VAN A. WENTE Naval Research Laboratory, Washington 25, D.
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Superfine Thermoplastic Fibers Hot-melt polymer extrusion
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Produces fibers as tow as 0.1 micron in diameter
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direct collection of filter mats or sheets with random fiber orientation
I N 1951 a project was initiated at the Naval Research Laboratory for the investigation of organic fibers that were less than 1 micron in diameter. Various manufacturing methods were to be considered and, after the feasibility of one became established, an evaluation was to follow of the submicron fibers as a filter media. The “fiber-forming’’ thermoplastic materials were of particular interest in this investigation because of their high strength and ability to undergo molecular orientation when stretched. Emphasis was placed on obtaining the extremelv small fiber size because such
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fibers were known to possess aerosol filtration efficiencies far in excess of ordinarily used materials ( 4 ) . Furthermore, superfine plastic fibers represented a new area of research where completely altered physical properties might be anticipated as a result of the nearly colloidal state of the substance. Description of Process
A manufacturing method evolved which yields fibers as fine as 0.1 to 1.0 micron for various thermoplastic materials including nylon, polystyrene.
INDUSTRIAL AND ENGINEERING CHEMISTRY
poly(methy1 methacrylate), poly(ethylene terephthalate), and polytrifluorochloroethylene. The apparatus developed for the production of fine fibers is essentially a ram-extruder that forces molten material through a row of fine orifices and directly into two converging high velocity streams of heated air or other gas. The illustrations in Figure 1 show the juxtaposition of the air streams and nozzles. Temperatures of the air and the hot-melt are controlled separately, and the velocities of the streams are also separately controlled. As shown in the cutaway- view of the nozzle tip (Figure 1) the individual orifices are actually slots which are milled into a flat surface and then matched with identical slots milled into a mating surface. When the two halves are placed together, they form a row of openings which, according to the preferred design are 0.014 inch wide and 0.020 inch apart, center to center. As a result of this spacing, the flat surfaces between the grooves are 0.006 inch wide and form “dead” spaces that separate the fibers in the blast. This straight-line arrangement of apertures permits the nozzle to be made with a large number of them depending on the width of fiber web which is desired. Most of the developmental work was done with a nozzle comprising about 15 individual orifices, but a larger unit containing 192 orifices was built and successfully operated. This larger unit, the disassembled nozzle section of which
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is shown in Figure 2, contained four separate groups of 48 orifices each. Each group was separated by a 1-inch space to facilitate proper clamping of the two nozzle halves. The assembled machine is shown in Figure 3, and a schematic diagram of the process in Figure 4. Insulated lines carry hot compressed air to the air nozzle and a hydraulically operated ram, ”4 inch in diameter, forces the plastic granules from a feed hopper into the melting section. The two elongated openings for the air streams generally have 0.010 inch as their smallest dimension and extend along the row of orifices slightly beyond the two extremities of the row. The point at which fibers are formed lies within the gas stream where cooling has progressed sufficiently to solidify the material. Since the hot-melt issues from the nozzle directly into the confluence of the two air streams, the greatest amount of attenuation occurs at this point of exit. Thus, size of the orifice has little importance provided it is large enough to pass the melt without plugging. The orifice diameter and length together determine the ram Dressure required to yield a given polymer feed rate. This back-pressure must be high enough to give good distribution and controllable flow to the molten material. An orifice length of 1 inch usually provides a n adequate back-pressure in the range 100 to 500 pounds per square inch. Depending on the exact temperatures and velocities used, the fibers cool to a solid form after being carried by the air stream to about 1 inch away from the nozzle tip. The newly formed fibers move away from the nozzle in a dispersed turbulent stream at very high velocity. With typical air conditions of 600’ F. and 50 pounds per square inch, this velocity might equal or exceed sonic velocity which would approximate 1600 feet per second. The air blast is separated from the fiber stream by passing through a 16-mesh collecting screen which moves with variable speed across a suction chamber (Figure 5). A random network of fiber is thereby deposited on the screen and then stripped off to be wound onto a reel either with or without being densified by press rolls. The nozzle angle-Le., the angle included by the two air streams-is an important factor in the performance of the equipment. When this angle approaches 90°, a high degree of fiber dispersion or turbulence results, and the fiber mat is deposited on the screen with the most random fiber distribution. A nozzle angle of 30°, however, yields a high proportion of “roped” or parallel fibers deposited as loosely coiled bundles which are considered undesirable in a filter media. O n the other hand a 30’ angle produces greater attenuation
Figure 2.
Dissassembled attenuating nozzle with 192 orifices
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Fiber attenuating machine (assembled) VOL. 48, NO. 8
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Schematic diagram of process equipment
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Operating Conditions for Various Thermoplastics Air Ram Pressme. Fiber ,Votzle Azr Pressurea, Lb./Sq. Diam., Temp., Temp,. Lb./Sq. Inch Microns F. Inch Polymer O F. 680 750 50 250 0.5 Nylon 66 600 720 50 250 0.5 Nylon 610 600 650 40 200 0.8 Nylon 6 600 750 40 175 0.5 Polv(ethv1ene terephthalate) 720 850 15 2200 1 .o Polytrifluorochloroethylene' 700 800 40 500 1. o Polystyrene Poly( methyl methacrylate) 650 750 40 1000 1.5 Polyet hylene 700 800 10 750 3.0 inch long with 0.010-inch opening. 5 Pressure given for upstream side of air nozzle slot Table
from a given gas velocity and, as a further advantage, has less tendency to form discontinuities in filament length. I t is an important advantage of the basic nozzle design, however, that all nozzles will resume attenuation whenever a break occurs. -4 compromise angle of 60" has been used for this work.
-i.e., air temperature, nozzle temperature? air pressure, or polymer feed ratt: (ram pressure). The fiber diameters given in the table are estimates made on an optical microscope with dark field illumination. Correlations were made with electron micrographs. An cxample of very fine nylon fiber is shown by an electron micrograph (Figure 6) where a high degree of uniformity in size is also notable. The diameters lie within the range 0.11 and 0.22 micron. As revealed in Table I both polystyrene and poly(methy1 methacrylate) must be produced with air temperatures well above their softening points. The use of 750" to 800" F. reduces the melt viscosity to a satisfactorily low value believed to be in the range of 1000 poises. Polyethylene and polytrifluorochloroethylene have sharper melting points, but both must also be attenuated a t high temperatures in order to reduce their viscosities. Where commercial polymers were offered in different molecular weight ranges: the types that possessed low molecular weight and hence low melt viscosity were most satisfactory. Poly(viny1 chloride) and polyacrylonitrile did not yield fine fibers by this process because they decomposed at temperatures well below those necessary to achieve adequate fluidity. Although operating temperatures as high as 600' F. would normally be considered destructive to all organic materials, here the exposure time at such high temperature is so short that negligible polymer degradation occurs. Short holdup time in the melting stage also reduces degradation and is effected by a cold water jacket around the feeding cylinder. Being cooled in this way, the plastic does not melt until it moves u p and contacts the heated face which then leads it directly into the slor feeding the nozzle orifices. Because flow properties of the hotmelts must conform to certain requirements for rapid attenuation, not all the polymers listed will form fibers in high velocity air. Both polyethylene and polytrifluorochloroethylene disintegrate
Attenuating Characteristics of Various Thermoplastics
Figure 5.
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Fiber-collecting screen
Among the factors that establish the ability of a given polymer to attenuate to a fine fiber are its melting point, viscosity-temperature characteristic, and surface tension which may be considered as incorporating intermolecular forces. The optimum operating conditions for a range of materials are given in Table I as those which yield the most continuous shot-free fibers. As a rule, linear polymers having fiber-forming characteristics are required for satisfactory operation. Diameters both larger and smaller than those given can be made by changing any of the four basic variables
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Figure 6. Electron micrograph nylon fibers
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mechanical entanglement and by frictional forces. The cohesive forces a t the fiber to fiber contacts are the major forces holding these synthetic mats together, and it follows that both the reduction of fiber diameter and the densification of the mats would increase the number of contacts within and consequently improve product strength. Strength of these products may of course be improved by addition of synthetic lattices or other binders. However, such techniques are not discussed here because of their deleterious effects on filtration efficiency of the mats.
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Figure 7. Effect of fiber diameter on strength of unbonded nylon fiber mat
almost entirely into small globules when air is used at the normal pressure of 40 or 50 pounds per square inch. Satisfactory fibers, however, result from them when low air pressure (10 to 15 pounds/square inch) and hence low velocity is employed. A typical requirement for producing 1-micron fiber might be 600 cubic' feet or 50 pounds of air per pound of such fiber. This air requirement is roughly proportional to the total length of fiber yielded by a given weight of material. As a result, the air needed to produce a fiber having a n average diameter of 0.3 micron is approximately ten times greater or 500 pounds of air. Strength
of Superfine Fiber
An important characteristic of fine fibers is that when they are used in unbonded webs or mats they acquire tensile strength rapidly as the fiber diameter is decreased. This property is illustrated in Figure 7 where also the effect of mat density is shown. The mats contained no added binder, and the higher densities were obtained by roomtemperature pressing. Strength of the fiber mats increased markedly as they were densified. For strength calculated in terms of pounds per square inch this was a natural result, but here the strength was greater by a factor in excess of the ratio of the sheet densities. I t is wellknown that at least part of the strength and flexibility of cellulosic paper arises from the ability of the cellulose fibers to subdivide or fibrilate and give a structure of submicron ultimate fibers. Thus, the contact area between fibers is extremely high and results in considerable fiber to fiber bonding. Although cellulose fibers are known to form strong paper by cementing together, these unbonded synthetic fiber mats derive their strength by
Fiber Diameter and Filtration Performance
To determine the manner in which fiber diameter depends on the various operating conditions it was necessary to use a test method which conveniently gave a direct indication of the average diameter. Such a method consists of determining the pressure drop across a pad of the fiber with air flowing through at a standard velocity of 14.2 cm./second. Since the flow resistance of fiber mats at constant porosity is proportional to the square of surface area (1), then this resistance alone may serve to evaluate fiber diameter without the introduction of corrections, assumed constants, and other sources of error. Flow resistance is proportional to pad thickness and hence, for a given polymer, proportional to pad weight. Values are reported in mm. of water/gram weight of a standard pad 82 mm. in diameter. Filtration data were taken using the conditions: Dioctyl phthalate particles of 0.3micron diameter Face velocity, 14.2 cm./second Initial concentration of about 70 micrograms per liter (3) For aerosol filtration through a homogeneous media, the following equation may be established ( 2 ):
where C = concentration of smoke at any point in the filter expressed as a fraction of the initial concentration; F = per cent of the smoke entering a layer (unit thickness) that is filtered (removed) by that layer; and X = number of layers through which smoke passed. The value of F may be taken as a measure of the true or basic filtering effectiveness if it is based on a unit thickness that is sufficiently small-for example, 2 or 3 microns. A more practical measure of filtering efficiency, however, depends on the amount of smoke removed from a given quantity of air per unit of energy expended in moving
that air through the filter. Such a value may be represented by the per cent of smoke removed ( F ) per unit pressure drop (AP). This unit of energy is set arbitrarily for comparison purposes at 1 mm. of water at the standard flow velocity (14.2 cm./sec.). Since a given homogeneous filter material possesses a definite linear relationship between its thickness and its resistance, the unit thickness or layer referred to in the original definition of F would be that which corresponds to 1 mm. of water pressure drop for the filter material involved. The value of F per mm. of water, AP, as reported here corresponds to the basic filtering rate or efficiency when energy consumption is the basis of comparison. Typical values of pressure drop and filtration performance are presented in Table I1 for mats made from thermoplastics at near optimum conditions. Fibers of the three polymers: nylon, poly(ethy1ene terephthalate), and polystyrene, possess high air resistance values which show extreme fiber fineness. As a consequence of their small size, these three exhibit high filtration efficiencies with nylon fiber being most effective in removing 9.470 of the smoke that passes to each filter layer. Poly(methy1 methacrylate) and polytrifluorochloroethylene, on the other hand, yield larger fibers as shown by their lower air resistance and poorer filtration. Densities of the mats vary widely from one material to another. However, when these densities are corrected for the specific gravity of solid polymer, as shown by the values for solid volume fraction, differences between the various mats are reduced. The effect of porosity is such that as the density of a fiber mat is increased by compression it is usual for measured penetration through the mat to decrease, but a t the same time filtration performance decreases because of raised flow resistance. Although differences in filtration performances when reported in this manner appear to be small, they can amount to large differences in actual service. For example, a filter material having a total resistance of 100 mm. of water and an over-all penetration of 0.01% (based on initial smoke concentration) has a n F value of 8.77@smoke removed per mm. of water. O n the other hand, a second material having a resistance of 100 mm. but an F of 6.7 would pass 0.10% of the smoke or ten times the amount of the first filter. Compared to the aerosol filtration performance of a n ordinary laboratory filter paper, however, all of these media are highly efficient. A typical paper-e.g., Whatman No. 1, has a n F less than one and gives correspondingly high smoke penetration. The filters described herein are generally comparable in performance to the high efficiency filter papers which are VOL. 48,
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Filtration Performances of Thermoplastic Fibers Density Solid Air Filtration of M a t , Volume Resistancea, Performance, Material Grarnlcc. Fraction M m . HzO/Gram F / M m . HzO Nylon 610 0,041 0.037 92 9.4 Polyethylene terephthalate 0,050 0.036 100 8.2 Polytrifluorochloroethylene 0.10 0.048 35 7.7 Polystyrene 0.030 0.029 91 8.0 Table II.
Poly(methy1 methacrylate) 0,041 0.035 58 6.5 a Air flow resistance at 14.2 cm./sec. velocity/gram weight of standard test pad, 82 mm. in diameter.
now available and which are composed, a t least partly, of either asbestos or superfine glass fibers. Effect of Manufacturing Conditions on Performance
Nylon 610 was chosen to illustrate the effects of various temperatures and pres-
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sures in the manufacture of superfine fibers because its behavior is typical of the fiber-forming materials. Variations of both air resistance and filtration performance with changes in the attenuating air pressure is straightforward (Figure 8). Both properties become greater as pressure is increased. As shown by Figure 9, however, flow resistance and smoke removed go through a maximum when plotted against air temperature. Thus, the plastic flow properties apparently reach optimum values when the air is near 700” F., and the result is that fibers of maximum fineness are produced. When the polymer melt is fed into the nozzle a t increasing rates. a general result is greater fiber diameter with a consequent opposite effect on flow resistance and smoke removed (Figure 10). However, a t the low feed rate of 0.1 gram ’minute/orifice, the filtration efficiency turns downward for lower feed rates; at the same time air resistance continues to rise. Although the efficiency drop was comparatively small it was nevertheless real, being repeated several times. An explanation for this surprising result might be based on the decreased continuity of fiber attenuation that results a t very low flow rates. Every time the flow of fiber is momentarily interrupted, a globule of solid ma-
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of air pressure On Air temperature, 720’ F.; o., grams/min. per orifice
nylon fiber.
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Figure 9. Effect of air temperature on nylon fiber. Air pressure, 50 Ib./sq. inch; feed rate, 0.1 1 grarns/min. per orifice
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The hot melt method of producing fine thermoplastic fibers may readily be applied to those polymers which possess reasonably low melt viscosities. Fiber diameters less than 1 micron and as low as 0.1 micron are obtainable from such fiber-forming materials as the linear polyamides and polyesters. Although not normally considered fiber-forming? such materials as polystyrene, poly(methyl methacrylate): and polytrifluorochloroethylene also yield submicron diameters. Mats or sheets with random fiber orientation may be collected directly from the heated air stream used to attenuate the fibers. Such mats serve as excellent filter media for fine aerosols and also have potential application as liquid filters. Filters made from special organic materials such as polytrifluorochioroethylene would have- unusual resistance to thermal and chemical degradation. As the fibers are made finer the sheets grow stronger and become more efficient filters. Sheets may be strengthened by densification or addition of binders, but a maximum strength usually exists a t the point where a n accompanying loss of filtration efficiency reaches its allowable limit. References
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terial appears to precede the next fiber. An abundance of such “shot” would naturally affect filtration performance adversely. Kozzle temperature is not shown as a variable herebecause its effect seems to be chiefly on polymer flow rate. Provided that this temperature is sufficiently above the polymer’s melting point, it may be used in conjunction with ram pressure to help control the feed rate. Temperature of the nozzle tip in the vicinity of the orifice outlets has been found to closely approximate the air temperature. Fibers made by this technique possess measurable electric charges and these, in turn, are known to influence filtration performance. Effects of the charges and means of controlling them are under investigation particularly on the more hydrophobic thermoplastics for which discharge rates are extremely low.
NYLON FEED RATE ( G R A M S PER MIN PER O R I F I C E )
Figure 10. Effect of nylon feed rate on fiber. Air temperature, 720” F.; air pressure, 50 Ib./sq. inch
INDUSTRIAL AND ENGINEERING CHEMISTRY
(1) Carman, P. C., J . SOG. Chem. Ind. 57, 225 (1938); 58,1(1939). ( 2 ) “Handbook on Aerosols,” chapt. 9 and 10, Atomic Energy Commission, Washington 25, D. C., 1950. ( 3 ) Knudson, H. W., White, Locke, NRL Rept. P-2642,Sept. 14, 1945. (4)Ramskill, E. A,, Anderson, W. L., $1. Colloid Sei. 6, 416 (1951). RECEIVED for review October 11, 1955 Division of Paint, ACCEPTED Plastics, March and 17, Printing 1956 Ink Chemistry, 128th Meeting, ACS
Minneapolis, Minn., September 1955.
