I
RICHARD D. CADLE and WILLIAM C. THUMAN Stanford Research Institute, Menlo Park, Calif.
Filters from Submicron-Diameter Organic Fibers Filters having widely varying characteristics can be made by spraying solutions of polymers in organic solvents. The solution iet is disrupted by an airblast into filaments which form fibers on evaporation
RECENTLY,
these laboratories were given the problem of finding or developing an organic fiber filter which could be used to collect solid particles from the stratosphere and upper troposphere. The particles, 0.01 to 1 micron in diameter and collected at a face velocity of 14 cm. per second with a pressure drop less than 100 mm. of water, were to be studied by neutron activation. Thk filters, therefore, had to be low in metallic contaminants and capable of being ashed. T o prepare such filters, a spray technique was developed in which a jet of polymeric material in solution, introduced into a high velocity air stream, is broken up and drawn into fibers from which the solvent evaporates. Fibers prepared in a similar manner are described by Francis (7), Till (4, and Till and Smallman (5). The process is simple and can produce fibers less than a micron in diameter, which in the form of mats have the desired filtering characteristics. The high performance of the filters prepared from 2-micron-diameter fibers compared with those prepared from smaller fibers is of interest and results from the low pressure drop produced by the larger fibers.
times between a glass plate and a 40inch-long, 35-pound glass roller filled with lead shot. Filter efficiencies were determined using lithium fluoride and ammonium sulfate aerosols. The lithium fluoride aerosols were prepared by dispersing aqueous solutions with a nebulizer in series with a bed of glass beads. Evaporation of the water from the droplets produced relatively nonhygroscopic lithium fluoride particles. Size distributions were obtained from electron micrographs. Monodisperse sulfuric acid aerosols were prepared by controlled condensation of sulfuric acid vapors. The size of the droplets produced under a given set of conditions was determined by allowing the particles in a sample to “grow” by absorption of water vapor at a controlled humidity, followed by size determination with a light-scattering technique (2). Aerosols having droplets of
Magnification, 25X
Experimental #
The adjustable spray nozzle of stainless steel (Spray Engineering Co., Burlington, Mass., Model 125, 1-inch diameter cone, F 2081) was constructed so that an annular blast of air surrounded the jet of solution. The solution of polymer in an organic solvent was placed in a container below the nozzle. Both the solution and the entrance to the nozzle air jet were subjected to the same pressure (usually 50 P A . ) . The fibers produced were drawn by suction against Dacron netting supported in a vertical position on a wire screen until a mat about 5 mm. thick accumulated. Most of the mats were pressed by rolling four
Proposed fiber-forming mechanism was substantiated by photomicrograph of fibers forming from solution leaving jet
w P
-01
0 2 03 04
06 08 10
FIBER DIAMETER - p
20
30 4050
the desired size were then allowed to react with ammonia. Collection efficiencies (100 x final concentration/original concentration) were determined by measuring particle number concentrations with a General Electric nuclei counter ( 3 ) . All of the polymers and solvents were obtained from commercial sources and were used without further purification. Short exposure time photomicrographs of the jet leaving the nozzle were obtained with a Beckman and Whitley framing camera equipped with a 24-inch focal length objective.
RA-2281..
Diameter distributions of fibers obtained from solutions of polystyrene in methylene chloride of various concentrations and air pressures Distributions do not include flber bundles
Results Fibers were prepared from cellulose acetate, cellulose nitrate, poly(methy1 methacrylate) polystyrene, poly(viny1 chloride), and poly(viny1 formal). The VOL. 52, NO. 4
APRIL 1960
3 15
Table 1.
Filter Efficiencies
Compressing the fiber mat increased collection efficiency and pressure drop
Filter Material Polystyrene, backing removed, no compression
Conc. in Solvent, Grams/100 M1. Solvent
Type LiF
6 6 8 10 12 12 20
Polystyrene, backing removed, compressed
6 6 8 8 8 12
..
C W S No. 6 asbestos flter a
Aerosol
Median Particle A,G EffiDiameter,b Mm. ciency, yo F, Mm.-1 P Hz0 0.04 0.04 0.04 0.04 0.04 0.04 0.04
33 29 34 22 22 10 8
99.1 98.0 98.8 98.4 95.0 92.9 99.5
(NH4)zSOa LiF
0.04 0.06 0.04 0.01 0.06 0.04
125 125 120 120 120 100
99.8 99.96 99.8 99.2 99.7 99.6
5.0 6.2 5.2 4.0 4.8
(NHa)zS04
0.06
110 110
99.96 99.0
7.1 4.2
LiF
LiF LiF LiF LiF LiF
LiF (“4)2S04
LiF
LiF
LiF 0.01 Pressure drop across filter measured at 14 cm./second linear flow.
solvents included acetone, methyl ethyl ketone, chloroform, ethyl acetate, methylene chloride, and methyl methacrylate. Beads of polymer tended to be formed along with the fibers, and for some combinations of solvent and polymer the beads constituted as much as 30% of the filter material. Other polymers, such as poly(methy1 methacrylate), produced rather brittle filter pads. T h e combination polystyrene-methylene chloride produced particularly promising fibers and this system was studied in detail. Fiber diameter distributions were obtained from electron micrographs by measuring the diameters at the points of interception of straight lines, drawn across the micrographs, with the fibers (see graph). Thus the question of fiber length was avoided. Increasing the polymer concentration and decreasing the pressure increased the median diameter. The distributions shown did not include bundles of fibers. When these were included, the median diameters were increased to 0.6, 0.9, and 3.0 microns for the 6, 10, and 20% solutions at 50 p.s.i., respectively, and 0.9 micron for the 10% solution at 20 p.s.i. The fibers were produced as segments varying from 2 or 3 mm. to 3 or 4 cm. in length.
14 13 13 19 14 27 67
5.5
from the present operation because the surface tension cannot overcome the viscous forces resulting from the polymer in the solvent. The above mechanism is substantiated by the photomicrograph of the fibers forming as they leave the jet (shown). Furthermore, if the fibers resulted from simple elongation of the initial jet of solution by air blast, the air blast velocity would have to be several orders of magnitude greater than the velocity of sound to maintain the rate of production which was achieved. A comparison of filter performance on the basis of collection efficiency alone can be misleading. A better basis is the use of a parameter F 12-hichis defined by the equation
Bundles not included.
For particles in the size range 0.01 to 0.06 micron the efficiencies always exceeded 90% (Table I). Compressing the polystyrene fiber mat somewhat increased the collection efficiency and greatly increased the pressure drop. Chemical Warfare Service filter paper No. 6 is included for comparison. Table I1 shows that there was a particle size of about 0.1 micron which resulted in lowest efficiency. An emission spectrographic analysis was prepared of a sample of polystyrene fibers and of the polymer from which they were made. Concentrations of metals in both samples were equal to or less than 1 p.p.m. by weight for all but aluminum, calcium, and silicon which were 2, < 10, and 7 p.p.m., respectively. Discussion
Aerosol formation by airblast atomization involves at least three stages, namely the initiation of small disturbances on the surface of the jet of liquid, the action of the aerodynamic forces on the disturbances to draw out numerous fine fibers and the collapse of the fibers into droplets as a result of surface tension. Presumably, fibers rather than particles result
where cI is the number concentration of particles leaving the filtrr and p is the pressure drop across the filter. This parameter is particularly useful for comparing various filters at a given face velocity. Thus the performance of the filters described here can be compared directly with those studied by IVente ( 6 ) and Wright, Stasny, and Lapple (7). Values of F are shown in Table? I and 11. Compressing the filters considerably decreasing their performance, as would be expected. The performance of the compressed polystyrene filters was approximately that of the CWS No. 6 filter and of the organic fiber filters prepared by Wenre. Acknowledgment
The solvent approach to fine-fiber preparation was suggested by C. E. Lapple. Electron micrographs were prepared and size distribution data were obtained by C. F. Schadt. The efficiency determinations were made by R. C. Robbins and I. S. Yaffe. Literature Cited (1) Francis, C. S.. Jr. (to American Viscose Corp.), U. S. Patents 2,483,4042,483,406 (Oct. 4, 1949). (2) LaMer, V. K., Inn, E. C. Y . , Wilron, I. B., J . Colloid Sci. 5, 471-96 (1950).
(3) Rich, T. A., Geojszca 31, 60 (1955). (4) Till, D. E., Modern Textiles Mag. 40, 36 (October 1959). (5) Till, D. E., Smallman, C. R. (to American Viscose Corp.), U. S. Patent Table II. Collection Efficiency as a Function of Fiber and Particle Size 2,810,426 (Oct. 22, 1957). (6) Wente, V. A., IND. ENG. CHEM.48, Lowest efficiency occurred with 0.1 -micron particles 1342-6(1955). Filter (7) Wright, T. E., Stasny, R. J., Lapple, Median Density, Efficiency for F for Indicated C. E., “High Velocity Filters,” Wright Diameter, Mm. -1 A p , Indicated Diameter, % Fiber Grams/ Air Development Center Techn. Rept. Diameter, Sq. Cm. Mm. H20 0.01 pa 0.1 p b 1 . 0 ~ ~0.01 M~ 0.1f i b 1 . 0 ~ ~ 55-457 (ASTIA NO.AD-142075) (October, 1957). 2.0c 0.0022 10 68 8 90 11 0.83 23 2.0 0.0105 17 97.2 57 99.0 21 5.0 27 RECEIVED for review July 20, 1959 0.4 0.0027 120 97.2 85.7 >99.99 3.0 1.6 ACCEPTED December 21, 1959 0.4 0.0050 408 99.9 98.3 >99.99 1.7 1.0 0.3
0.0032
LiF aerosols.
316
680
* (NH&304
aerosols.
99.2
99.2
>99.99
0.71
0.71
Not including bundles: mats compressed.
INDUSTRIAL AND ENGINEERING CHEMISTRY
..*. ..
Work supported by Contract No. AF 19(604)-2644, Air Force Cambridge Research Center.
