effects on synthetic fabrics Like other organic materials, the polymer yarns used in filter bag fabrics and sewing thread are susceptible to dam- age from moisture, heat, and ultraviolet (UV) radiation. To optimize bag performance and life, your bag management procedure must mitigate the effects of these factors. Companies often select filter bag fabrics for flu- id-bed applications based on performance factors such as air permeability and nominal particle retention, along with criteria such as dimensional stability, strength, and chemical compatibility. Further limita- tions are found in 21 CFR Part 177, which includes a list of polymers acceptable for use in applications where materials are in contact with food and drugs, taking into consideration such factors as reactivity, leaching, and even the reaction processes used to pro- duce the polymers. As a result, a limited number of polymers are suitable for filter construction. For example, the combination of moisture and heat can affect the dimensional stability of polymer fibers. Polymers absorb and release water, which—along with changes in temperature—can cause the fibers to alter- nately swell and contract. As with other mechanical structures, the repeated stress of swelling and contraction may lead to cracking and eventual failure. Extremely low temperatures can make the fibers brittle and increase the likelihood of cracking, especially at points where the fabric is folded or creased. In addition, swelling of the yarns can decrease the fabric's porosity, essentially reducing the area of the openings in the mesh and even closing off the openings in closed-weave fab- rics. This may lead to unacceptable changes in airflow and pressure drop and may cause filter blinding and poor process performance. Continuous exposure to intense UV radiation may create unwanted chemical reactions, breaking down the polymer chains and reducing the fabric strength. In addi- tion, these reactions often yield unwanted by-products that may leach and contaminate bags that are placed into storage after they are washed. In almost all operations, technicians remove the bags from storage and install them directly in the fluid-bed unit with no opportunity for removing contaminants in between. Mechanical factors affecting filter fabrics Folds and creases in the filter fabric are of special con- cern in long-term storage, where the creases might "set" into the fabric. Sharp bends and folds may create local- ized areas of mechanical stress concentration that can cause yarn failure during operation and especially during the repeated abrupt mechanical shaking action of the fil- ter cleaning process, which can lead to localized wear, fraying, and tearing of the filter fabric.