30 minutes. At this stage a "latent image" of colorless silver ions is present in the glass surface. 3. Heat the iece in reducing atmosphere-for example, a flowing stream or20 to 50% hydrogen in nitrogen-for 15 to 60 minutes a t 450' to 500' C., to reduce the silver ions to metallic silver. Clean the surface by washing.
Name Plates and Dials in Photosensitive Opal Glass
APPLICATIONS FOR PHOTO-STAIN.The photo-stain process is believed to be a practical method for photographic reproduction of two-dimensional images which are permanent, dimensionally stable, scratch-resistant, and impervious to chemical attack and heat. It is a relatively inexpensive method and when the manufacturing techniques are sufficiently perfected it is expected to be suitable for manufacture of accurate scales, engineering drawings, calibrated instrument faces, dials, reticles, and cross-hairs in transparent glass. It may be employed in making reproductions of halftone or continuous tone prints, as well as line drawings.
Designs are i n three dimensions, white translucent patterns i n clear glass. May be used with or without colored glazes
CONCLUSION
PROCEDURE. 1. Produce the photographic image in a silver halide emulsion by conventional methods. Fix and wash thoroughly. The emulsion may be applied to the glass either beforehand or after the image is developed, as stripping film is now employed in the printing industry. The emulsion must contain enough silver to prevent the image from being carried away from the glass during the heat treatment in step 2. Some emulsions which have been used successfully are Kodak 33, Kodak L-1 spectrographic emulsion, Kodalith (Eastman), and Reprolith (Ansco) stripping films. 2. Apply a thin coat of paste (24 weight % ferric sulfate plus 75 weight % yellow ochre, dispersed in an oil or a water vehicle) over the photographic emulsion and allow it to dry. (The ferric sulfate aids in oxidizing the silver during heat treatment, and the ochre aids ion exchange and prevents distortion of the silver image as the emulsion decomposes.) Heat the piece rapidly (within 10 minutes-slow heating carbonizes the gelatin and makes it difficult to fire out) to about 450" C. and hold it for
A number of new products and processes have resulted from the discovery that actinic radiation can cause photochemical reactions in glass. It seems saje to predict a useful and expanding future for photosensitive glasses and related products. Sun and Kreidl ( 4 ) have discussed coloration of glass by radiation, and provided an exhaustive bibliography. LITERATURE CITED
(1) Schulman, Ginther, and Evans, U.S. Patent 2,524,839 (Oct. 10, 1950).
(2) Stookey, S.D., IND.ENG.CHEM., 41, 856 (1949). (3) Ibid., 45, 115 (1953). (4) Sun, K. H., and Kreidl, N. J., Glass. Ind., 33, 511 (October 1952).
(5) Weyl, Schulman, Ginther, and Evans, J. EZectrochem. Soc., 95, 70 (1949). RECEIVED for review March 23, 1953.
ACCEPTEDOctober 10, 1953.
Application of Glass Fibers in Filtration Processes CLAYTON A. SMUCKER AND WAYLAND C. MARLOW, JR. Owens-Corning Fiberglas Corp., Newark, Ohio
Glass fibers have properties that are significant in filtration applications. Great versatility of products is obtained through combination of various fiber sizes, structures, and bonding materials. Further investigation of glass fiber products is needed for the solution of specific filtration problems.
T
HE ancient art of glass making was known to the Egyptians and has been practiced as a craft or an industry for almost 5000 years. The fact that glass can be made in fiber form has also been known for a long time, but such fibers were first produced and sold on a broad commercial scale less than 20 years ago. In the few years since its appearance on the commercial market, glass fiber has been produced in a variety of forms for many unrelated applications, and its production now assumes the status of a major business. The fundamental properties of glass fiber led t o the firet commercial application in the field of filtration, but the present rate of consumption for this use represents only a small portion of its ultimate potential.
176
BASIC PROPERTIES OF GLASS FIBERS
Glass fibers possess a combination of unique properties which distinguish them from both natural and synthetic organic materials. Glass will not burn. Glass is heat-resistant. The degree of heat resistance is a function of composition and ranges from 1000' to 1800' F. The physical properties of glass are not adversely affected by extremely low temperatures. Glass is resistant to attack by chemical agents. Although tremendously large surfaces are exposed when glass is produced in fibers of small diameter, it is nevertheless very resistant to most
INDUSTRIAL AND ENGINEERING CHEMISTRY
VoI. 46, No. 1
-Ceramics chemicals except hydrofluoric acid and strongly alkaline solutions. Compositions have been developed that will withstand contact with hot concentrated mineral acids indefinitely. Glass fibers will not absorb water or any liquid or gas. Unmodified glass surfaces d l adsorb infinitesimal quantities of water, dependent upon equilibrium conditions of temperature and humidity. Glass surfaces can be modified by various chemical treatments. The surface is normally hydrophilic, but can be made permanently hydrophobic by the application of silicone materials ( 5 ) These surface treatments cannot be removed by the action of ordinary detergents or by organic solvent extraction and will resist heat up to approximately 800” F. RANGE OF FIBER DIAMETER
The real utility of any filter medium depends upon its adaptability to a wide range of process and operating requirements. Glass fibers are commercially available in a wide range of fiber diameters. They can be produced or fab$cated in a variety of densities and structural forms and can be combined with an unlimited variety of organic and inorganic bonding agents and thus made suitable for use with specific gases or liquid8 under various conditions of temperature, particle size, and flow resistance. The key to the versatility of glass lies in the wide range of fiber diameters which are now commercially available. This range includes fibers produced to a specification of 0.5- to 250micron average diameter and includes a selection of the entire range of intermediate diameters lying between these extremes. Perhaps a comparison in terms of more familiar dimensions will better illustrate this remarkable range. Assuming a 50,000 times magnification, the 0.5-micron fiber would have an apparent diameter of approximately 1 inch while the 250-micron fiber would appear over 40 feet in diameter. A significant portion of the material of 0.5-micron average specification is less than 0.05 micron in diameter, which on the basis of the above magnification would be equivalent to 0.1 inch. On the above scale of magnification, a typical cotton fiber would have an apparent diameter of 3.5 feet and an average human hair 10 feet. Another interesting characteristic of these extremely fine fibers is the very large surface which they possess. If 1 cubic foot of bulk glass were converted to fibers possessing an average diameter of 0.5 micron, the surface exposed would be 2,400,000 square feet or approximately 55 acres! VARIETIES OF PHYSICAL STRUCTURE
Filtration properties of glass fibers may be further modified by structural arrangement and assembly of the fibers to meet specific requirements of various applications. Many varieties of construction are possible, and they can be broadly classified as follows: Bare unbonded glass fibers arranged in a random jackstraw pattern with proper mechanical support may be placed in the filter assembly. Tn such systems the ultimate heat and chemical resistance of the glass fibers and their mechanical support may be fully utilized without limitations imposed by organic bonding materials. Efficiency and flow resistance are dependent upon the density of packing, area filter surface, and selection of fiber diameter. Resin-bonded glass fibers arranged in a random jackstraw pattern may be formed with the complete range of available fiber diameters and a t practical densities from 0.5 to 12 pounds per cubic foot. Based on the bulk density of glass a t 150 pounds per cubic foot, the free space or voids in these structures correspond to a range of 99.6 to 92%. Resin-bonded glass fibers possess a degree of rigidity and ‘dimensional stability not attainable in unbonded form, but the heat and chemical resistance of these structures is determined and in certain cases application is limited by the characteristic properties of the bonding resin. Glass fiber papers and mats with a density range from 10 to 15 pounds per cubic foot can be effectively utilized as a filter medium. The smaller fibers with diameters ranging from 0.5 to 5 microns are most commonly used in this form. These papers have been produced with and without the use of bonding mateJanuary 1954
and Glass
rials for application in air, gas, and liquid filtration. Such papers can be folded or pleated in order to multiply the filter surface area, thus minimizing the effect of high flow resistance which is inherent in dense, fine-fibered structures required to remove small particles suspended in gas or li uid streams. Glass fibers are produced in %e form of staple and continuous yarns over a range of filament diameters and woven into a variety of fabric structures, These fabrics may be used in plate and filter frame presses where conditions of temperature or chemical attack preclude the use of organic fiber media. The efficiency and flow resistance of these fabrics are determined by filament diameter, yarn construction, and fabric design. It is also feasible to combine glass fabrics with the other forms described above in order to obtain strength and mechanical support required for certain operating conditions. EFFECT OF CHEMICAL BONDING MATERIALS
For many filter applications structural properties and mechanical strength are important. Glass filaments are essentially smooth cylinders with extremely high tensile strengths, in excesa of 200,000 pounds per square inch. There is, therefore, no problem in creating woven fabrics of more than adequate strength requirements for most applications. However, paper structures consisting of a jackstraw arrangement of glass fibers unmodified by any bonding materials cannot be produced with strength properties equivalent to a cellulose fiber paper of comparable weight or thickness. The mechanical strength of these glass papers or mats is substantially improved by the addition of bonding materials. These bonding materials, which may be either organic or inorganic, modify the chemical and heat resistance of the resultant paper or mat and may, therefore, limit the range of their application. However, by proper selection of bonding materials, a wide range of chemical and heat resistance can be obtained. The important categories of bonding materials which can be used in combination with glass fibers are classified aa follows: Organic bonding agents, which may be combined with glass fibers in either low density resin-bonded or the higher density paper forms, include Teflon, and silicone, phenolic, synthetic rabber, vinyl, and cellulose derivatives. These materials d o not by any means complete the array of bonding agents available, but only illustrate the wide range of heat and chemical resistance possible ’for specific applications a t various cost levels. Each may be utilized with the fiber diameter required for a specific application. Glass papers with remarkable strength properties have been made with certain inorganic binders. These materials will undoubtedly find application where the use of any organic bonding material is precluded because of its limitations of heat and chemical resistance. Glass papers consisting of 100% glass fiber and containing no bonding materials have been made. These papers are relatively weak, but the product has performed a very useful service in certain applications where the presence of any binder is objectionable and where adequate mechanical support can be devised. GAS FILTRATION CHARACTERISTICS
In order to evaluate the performance and determine the proper selection of glass fibers for a specific filtration problem, various methods of rating efficiency, life, and other important requirements of the filter media have been devised. Two widely different test methods designated as the “Rowley dust removal test” and the “0.3 micron smoke removal test” have been selected and typical results of tests on several types of glass fiber filter media indicated. These data should be considered only as evidence in support of the versatility of glass fiber filter media, as they are obviously inadequate for engineering design or specification purposes. In the Rowley dust removal test a standard dust iniuture of the composition indicated in Table I is spread on a revolving disk and progressively fed into an air-intake chamber ( 9 ) . The dust-laden air of known concentration is passed through the test filter a t a specified rate and an aliquot sample is taken in the air stream which has passed through the test filter. The weight of
INDUSTRIAL AND ENGINEERING CHEMISTRY
177
the dust in ‘this sample is then compared with the weight in an equal volume of dust-laden air before passing through the air filter. This comparison, which is expressed as weight percentage, is designated as the efficiency of the air filter a t the specified operating condition.
TABLE I. ROWLEY DUSTMIXTURE Dust Composition Pocahontas coal ash Lamp black Fly a& Fuller’s earth
Screen Mesh 200 100 200 100
%
by Weight 50 20 20 10
TABLE 11. AIR FILTEREFFICIENCIES Fiber Diameter of Media, Microns 125
EfFcienoy, % Rowley dust 0.3-micron smoke
20 4 1
0.5
In the 0.3-micron smoke removal test, dioctylphthalate or another suitable high boiling liquid is vaporized and passed through a chamber where the vapor is condensed to get droplets or smoke (4). This smoke is then blended with clean air in a chamber and its particle size and concentration are measured by a n optical method. The smoke-laden air is then carried through the test filter, from which it emerges into another chamber where the density of smoke in filtered air is measured in a similar manner. The observed readings can then be converted to smoke particle density, which in turn is compared to the original concentration of smoke in the condenser chamber. The resulting comparison is a measure of the filter efficiency. Table I1 indicates the range of efficiencies obtained with glass fiber media of various fiber diameters and structures. The 125micron glass fiber filter is the type commonly used in forced warm-air heating and ventilating systems. These are impingement-type filters and depend for their efficiency upon a viscous oil-dust adhesive. From the standpoint of commercial application, they are designed to provide optimum combination of efficiency, life, and other desired characteristics within the space permitted for the installation. Approximately 80% of the standard Rowley dust is removed by a 2-inch filter of this type, but it removes none of the 0.3-micron smoke. Thin mats made from 20-micron fiber will remove over 90% of Rowley dust, but again exhibit practically zero efficiency in the smoke removal test. A glass fiber mat constructed from 4-micron fiber will have an efficiency of 95 to 99% by the Rowley method, depending upon the thickness and density of fiber used, and is, therefore, a highly efficient dust remover, but only 10% of the 0.3micron particles are arrested by the filter media. The 1-micron fiber mat with approximately the same thickness and bulk density as the 4-micron mat above will remove 80% of the smoke particles, while the 0.5-micron fiber made into a paper sheet is so efficient that it removes practically all of the tiniest smoke particles. A sheet of this paper of an area equivalent to the commercial dust-removal filter would have an extremely high initial air-flow resistance and would quickly become plugged, and is, therefore, unsuited in this form for such an application. From the above data, it is evident that solid and liquid particles over a wide range of particle size can be either partially or
178
completely removed from a gas or air stream by use of the proper glass media and equipment design. LIQUID FILTRATION APPLICATIONS
The fundamental properties of glass fibers and their availability in various sizes and forms also make their use as a liquid filter medium attractive. While the current rate of consumption in this field is of much less commercial significance than applications in air or gas filtration, it is anticipated that recent technological progress in the production of low-cost fine fibers (0.5 micron) will accelerate these developments. At the present time, investigations are in progress to determine the relationship between fiber size, density, structure and pressure drop, flow resistance, and filtration efficiencies in liquid filtration. Glass fibers have been used for many years in chemical laboratory manipulations. The recently available paper materials have been suggested for use in connection with investigations in the field of biochemistry and colloid chemistry. They may also find application in chromatographic analysis. Outstanding results have been obtained in the removal of finely divided solids suspended in liquid fuel and other petroleum products. A recent significant application of fine glass fiber mats (4 microns and less) is in the removal of water from hydrocarbon fuels. Virtually complete removal of both suspended and dissolved water is effected when gasoline and Diesel fuels are passed through the proper assembly of these fibers. There is outstanding improvement in the operation of, and reduction of maintenance in, equipment using such desiccated fuels Glass fibers are also used as a coalescing medium in the desalting of crude oil (1). When the crude oil is passed slowly through a bed of glass fibers, the salt content is reduced to acceptable levels by coalescing the droplets of brine. Tower packing is another interesting application of glass fibers ( 8 ) . Bulk densities in the order of 3 to 6 pounds per cubic foot of coarse fiber (150 microns) provide a surface area of over 200 square feet per cubic foot of fiber pack with void spaces of the order of 97%. Bubble cap columns have been replaced by glass fiber-packed columns with a resulting increase in throughput because of less tendency to flood. Glass fiber-packed columns are frequently used in liquid extraction processes and scrubbing processes. Glass cloths are used in plate and frame, rotary, and centrifugal filters. When the proper acid-resistant glass composition is selected, these cloths can be used in the filtration of hot acids OTHER APPLICATIONS OF GLASS PAPER
The development of low-cost processes for fine glass-fiber production will greatly stimulate the use of these fibers in the specialty paper field. At the present time some of the more promising fields for these papers are special high temperature gaskets and electrical insulation materials. LITERATURE CITED
(1) Hayes, J. G.,Hays, L. A,, and Wood, H. S., Chem. Eng. Progr.,
45,236 (1949).
(2) Minard, G . W., Koffolt, J. H., and Withrow, J. R., Trans. Am. Inst. Chem. Engrs., 39,813 (1943). (3) Rowley, F. B., and Jordan, R. C., Engineering Experiment Station, University of Minnesota, Bull. 16 (1939). (4) Sinclair, D., “Handbook on Aerosols,” Chap. 8, U. S. Atomic Energy Commission, Washington 25, D. C., 1950. (5) Spitze, L. A,, and Richards, D. O., J. A p p l . Phys., 18, 904
(1947).
RECEIVED for review dpril 15, 1953.
INDUSTRIAL AND ENGINEERING CHEMISTRY
ACCEPTEDNovember 5 , 1963,
Vol. 46, No. 1
