In Australia, the use of gel seal HEPA filters has become the “default” choice. This is driven by the inherent performance advantages of the gel seal; a soft material that results in filter installations that are easy to seal and require low clamping pressure compared to typical foam gasket systems.
Polyurethane gels may be used, however the general chemical and environmental stability of Silicone gels have gained in popularity within HEPA applications.
In general, gels are formed by carefully mixing two equal parts of the co-polymer and adding it to the HEPA channel. The nature of these lightly cross-linked systems is that they are unforgiving to errors in processing – with significant variations in the final characteristics resulting. However, if reasonable care is exercised in manufacturing, stable gels that give excellent performance result.
Mechanical sealing methods have fallen out of favour for very practical reasons. With larger HEPA housings, the flatness of sealing surfaces is practically impossible to achieve without excessive crush forces on the seal, or impractically long installation times to prevent any distortion of the housing frame.
The gel seal process is simply a more mechanically forgiving, reliable and versatile solution. Misguided attempts to retro-fit mechanical seal to gel seal (or the reverse) to existing housings of one type are cost and time ineffective. If there is an application specific reason for using one particular sealing type, that needs to be determined on installation – or if change is needed, replace the entire housing and filter style.
These two gel types provide a physically compliant, air tight seal between the HEPA filter frame and its housing. Both contain similar characteristics in terms of elasticity, surface tack and softness and will perform the basic job of sealing HEPA filters to the air delivery system. However there is a wider acceptance of Silicone gels in general HEPA applications.
Silicone gels are produced by several major corporations and are included in a range of electrical, electronic, vibration damping and sealing applications. They generally offer good chemical resistance and enable higher usage temperatures; but are also relatively high in cost, and may suffer cure inhibition when applied directly to some incompatible substrates.
Polyurethane gels are not commonly used by many HEPA filter manufacturers. Although they are relatively low cost and offer fairly good chemical resistance, commonly cited disadvantages include high moisture sensitivity of the unreacted components resulting in a limited raw material shelf-life, and incompatibility with certain substrates that may result in cure inhibition.
Multiple reports of issues with limited batches of HEPAs from multiple vendors have been documented over the years. Some of these include:
These issues may be divided into some basic categories:
As a trivial case, a filter may be loaded to excess, perhaps by uncleaned ductwork after construction, so the fully compliant filter gel is pushed out by excessive static across the filter. This case is simply one of the filter being exhausted – not a failure.
Gel chemistry is fragile and unforgiving to errors in processing. Manufacturing related process problems include liquefaction, unacceptably large variability of final gel properties, cure inhibition (wet spots), and gel dripping.
These problems may be associated with using a gel that is past its shelf life, is not metered and/or mixed properly, is not stored properly, or is applied to an incompatible or contaminated substrate. Numerous vendors have occasional examples over the years, generally over a small batch of filters. Once identified, corrective action can be swiftly taken, and affected filters discarded.
Installation problems most likely include insufficient penetration of the knife edge into the gel OR cutting of the gel by excessive penetration. Tearing or gel damage can result if a HEPA is pushed onto the knife edge then moved aggressively to enable seating.
Exceeding the elastic limits of the gel will result in splitting and cutting of the gel. Below the elastic limits, the gel will “snap back” and return to nearly its original state when the filter is removed (“self-healing”). Self-healing is somewhat misleading as once the gel is cut or fractured, it can stick back together and form a seal, but it will not reform chemical bonds.
Finally, incorrect (excessive) air flow rates or mis-designed ductwork can raise a host of issues that impact overall performance. These range and scope of these issues is well dealt with in other publications.
There is some evidence to suggest that a range of chemicals and environmental conditions may contribute to premature gel failure. Oxidizing chemicals (e.g. bleaches) used for disinfection, when sprayed aggressively through protective meshes may overcome outlet airflow and impact on the media or gel surfaces. Powerful oxidizers, acids, solvents etc. will compromise the materials. It is possible that if materials are at the edge of their passing performance specification – these chemical attacks will be especially significant.
Some test oils have been speculated as at issue, most modern non-PAO test oils have proven reliable. Excessive loads of oil or inferior oils may create chemical attack or excessive pressure spikes that damage the filter. There is some literature evidence that this chemical damage has occurred with PAO test oils.
The issue of gel dripping can be caused by a combination of multiple factors which include; the original choice of gel, the quality of the components, the reliability of the mixing process and the exposure of the gel to oxidizing chemicals or other detrimental environmental stressors. A significant excess exposure to oil or elevated temperature may exacerbate this problem.
In addition to batch testing of HEPAs for arrestance and pressure drops, the ultimate assurance of filter performance is NATA certified testing on site. Irrespective of how rigorously filters have been tested in the factories, these fragile items may suffer considerable damage in shipping or storage. Thus, the yearly certification testing is the most cost effective way to assure performance at a site.
In some applications (such as TGA sites) further “traceability” may be required for documentation purposes. While this is available at an increased cost – it does not relieve the importance and necessity for site testing and NATA certification to assure performance. If a filter fails its primary arrestance test – it should be repaired or replaced at the first opportunity.
Gel seals, silicone based or even polyurethane, offer reliable and versatile sealing solutions for most HEPA applications. If due care is applied to the factors described above, the failure rate for these units is vanishingly low. However, despite the simple appearance of the HEPA filter, it is a complex assembly of parts, with a highly cross-linked polymer that requires careful manufacturing, transport, storage and installation to achieve its design purpose.
Written by Dr Allan Heckenberg (PhD.) and Shannon Roger (B.Ed.) for Airepure Australia 2016.
Airepure Australia offer a range of products, services and consulting expertise that can assist you with your compliance to ACHS, DHS VIC Guidelines (and equivalent for QLD, WA and NSW), ISO/IEC 17025:2005 Requirements, AS/NZS 2243.3:2010 and AS/NZS 2243.8:2014. Airepure is a leading national air filtration company providing unique, powerful and integrated air filtration solutions, ranging from basic HVAC filtration and odour control right through to high end HEPA/ULPA filtration and airborne containment technologies. For more information, visit www.airepure.com.au or call 1300 886 353.
References: Devine, S. “Technical Bulletin: A Brief Discussion of Gel” 2012