Potting Material Failure
Potting materialis the urethane component that is poured into the filter frame during filterassembly; the filter media is then placed into the potting material to create aleak free bond between the paper element and the filter frame.
A case studyinvolved occasions when leaks were found at this bond only a few months afternew filters were installed and initially passed a leakage test. It wasdetermined the manufacture of the potting material supplied to the filterassembler/manufacture had changed their formulation. The new formulation hadinsufficient quality control before it was released for use. The inferiorpotting material over time would shrink, eventually pulling on the filter elementand breaking the bond and seal.
When selecting afilter manufacturer, it is prudent to inquire about their quality controlmeasures and those applied to their material suppliers. It is also imperativethat the purchase agreement includes a requirement that if at any time thefilter manufacturer suspects a possible universal problem, all end-users of theimpacted filters be notified.
Insufficient Amount of Gel in the Filter Track
There have beensome cases observed in which the HEPA/ULPA filters are delivered from thefactory with an insufficient amount of gel in the track such that the knifeedge does not penetrate the gel sufficiently, resulting in air bypass leakage.
Loss of Gel Adhesion
Loss of geladhesion is not a common failure mode. Adhesion between the gel and othersurfaces may becompromised if the surface incontact with the gel is contaminated with foreign substances or oils. Off-ratiomixing of the gel, or other manufacturing errors or defects with the gel itself,can lead to surface blooming and exudation of unbound components that caninterfere with the surface tack (stickiness) of the gel.
Knife Edge Alignment
The idealposition for the installed filter is with the knife edge at the center of thegel track. In some cases, the knife edge contacts the metal edge of the geltrack; this may result in air bypass (a seal leak) in the immediate area. Spacersor guides may be used to position the filter properly.
Degradation of Filter Gel Performance
There have beenobservations in pharmaceutical applications in which the filter gel materialsappeared to have degraded into a flowable, viscous material rather than a firmgel. This behavior has been studied extensively in the industry and is theresult of several factors that contribute to this failure mode. This failure isprimarily due to thediffusion of unbounded gelcomponents migrating to the surface of the gel. Therefore this migration is adegradation of gel performance; technically it is not a degradation of thebonded gel polymer since the polymer bonds are not broken. The filtermanufacturers have each developed their own gel systems to reduce the risk ofthis mode of gel performance degradation, and it is advised to work with thefilter vendor to understand the appropriate gel system and filter design forthe specific application.
It should also be noted that when gel materials in pharmaceuticalapplications are observed to lose or change color, this does not necessarilyequate to degradation in gel performance. Cleaning and sanitizing chemicalsoften used in pharmaceutical cleanrooms can discolor the gel by bleaching thecoloring agent in the gel, but do not usually break the bonded silicone polymers.
Even though gel systems mightappear on first observation to be similar to one another, they can have otherproperties which make them very different when subjected to operatingconditions in the pharmaceutical industry. However, all gel systems need tomeet the required physical properties to perform the task of sealing the filterframe to the filter housing, such as adhesion and stiffness to allow the correctknife edge penetration and sealing.
Identified factors that contribute to this modeof degradation of gel performance include the following:
The amountof cross-linked (bonded) gel material should be optimized since it isdesirable to minimize the amount of polymer that is unbounded. This needs to bebalanced with the mechanical properties required for the gel to properly sealthe filter. Controlling the ratio of the components and the mixing operationduring manufacturing is critical to a properly cross-linked gel.
The molecularweight of the gel system is important. A higher molecular weight gel systemcomponent reduces the diffusion of unbounded polymer. A narrow distribution ofmolecular weight is most desired. This needs to also be balanced withproperties such as viscosity of starting components and final surface tack ofcured gel.
As much as possible, minimize exposure of the gel to PAO and other nonvolatile oil-basedtest aerosols as excessive PAO may accelerate the rate of unbounded polymerdiffusion by swelling the gel and increasing free volume. Although the liquidtest aerosol will not break apart bonded polymer material, it does act as asolvent to increase the migration of unbounded polymer components. Testing atlow aerosol concentrations, good aerosol distribution, minimizing the amount oftime that the filter is subjected to the aerosol challenge, and alternativetest methods are steps that can be taken.
The gel components need to be manufactured andmixed under a rigid quality controlsystem. This needs to include strict control of the manufacturing andmixing environments, testing of all relevant chemical and physical propertiesof every batch of gel material including limiting any impurities.
Miter jointintegrity of the frame is critical for filter performance. It needs to contain anygel failure from leaking into thecleanroom. Use the correct sealing compounds at the miter joints to preventleakage at the joints.
Silicone gels are moreresistant than urethane gels to oxidizing chemicals normally seen in thecleaning and sanitizing ofpharmaceutical cleanrooms, and thus are generally the preferred gel system.Oxidizing chemicals can attack the urethane gel and create a hardened surface.Semiconductor cleanrooms generally do not use these oxidizing chemicalsroutinely and urethane gels are common in those rooms. Silicone gels willgenerally not degrade in the presence of these oxidizing chemicals. However,there may be some specific applications, such as when acid or caustic chemicalsare present, that the silicone gels may be subject to hydrolysis (rather thanoxidation) and it may be possible that a urethane gel system might be apreferred system.
Filters may be mated to installation structuresusing gaskets to form a reliable pressure boundary seal and to avoid air bypassof the filter element. A variety of gasket materials is available includingneoprene, polyurethane, silicone sponge, etc. Gaskets may be applied manuallywith brushed-on adhesive or pressure sensitive adhesive. Gasketsections should be joined by a flexible adhesive. Alternatively, liquidgasket material can be directly applied in a continuous length along the filterperimeter and allowed to expand into foam and cure in place forming aone-piece, continuous gasket.
Silicone gel has been used toseal filters successfully for many years. It exhibits good qualities, servicelife, and resistance to oxidation. Care needs to be taken when selectingsilicone gels to anticipate the environment to which they will be exposed. Siliconegel should not be used where frequent or prolonged exposure to acids or basesis reasonably anticipated. Silicone gels are generally more resistant tooxidizing chemicals used for cleaning and sanitizing pharmaceutical cleanrooms(such as bleach, sterilant, phenolic cleaner/disinfectant) than are urethanegels.
Polyurethane gel materialsare commercially available and often used when low outgassing properties arerequired (such as in semiconductor cleanrooms), where silicone cannot betolerated, or where the use of silicone gel is known to be a problem. Polyurethanegel has proven to be a good alternative to silicone gel in some applications.Polyurethane gel does exhibit the formation of a thin surface skin over timeand may undergo surface oxidation or limited shallow stress crack formation (asit is exposed to PAO); however, these aging effects have been shown to notcompromise sealing of the filter over time. Aging of the gel may be acceleratedif the gel is exposed to high doses of vapors from oxidative cleaning agents.Urethane gels in general tend to be more resistant to degradation in applicationsin which acid or bases are present than are silicone gels. Filters with gelolder than about five years, or that show signs of gel aging, should bereplaced and not reinstalled if removed from the system.
In summary, in order tominimize the risk of gel degradation in pharmaceutical applications, it isgenerally advised to select a silicone gel system with the followingcharacteristics:
High molecular weight
Lower percentage of unbounded gel components
Manufactured and mixed under a rigid quality control system, withproperly designed miter joints in the frame
Exposure to a minimum amount of PAO during filter testing
There may be specificapplications in which urethane gels might be appropriate. It is advised to workwith the filter vendor to select the most appropriate gel for the specificapplication.
Figure 8.1 illustrates the superior adhesionproperties of a silicone gel.
Figure 8.1: Example of the Superior Adhesion Properties of a SiliconeGel
Photo credit: Ronald Roberts, Bayer U.S.
Figure 8.2 shows an example of the liquificationof filter gel.
Figure 8.2: Example of the Liquification of Filter Gel
Figure 8.3 illustrates urethane gel developing to ughening and cracking on the surface.
Figure 8.3: Example of Urethane Gel Developing Toughening and Crackingon the Surface
Photo credit: Ronald Roberts, Bayer U.S.