Article pubs.acs.org/IECR
High Performance Air Filters Produced from Freeze-Dried Fibrillated Wood Pulp: Fiber Network Compression Due to the Freezing Process Alan L. Macfarlane,† John F. Kadla,‡ and Richard J. Kerekes*,† †
Chemical and Biological Engineering, and ‡Wood Science Department, The University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z4 ABSTRACT: High efficiency air filters to capture submicrometer particles have been produced from fibrillated pulp fiber by a wet beating and freeze-drying procedure. Under appropriate conditions, air permeability and particle capture efficiency meeting the N95 standard can be achieved. Pulp type, beating time, and the freezing process were examined. Particle capture efficiency improved with increased beating, but excessive beating resulted in the formation of a low permeability layer (LPL). This LPL was the result of fiber network compression induced by the freezing process and formed farthest from the cold source. Adding a formation aid commonly used in paper making prevented compression of the fiber network, resulting in higher quality filters. to capture by filtration. As described in a review by Thiessen,6 this represents a minimum efficiency. As particle size decreases, capture by the diffusion mechanism increases while capture by interception decreases. Inertial impaction plays a minor role in the capture of submicrometer particles. At the MPPS, capture is achieved equally by diffusion and interception. Increasing air velocity increases capture by interception and decreases capture by diffusion, resulting in a lower MPPS.6 Some authors measure air filtration efficiency at particle diameter of 300 nm as this is close to the MPPS.6,7 It may be noted here that some studies suggest alternate sizes for MPPS. For example, Eninger et al.8 found that MPPS was between 20 and 90 nm for both N95 and N99 commercial respirators and Balazy et al.2 found that MPPS was
