MC² Market & Competitive Convergence

• The Journal of Powder/Bulk Solids Technology, March 1998, p. 26-33, 'Enhanced Dust and Powder Collection with Electret Composites'
• Filtration & Separation, March 1998, Vol. 35, No. 2, p. 118-122, 'Cleaner, Healthier Environments: AFFCO Enhances Air Filter Performance with Electret Composites'
• Nonwovens in Filtration Conference, March 1997, Stuttgart, Germany, 'Performance Enhanced Electret Composites'
• Fluid/Particle Separation Jr., August 1997, Vol. 10, No. 2, p. 148-153, 'Performance Enhanced Electret Composites'

The operational envelope of conventional nonwovens in air filtration applications can be enlarged by incorporating electret media. Several new composite media have been developed by combining small increments of electret with conventional nonwovens, resulting in significant performance gains. Data showing initial particle removal efficiencies by particle size, initial pressure drop and loading are included. Potential applications that can utilize this innovation are also discussed.

Electret
The term electret is used here to describe fibrous air filtration media whose individual fibers carry an electrostatic charge, while the media, as a whole, is essentially neutral. Such fibers may behave like a mini capacitor with one side of the fiber being negative and the other side being positive. The electret tested here is another quite common configuration where half the fibers are negatively charged and the others positively charged.

The addition of an electrostatic charge to the fibers results in increased particle removal efficiency while the pressure drop remains unchanged. The increase in efficiency is primarily attributed to the charged fibers attracting particles having an opposite charge, and the attraction of neutral particles when a dipole is induced by the charges on the fibers.

Needleunch Media
The media used in this experiment is made from staple polyester fibers made from two denier sizes. In the manufacturing process the fibers are carded, needlepunched on one side using barbed needles that intertwine the fibers, creating the tortuous path for particle capture.

These filters are strong, resist abrasion, and can remove large quantities of particulate. They can be fabricated using a wide variety of fibers and surface finishes. This allows their use in high temperature and/or corrosive environments. Surface treatments allow particulate to build up on the surface of the filter forming a cake that can be released by back pulsing the filter with a blast of air which regenerates the filter.

So many variables exist in the fabrication process -- fiber diameter, needling, machine speed, fiber type, surface treatment -- that true mass customization is possible. It's feasible to have tailor-made filters to match the operating environment and optimize flow and particle retention.

Needlepunch/Electret Composites
Adding electret media to needlepunch media was considered for several reasons. First,it has been reported that the electrets can hold higher quantities of particulates at a given pressure drop than filters that rely primarily on mechanical means for particle removal. This has been attributed to the way the particles load on the electret fibers: The particles tend to load more evenly over the entire surface of the fibers, leaving more space between fibers for air flow. Further, one of the mechanisms of electrostatic particle removal is the polarization of particles as they approach the charged fibers. The induced dipole would then be capable of polarizing sites on the downstream needlepunch fibers which in turn could attract and retain particles. This assumes that the electret is upstream of the needlepunch media.

For some applications it is preferred to have the electret downstream of the needlepunch media. For instance, the surface finish on the needlepunch may be utilized for cake buildup and release and, in this case, it would be facing the air flow. In the beginning of the filtration process, prior to cake formation, particle removal efficiency can be very low. Often, it can be a matter of days before the efficiency builds up to an acceptable level. Incorporating a layer of electret media on the downstream side could reduce the duration of this break-in period or even eliminate it.

Media tested:
(a) NP (400 g/m² polyester needlepunch filtration media)
(b) 30 30 g/m² electret
(c) 30/NP 30 g/m² electret over NP
(d) 70 70 g/m² electret
(e) 70/NP 70 g/m² electret over NP

Challenge Particles
Loading:
Neutralized PTI Fine (ISO Fine) with a mass concentration of 200 mg/m³

Efficiency testing:
Neutralized polydispersed KCl

Particle Removal Efficiency
Filter media a, b, c, d, and e were tested for particle removal efficiency by particle size at a media face velocity of 15 cm/s. Their pressure drops were determined at 5, 10, 15, 20, 25, and 30 cm/s. The effective filtration area of the tested media was 0.186 m².

Loading
Filter media a, c, and e were tested for loading at a media face velocity of 15 cm/s. The terminal pressure drop was 22 - 23 mm water gauge, and an absolute filter was placed downstream of the test media to capture particles penetrating the test media.

Gravimetric efficiency (GE) was calculated as follows:
GE (%) = weight increase of test sample ÷ (weight increase of test sample + weight increase of the absolute filter)

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EXPERIMENTAL RESULTS

Pressure Drop (mm w.g.) vs. Face Velocity

Particle removal efficiency (%) @ 15 cm/s using KCl aerosol

Pressure Drop (mm w.g.) vs. Loading

Gravimetric efficiency

SUMMARY

The pressure drops of the composite media were approximately 10% more than the sum of the individual components. The additional needling used to combine the two components caused this.

The composite structures exhibit much better particle removal efficiencies, suggesting that then mechanisms of particle removal operating in the individual components are somehow synergistic in nature, far exceeding what one would expect from merely judging the performance of the individual components.

The loading results demonstrate to a greater degree this synergistic phenomenon. The final weight increase for the 70/NP was greater than 2.5 times that of the NP alone, yet its initial pressure drop was only 1.6 mm of water gauge higher.

These composites should prove to be useful in industrial dust collection applications, including pulse-jet bag houses, equipment protection, and at the point-of-emission in the workplace. Lower basis weight components could extend their use to other application, including HVAC, room air purifiers, cabin air filters, and vacuum cleaner filters.

Hsieh, K.C., Test Reports, Particle Tech, Inc., Vadnais Heights, Mn., USA, 1997

Frederick, E.R., Utilizing Electrical Effect in Nonaqueous Filtration, Filter Media Specification, Pittsburgh, Pa., USA, 1996

Purchas, D., Handbook of Filter Media, Elsevier, Oxford, England, 1996

Brown, R.C., Air Filtration, An Integrated Approach to the Theory and Applications of Fibrous Filters, Pergamon, Oxford, England, 1993

Hines, William C. Aerosol Technology, Properties, Behavior, and Measurement of Airborne Particles, Wiley, New York, USA, 1982