US 3382305 A
Description (OCR text may contain errors)
A. L. BREEN May 7, 1968 PROCESS FOR PREPARING ORIENTED MICROFIBERS Filed 001:. 29, 1954 INVENTOR ALVIN L. BREEN BY 5. m
ATTORNEY United States Patent 3,382,305 PROCESS FOR PREPARING ORIENTED MICROFIBERS Alvin L. Breen, West Chester, Pa., assignor to E. I. du
Pont de Nemours and Company, Wilmington, Del., a
corporation of Delaware Filed Oct. 29, 1954, Ser. No. 465,538 8 Ciaims. (Cl. 264-171) This invention relates to new fibers and a process of making such fibers.
As is commonly known, filaments can be made by extruding melts or solutions of fiber-forming materials through an orifice. By stretching such filaments after spinning, or while spinning, or both, the length of these filaments is considerably increased; usually also the strength and certain other physical properties are considerably improved. These methods allow the preparation of relatively fine fibers. However, with increasing fineness of the filaments more and more difiiculties arise during spinning and drawing. The number of breaks increases rapidly; spinning and drawing become impractical when trying to prepare fibers with diameters considerably below the range of ordinary textile fibers.
Another method which has been extensively used especially for the production of very fine inorganic fibers and which has also been applied to organic materials, consists in attenuating the molten material for instance by the application of jets of heated gas. This process and its modifications allow the production of very fine fibers with diameters as low as 1 micron or less. However, the resulting fibrils show very little or no orientation along their axes, any orientation present being so slight that it is insignificant.
It is, therefore, an object of this invention to provide oriented filaments having very small diameters. It is another object of this invention to provide novel filaments which are composed of two polymers in random distribution, one being present as microfibers. It is another object to provide new oriented fibers from condensation polymers, such as polyesters, which fibers have a very small diameter compared with ordinary textile fibers. It is still another object of the invention to provide a new process for making the fibers, such as isolated oriented fine fibers or the composites.
The objects of this invention are accomplished by dispersing at least two different fiber-forming polymers incompatible with each other and extruding the resultant mixture through .a shaped orifice into a medium which sets or fixes the extruded material in the shape desired. This structure is drawn and may be used as such, or, the structure may be treated with a solvent for the matrix material which solvent has little or no solvent action on the other polymer used in forming the microfibers. The desired oriented microfibers remain after removal of the solvent and dissolved matrix polymer. The microfibers are then washed and dried by normal techniques. Orientation is easily demonstrated by the usual X-ray diffraction measurements. Such were made on both the composites and on the isolated microfibers, and the diffraction patterns showed orientation resulted in the drawing steps and was retained by the microfibers after the isolation and purification steps. The diffraction patterns correspond to those made on the well-known continuous oriented filaments of the polymer being examined. In the composite filaments the microfibers, as for example, the polyester fibers, are arranged substantially in the direction of the long axis of the composite filament.
In the composite structures from which condensation polymer microfibers are to be isolated, the polymer leading to the microfibers constitutes from about 25% to about by weight and the matrix-forming component from about 75% to about 25 based on the weight of the structure. Preferably, the microfiber-forming component will be present in amounts of about 25 to about 60% and the matrix component in amounts of about 75 to about 40%. The composite structures are stretched at least twice their original lengths. The resultant microfibers have diameters of about 0.01 micron to about 3 microns with those having diameters of about 0.01 to about 0.1 micron being preferred. The ratio of the length to the width is more than about 50, and microfibers of considerable length can be prepared.
Useful composite structures having microfibers embedded in a matrix are also prepared by this invention, the microfibers stemming from addition, condensation or natural polymers. The microfiber content of the composite may vary from about 5% to about 50%, the matrix being present in amounts of about to about 50%. There are instances where surface tensions of the polymers involved permit the preparation of useful composites having as high as 75 microfiber content. Usually the microfiber .content will be between about 10% to about 30%. These oriented composite fibers have good strength and are generally delustered and have a more wool-like structure than synthetic polymers generally have. Their rough surface leads to greater dimensional stability in certain fabric forms.
In the figures FIGURE 1 is a micrograph of cross-sections showing a filament containing microfibers distributed at random in the matrix and FIGURE 2 is a micrograph, in plan view, showing the oriented microfibers remaining after the matrix has been removed from an oriented filament containing the microfiber and matrix.
The invention is further illustrated by the following examples in which the proportions of the ingredients are expressed as parts by weight and which are given for illustrative purposes only.
EXAMPLE I Fifty parts of poly(ethylene terephthalate), being of 20 mesh particle size and having an intrinsic viscosity of 0.52 in a solvent mixture consisting of 60% tetrachloroethylene and 40% phenol, was mixed for 2 hours at room temperature with 50 parts of poly(hexarnethylene adipamide) which was of 20 mesh particle size and had an intrinsic viscosity of 1.03 in the said solvent. The polymer mixture was heated to 280-294 C. just long enough to produce a melt and the melt was passed through a multilayer sandpack wherein the sand was arranged in order of diminishing particle size in the direction of flow of the polymer. The top layer of this pack was formed by sand of 20 to 40 mesh particle size and the bottom layer of sand of to mesh particle size. Such sand packs are described for instance in US. 2,266,368. The filtered polymer mixture coming through the pack was extruded through a spinneret having 10 holes of 0.009 inch diameter each. After drawing the resulting yarn on rolls heated to 80 C. to 5 times its original length, the yarn had a tenacity of 3.64 grams per denier and an elongation at break of 21%. Microscopic examination showed that the composite filaments consist of a matrix in which are embedded at random, the polyester fibrils having microscopic dimensions. This is shown in FIGURE 1, wherein reference number 1 shows the polyester material (dark) and 2 designates the polyamide matrix (white).
EXAMPLE II Equal parts of the polymers used in Example 1 were mixed at 280 C. under nitrogen of a pressure of 0.5 millimeter mercury with slow stirring for 30 minutes.
The resulting melt dispersion had an intrinsic viscosity of 0.94 in the solvent mixture of Example I. The melt was passed through a sandpaek containing one layer of sand of 20 to 40 mesh particle size and thereafter extruded through the spinneret used in Example I. After drawing the resulting yarn on a roll heated to 80 C. to 3 times its original length, the yarn showed a tenacity of 1.3 grams per denier and an elongation at break of 99%.
The drawn, composite yarn was then treated with formic acid without applying tension to the yarn. The composite filaments disintegrated completely to give oriented polyester microfibers, most of which were 100 microns to several millimeters long having diameters of about 0.1 to 2 microns. Such filaments 3 are shown in FIGURE 2, some of which lie singly while others are in groups or are clustered together, some heavily.
EXAMPLE III A mixture of 25 parts of the polyester and 75 parts of the polyamide of Example I was melted under the same conditions as described in Example II. The resultant dispersion was spun at 286-287 C., after passing through the sandpack described in Example I, using a spinneret having 30 holes of 0.009 inch each. After drawing the resulting yarn on a roll heated to 80 C. to 3 times its original length, a yarn was obtained which had a tenacity of 2.1 and an elongation at break of 130%.
The yarn was then immersed in formic acid with the application of slight tension and mild rubbing. After a short time the composite filament had completely disintegrated to yield oriented polyester microfibers which were about 20 to 40 microns long and which had diameters varying between 0.5 and 2 microns. As in the previous examples, the X-ray diffraction pictures showed that the extent of orientation was substantially retained. Similar results attain when films instead of filaments are processed.
EXAMPLE IV The polymers used in Example were mixed at 280 C. in a ratio of 75 parts of the polyester and 25 parts of the polyamide under nitrogen at atmospheric pressure. After passing the melt through the sandpack of Example I, the melt was extruded at 280-283C. through the spinneret used in Example III. The resulting yarn was drawn on rolls heated to 80 C. to 3 times its original length, and a yarn having 1.5 grams per denier tenacity and 137% elongation at break was obtained. On immersing the yarn in formic acid without applying tension and without rubbing the yarn, the composite filaments partly disintegrated.
This example illustrates the applicability of composites containing large amounts of the polymer being formed into microfibers and relatively small amounts of the matrix polymer.
EXAMPLE V The polyamide in each of Examples I to IV was substituted by the same weight of polyethylene. With the procedures set forth in the above examples composite filaments were obtained, the structures of which were similar to those obtained with polyamide as matrix material.
The composite polyethylene-polyester fibers yielded on immersion in xylene oriented polyester microfibers of dimensions similar to these obtained in the foregoing examples.
EXAMPLE VI One part of poly(hexamethylene adipamide) flake was mixed with 3 parts of polyethylene flake. This mixture was melted and further mixed in a screw extruder and forced through a conventional nylon spinneret and pack to form continuous filaments of the polymers dispersed in each other. The filaments were drawn to about 5 times their original length and the polyethylene component was dissolved in hot xylene. Discrete microfibers remained which were not soluble in the xylene and these polyamide microfibers displayed a high degree of orientation between crossed polarizers. If desired the xylene could be evapo rated to deposit polyethylene about the microfibers.
EXAMPLE VII A solution of N.N-dimethylformamide containing 22% solids was prepared using polyacrylonitrile and cellulose acetate in a 75 %/25% relation, respectively. This solution was extruded at 137 C. into a cell heated to 240 C., and the resultant dry spun fiber was drawn 2 over a hot pin heated to a temperature of 100 C. The resultant composite, oriented fiber was delustered and had a slightly rough surface. The usual examination of the composite filament indicated that it was oriented and contained microfibers of cellulose acetate embedded in polyacrylonitrile.
EXAMPLE VIII A mixture containing 25% cellulose acetate and of the polyurethane from piperazine and ethylene glycol bisehloroformate was dissolved in tetrafiuoropropanol. A fiber was dry spun by extruding a solution containing 25% solids heated at C. into a cell the temperature of which was 160 C. The resultant fiber was drawn 3X on a hot pin heated to 160 C. The composite fiber thus formed was delustered, opaque and porous, and had a rough surface. Again, examination revealed that the oriented composite contained microfibers, these being from the cellulose acetate polymer and being embedded in the polyurethane.
Using about 10% of the polyurethane and of cellulose acetate composites could be produced in which the microfibers were derived from the polyurethane and embedded in the cellulose acetate.
EXAMPLE IX Formic acid solutions of poly(hexamethy1ene adipamide) and the polyurethane derived from piperazine and ethylene glycol bis-chloroformate were blended to produce solutions containing 27% solids, the amide and the urethane being in a 25/ 75 relationship. This solution was dry spun at a temperature of 90 C. using a cell heated at a temperature of 180 C. The resultant composite filament was drawn 5 X over a hot pin the temperature of which was C. The resultant oriented composite comprised microfibers of poly(hexamethylene adipamide) embedded in the polyurethane matrix.
These fibers were similar in characteristics to those prepared in Example VIII and had higher strength and required unusually high work to break as compared to fibers made from only the polyurethane under similar conditions.
'In the preparation of isolated microfibers any combination of polymers behaving similarly to those described in the above Examples I to VI may be used. Generally, the polymers used in the isolating of microfibers are condensation, melt-spinnable fiber-forming polymers such as polyamides, olysulfonamides, polyester-amides and polyesters. Any of these may be used in combination with the other, similar or different, polymers so long as the composites prepared are dispersions, or so long as the polymers to be made into microfibers do not dissolve in the other polymers used in making the composites or in the solvents applied after the drawing step. If desired, a mixture of microfibers can be prepared. The polyamides and polyesters of the present invention comprise such polymers which have recurring amide groups or ester groups respectively and which are known to form fibers. The polyamides useful in the present invention comprise such polymers as described for instance in US. 2,071,251 and 2,071,253. The polyesters of this invention comprise such compounds as described for instance in US. 2,071,250 and 2,465,319. Polysulfonamides and polyester-amides which may be used have chemical compositions given in such patents as US. 2,321,890, 2,321,891, 2,224,037, 2,312,879 and 2,396,248.
In the preparation of oriented composite structures in which the microfiber is to remain embedded in the matrix, the above polymers may also be used as well as addition polymers, naturally occurring polymers and derivatives of naturally occurring polymers such as derivatives of cellulose. In all instances however, the specific polymers to be used together are incompatible with each other and are capable of being shaped either by melt, dry or wet spinning techniques. Further examples for the composites include poly(vinyl acetate), polystyrene, poly(vinyl chloride), poly (vinylidene chloride), and polymers of acrylic and methacrylic acids and their derivatives such as the esters, as well as copolymers thereof. Usually the polymer having the highest surface tension will form the microfiber, but this depends to some extent on the amount present.
The polyamides include such polymers as poly(hexamethylene adipamide), or poly(hexamethylene sebacamide) and poly(epsiloncaproamide) and copolymers thereof, and the polyesters include, besides poly(ethylene terephthalate), poly(trimethylene terephthalate), poly (ethylene hexahydroterephthalate), and copolymers containing sebacic or adipic acid, for example up to as well as the polyesters containing recurring units derived from glycols with more than three carbons in the chain.
It is well known that polyesters and polyamides can be homogeneously blended in melts, the prolonged heating actually bringing about interaction. But this reaction is relatively slow and it takes at least several hours to produce a homogeneous reaction product of the two starting polymers. In the process of the present invention the time needed and applied for dispersing the melt is very short. Therefore, no substantial interaction between the components occurs. Both polymers retain their original physical and chemical properties.
In this invention the polymers, such as polyamides and polyesters, that are melt dispersed and spun do not molecularly mix one with the other in the molten state; in other words they are not compatible under the conditions used. As shown in British Patent No. 610,140, it is possible to use sufficient heat and time to blend the polymers and produce a homogeneous composite, but such a result is not desired in this invention and is avoided by adjusting such factors as time, temperature and choice of materials. For example, the melting temperatures and the viscosities of the molten polymers should not be too far apart. Furthermore, the polymers should not react or degrade substantially at temperatures above the melting point in the short time of contact in the molten state necessary for producing the fiber. Generally, it is suificient to mix in the dry state and then carry out the melting and extrusion in a short time.
It is preferred in carry out the present invention to use together poly(hexamethylene adipamide) with poly(ethylene tcrephthalate). These two polymers do not react with each other to any appreciable extent during the lending and spinning operations. The polyamide can be readily dissolved out of the composite filament by the application of formic acid without the polyester being affected. In the place of polyamides any other synthetic linear polymer, which fulfills the above requirements, may be used as the matrix material in this invention. When using, for instance, polyethylene as the matrix forming material, this may be dissolved out with xylene in order to isolate the polyester microfibers. Other hydrocarbon soluble polymers may be used instead of polyethylene such as polyisobutylene. Also, hydrocarbons other than xylene can be used in the dissolving step as, for example, toluene.
The dispersing of the polymers can be done in conventional manners, for instance, by mixing the dry powders or flakes of the polymers and melting this mixture or, as already stated, by melting the polymers separately and mixing them in the molten state. Fine dispersion of the polymers in the matrix material can be promoted by rapidly stirring, shaking, or other means and is further promoted by filtering the melt before extruding through the customary sand filters or similar devices. By one or more of these means dispersions approaching colloidal dimensions can easily be obtained. In general, the time of contact of the two polymers in the molten dispersed form is only a few seconds to a few minutes. The contact time can be kept short if necessary by special means in the spinning device. The molten polymers may be fed separately to a mixing chamber which has a relatively small dimension compared with the volume of output per minute of the spinning unit. Dispersion of the polymers in the mixing chamber may be promoted by the application of mechanical agitation or by the action of ultra-sonic waves.
In the composites containing polyester, polyester usually forms the dispersed phase in the composite. The dimensions of the final microfiber, such as those of polyesters, are controlled by such factors as the ratio of matrix and other polymer applied, and by the extent of mixing which influences the size of the single dispersed particles in the melt. The attenuation during the spinning operation and the draw ratio of the after-drawing or stretching procedure controls the diameter length relation of the final microfiber.
Though the components may be used in almost any desired ratio, compositions of the melt from 25 polyester and 75% matrix, as polyarnide, to about polyester and 40% matrix are preferred, these percentages being in reference to the melt dispersions and to the composite articles therefrom prior to leaching. In order to avoid agglomeration of the single dispersed polyester particles in the melt, the ratio of the polyester to matrix should not exceed a certain limit which varies somewhat with the nature of the polymer used, and the specific process conditions applied. In melt dispersions varying from a minor amount of polyester and a major amount of polyamide up to about equal parts of these polymers practically all the polyester forms discrete dispersed particles. Therefore, practically all of the polyester applied in this range is transformed into microfibers. With increasing amounts of polyester more and more of the dispersed polyester particles adhere together thus reducing the yield of polyester in discrete microfibers. The solubility of the matrix of the composite fiber especially after stretching is reduced with low content of matrix. The ratio of the polymers has, furthermore, some influence on the dimensions of the fine filament. With minor amounts of polyesters in major amounts of the matrix mostly discontinuous microfibers are obtained. A typical example of an oriented polyester microfiber obtained in a process using 25% polyester and polyamide is a fibril being 50 to 1000 microns long and having diameters up to 0.5 to 1.0 micron. Thi corresponds to a length-width ratio of more than 50. By varying the process conditions the length-width ratio can be increased and the microfibers may become continuous. When melt dispersions of about 75% polyester and 25% polyamide are used, the proportion of the polyester transformed to microfibers decreases rapidly so that higher ratios of polyesters are not recommended. However, this may vary somewhat wih the nature of the polymer as applied and the combination of materials used.
The melt dispersions may be spun into room-temperature air for solidification, or the temperature may be raised or lowered depending, in part, upon the properties of the melt dispersion. Generally, room temperature is used for the convenience it afiords, and the filaments travel about 6 feet between the spinneret and wind-up. A transverse air stream or other quenching means may be used, if desired.
Drawing or stretching of the composite fibers may be carried out while or after spinning as is commonly known. The common devices may be used, for instance,
stretching between hot or cold rolls driven at different speeds. The draw ratios applied may vary widely. Generally, it is sufiicient to after-stretch or draw the spun composite fiber 2 to times its original length to obtain high orientation of the polyester microfiber contained therein. Sometimes it may be desirable to apply even higher draw ratios in order to obtain the desired orientation of the microfibers. While lower draw ratios may be used, usually the composites are drawn to at least 2 times their original length. If the composites are desired to have low elongations, preferably the higher draw ratios, say at least 4X, are used.
As is commonly known, the orientation and improvement of physical properties of fibers is mostly brought about by stretching at room temperatures or at temperatures considerably below the melting temperature of the polymer. The original particles of polymer forming the microfiber in the melt dispersion, which are thought to be substantially spherical, or attenuated considerably in the spinning process. This attenuation, together with the afterstretch or draw, determines the length-width ratio of the final microfiber, the volume of which is substantially equivalent tot he volume of the original dispersed polymer particle in the melt. With increasing ratios of the fiber component in the melt the, attenuated particles, while still in the molten or softened state in the spinning operation, may fiow together. This may result in considerably increased length compared wi h the diameter. Therefore, with relatively high ratios of the polymer forming the micropolymer, microfibers are obtainable with very high length-width ratios.
As already stated, the microfibers are isolated from the drawn composite fibers by a leaching procedure with a solvent for the matrix. Such a solvent suitable for the present process is formic acid which has proven especially e fective when the matrix is formed by a polyamide such as poly(hexamethylene adipamide). Other solvents useful in the present invention which dissolve the polyamide matrix material and which do not affect the polyester microfibers substantially are phenol or mixtures of phenol with water. The added water reduces the solvent power of the phenol. The amount of water is so adjusted that under the prevailing conditions the polyamide is dissolved preferentially and the polyester is substantially not affected. Similarly, mixtures of phenols or cresols with alcohol or tetrachloroethane can be used. The dissolving or leaching action of these solvents varies with the ratio of the polymers used in the melt blend. The lower polyester content of the composite fiber requires less leaching or dissolving action by a given solvent than higher ratios of polyesters. This increases gradually, and when ratios of 75% polyester and polyamide are approached the action is so slow that it is preferable to apply simultaneously some mechanical action like pressing or rubbing the composite fiber. The isolation of the oriented polyester microfiber from the drawn or stretched fibers may be carried out by submerging the fibers in the solvent. The dissolving action may be improved by shaking, stirring, circulating the solvent or moving the fibers in the solvent. The leaching may be carried out while the fiber is held under tension or also without tension. The resulting oriented microfibers are obtained in the form of a slurry or dispersion, or of a matted yarn or loose network of microfibers, depending on the size and especially length of the fibers and the conditions of separation applied.
In the leaching step elevated temperatures may be used, but solvent at room temperature (about 25 C.) is frequently very effective, requiring only a few seconds to a few minutes. The leaching may also be done in closed systems under pressure but generally elevated temperatures are not needed. The leaching is effected at temperatures below and usually far below the melting or softening point of the microfibers and the boiling point of the solvent, so that no difficulties are involved.
It follows from the above description that by proper modification of polymer ratios, dispersing action, spinneret, attenuation while spinning and after-stretching, dimensions of the filter and similar factor, oriented microfibers of any desired length-Width ratio and of any desired dimension can be made according to the process of the present invention. Oriented microfibers having diameters of a few microns to about a tenth of a micron, or even as low as a hundredth of 21 micron or less and having length-width ratios of more than about 50 are preferred. These include microfibers being only a few microns long the dimensions of which are so small that when dispersed in water or another non-solvent they show Brownian motion. Such oriented fibers have not been produced by any other known technique. Also, microfibers having lengths of one or more millimeters to several centimeters and even continuous lengths of microfibers may be made according to the present invention. Which of these dimensions are preferred depends mostly on the intended use of the oriented microfibers.
The length of the microfibers can be limited by cutting the drawn or stretched composite fiber into staple of corresponding length for instance 1 mm. or longer. This modification of the process has sometimes the advantage of promoting the leaching action of the solvent. This is of special importance when composite filaments of a relatively high denier are spun from melts wherein the content of the polymer forming the microfiber is high.
The new oriented microfibers may be used alone or together with conventional textile fibers for the production of various kinds of textile materials. They may be used in insulation of clothing, for instance, in form of a thin layer between ordinary textile fabrics. Other applications comprise such uses as in very fine filters, for instance, for filtering aerosols. Other very important uses comprise their application for acoustical insulation. The small dimensions and the extreme surface-volume ratio of the microfibers make them especially suited for applications where high insulation effects are desired combined with a very low weight and high mechanical resistance. For this purpose, they may be applied by help of adhesives to construction surfaces so that the single filaments adhere substantially perpendicularly or also in smaller angles to the surface. Or they may be applied to construction surfaces in the form of resilient sheets or mats.
Because of the extremely high surface to volume ratio these microfibers have strong mutual attraction and may be formed into paper-like structures with little or no sizing. Partial removal of the matrix phase may be used to make strong paper-like structures in which the residual matrix serves to hold the microfibers together.
Any departure from the above description which conforms to the present invention is intended to be included within the scope of the claims.
1. A process for the formation of oriented articles containing microfibers having an average diameter of from about 0.01 micron to about 3 microns and having a ratio of length to width of more than about 50 which comprises blending at least two incompatible fiber-forming polymers; extruding the resultant mixture through a shaped orifice into a setting medium; and drawing the resultant shaped article to produce orientation of both of said fiber-forming polymers sufiicient to achieve said specified microfiber dimensions.
2. A process in accordance with claim 1 wherein the said microfibers are present to the extent of about 25% to about by weight of the resultant shaped article.
3. The method which comprises extruding a filament of an intimate mixture of two mutually incompatible, fiber-forming, organic polymers, moderately stretching said extruded filament at a temperature near the extrusion temperature, and thereafter further stretching said filament.
4. The method of claim 3 including the further step of selectively dissolving one of said polymers from said filament.
5. A process for the formation of oriented articles containing oriented microfibers having an average diameter of from about 0.01 micron to about 3 microns and having a ratio of the length to Width of more than about 50 which comprises melt blending at least two mutually incompatible fiber-forming organic polymers to form an intimate mixture, extruding the said intimate mixture into a filament, stretching said extruded filament at a temperature near the extrusion temperature, and thereafter further stretching said filament.
6. A process for the formation of an oriented filament containing oriented microfibers having an average diam eter from about 0.01 micron to about 3 microns which comprises melt blending at least two mutually incompatible fiber-forming organic polymers to form an intimate mixture, extruding said intimate molten mixture into a filament, stretching said extruded filament at a temperature near the extrusion temperature, and thereafter furstretching said filament from about 2 to 10 times its original length to molecularly orient the filament and the microfibers.
7. A process in accordance with claim 6 wherein the said microfibers are present to the extent of about 25% to about 60% by weight of the resultant shaped article.
10 8. A process in accordance with claim 7 wherein said intimate mixture of said incompatible fiber-forming organic polymers comprise a mixture of a polyester and polyamide.
References Cited UNITED STATES PATENTS 2,385,358 9/1945 Hanson 264-138 2,700,657 1/1955 Zook et al. 260-455 3,016,599 l/l962 Perry 161-181 X 2,531,234 11/1950 Seckel 18-54 X 2,676,164 4/1954 Charlton et al. 260-45.4 2,776,465 1/1957 Smith 28-82 2,577,915 12/1951 Pillar 18-54 2,681,266 6/1954 Kurnmel 18-54 2,637,893 5/1953 Shaw 28-82 2,674,025 4/ 1954 Ladisch 28-82 FOREIGN PATENTS 610,170 10/ 1948 Great Britain.
ROBERT R. MACKEY, Primary Examiner.