US3565979A

US3565979A - Flash spinning

Info

Publication number: US3565979A
Application number: US760534A
Authority: US
Inventors: Louis C Palmer
Original assignee: EI Du Pont de Nemours and Co
Current assignee: EIDP Inc
Priority date: 1968-09-18
Filing date: 1968-09-18
Publication date: 1971-02-23
Anticipated expiration: 1988-02-23

Abstract

POLYMERIC FLASH-UPON STRANDS ARE ELECTROSTATICALLY CHARGED BY PASSAGE BETWEEN AN ION GUN AND TARGET PLATE, THEN COLLECTED ON AN OPPOSITELY CHARGED OR GROUNDED SURFACE. LOSS OF ELECTROSTATIC CHARGING OF THE STRANDS, DUE TO POLYMER BUILD-UP ON THE TARGET PLATE, IS PREVENTED BY MAINTAINING A CONDUCTIVE SUBSTANCE ON THE PLATE SURFACE. THE CONDUCTIVE SUBSTANCE CAN BE APPLIED CONTINUOUSLY BY WIPING A LIQUID COMPRISING IT ONTO THE SURFACE AS THE PLATE ROTATES.

Description

Feb. 23; 1971 L. c. PALMER 3 3,565,979

- FLASH SPINNING Filed Sent. '18; 1968 2 Sheets-Sheet 1 NEGATIVE DC' v SOURCE Fl G'- 1 INVENTOR LOUIS C. PALMER ATTORNEY Feb. 23, 1971 L. C. PALMER FLASH SPINNING Filed Sept. 18, 1968 FIG- 3 2 Sheets-Sheet 2 INVENTOR ATTORNEY United States Patent Ofice 3,565,979 FLASH SPINNING Louis C. Palmer, Richmond, Va., assignor to E. I. du

Pont de Nemours and Company, Wilmington, Del., a

corporation of Delaware Filed Sept. 18, 1968, Ser. No. 760,534 Int. Cl. B29d 7/02; D01d 1/10; D01f7/00 U.S. Cl. 264-24 7 Claims ABSTRACT OF THE DISCLOSURE Polymeric flash-spun strands are electrostatically charged by passage between an ion gun and target plate, then collected on an oppositely charged or grounded surface. Loss of electrostatic charging of the strands, due to polymer build-up on the target plate, is prevented by maintaining a conductive substance on the plate surface. The conductive substance can be applied continuously by wiping a liquid comprising it onto the surface as the plate rotates.

BACKGROUND This invention relates to a method for preventing the loss of electrostatic charging during the preparation of uniform webs of endless fibrous structures in which the endless fibrous structures are obtained by flash-spinning, are passed through an electrostatic charging zone, and are collected upon an oppositely charged web-collection device. It is more particularly directed to such a process wherein the endless fibrous structures are plexifilaments.

Suitable endless fibrous structures include monofilaments, bundles of filaments, yarns, plexifilaments, and the like. By endless is meant that the fibrous structures are continuous along their length, but not necessarily that each filament in a bundle is continuous.

A plexifilament, as meant herein, is a yarn-like three-dimensional integral plexus of synthetic organic, polymeric, fibrous elements, the elements being coextensively aligned with the axis of the plexifilament and having the structural configuration of oriented thin filmfibrils. Detailed description of preferred plexifilaments is found in Blades et a1. U.S. Pat. No. 3,081,519, and a preferred process for preparing such structures is given by Blades et al. in U.S. Pat. No. 3,227,784.

Steuber, U.S. Pat. No. 3,169,899, describes sheets of randomly collected plexifilaments and a method by which such sheets can be continuously and uniformly collected. A plurality of plexifilaments is produced at points above and laterally spaced across the width of a moving belt. Each plexifilament is directed toward the belt and passes through a zone where it is electrostatically charged. By oppositely charging or grounding the belt, or a conductive plate supporting it, the plexifilaments are attracted to and held upon the belt such as not to be further displaced by surrounding currents of gas.

Further details of a preferred means for charging a plexifilament are disclosed by Owens in U.S. Pat. No. 3,319,309. The horizontally flash-spun plexifilament immediately contacts an arcuate surface which directs it downward toward the moving collection belt. As the plexifilament continues downward, it crosses over an electrically conductive grounded target plate. Spaced oppositely from the target plate, and aimed at it, is a source of ionizing current (hereinafter designated ion gun) passing to the target plate through the gas-filled gap therebetween. Passage through the narrow gap between ion gun and target plate produces an electrostatic charge on the plexifilament. By the process generally as described hereinabove, excellent, uniform, strong, and broadly useful webs and sheets are prepared.

Patented Feb. 23, 1971 Associated with the flash-spinning of fibrous structures is the generation of a small amount of tiny separated particles of solidified polymer. While this is particularly true of the production of plexifilaments, it is true to varyng degrees for all flash-spinning. These tiny particles become electrostatically charged like the endless fibrous structure, but, owing to their small mass and momentum, become attracted to the target plate, rather than to the collection belt, and gradually form thereupon an electrically insulating film. As the thickness of the film increases, the potential drop across it eventually exceeds its dielectric strength. Tiny craters then form through the film, which craters become sources of back corona, i.e., large current-density plasma jets of both polarities. This situation develops rapidly, when it occurs, and can be detected as a precipitous increase in current from the ion gun. It causes neutralization of the charge on the plexifilament. Thereafter, the plexifilament does not spread out due to electrostatic repulsion of its elements, is not attracted to and held upon the collection belt, and tends to float above and to collect nonuniformly on the collection belt. The target plate at this point is said to be fouled, and the condition is described as loss of electrostatic charging.

In order to postpone fouling, the target plate has been constructed as an annulus and rotated so that the lowermost active area of its face continuously changes. An insulating film eventually forms over its whole face, nonetheless. Further postponement of fouling has resulted when a scraper blade has been mounted near the top, inactive portion of the rotating target plate to scrape off the deposited film. Scraper blades, however, tend to grind circular grooves into the face of the target plate and, thereby, to promote accumulation of polymeric film. Scraping sufficient to remove all the deposited film quite rap1dly wears away, also, the surface of the target plate.

In practice, the use of only a scraper delays fouling only moderately.

SUMMARY OF THE INVENTION This invention is an improvement in a process of preparing Webs of endless fibrous structures wherein the structures are obtained by flash-spinning of a polymer solution, are electrostatically charged by passage through the gap between an ion gun and a target plate, and are collected upon an oppositely charged collection device. The improvement provides for preventing, or at least greatly postponing, loss of electrostatic charging of the structure due to fouling of the target plate, i.e. build-up of an insulating layer of polymer thereon. The improvement comprises maintaining on the surface of the target plate a non-volatile conductive substance by applying to the surface a liquid which comprises the substance and which wets the surface and the polymer thereon sulficiently to uniformly distribute said substance thereover. Conveniently, the target plate is rotated during the process so that a portion of its surface is inactive, i.e. does not receive current from the ion gun, and the non-volatile conductive substance is applied by wiping the liquid comprising it onto this inactive portion continuously as the plate rotates.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic elevation view in cross-section of a flash-spinning process in which the present invention is particularly useful.

FIG. 2 is a view of the ion gun, deflection device, and target plate taken as indicated at 22 of FIG. l.'

FIG. 3 illustrates in cross-sectional side elevation a preferred means for wiping conductive liquid onto a target plate.

FIG. 4, shown as in FIG. 3, represents an alternative method for applying conductive liquid.

FIG. is still another alternative to the methods illustrated by FIGS. 3 and 4.

DETAILED DESCRIPTION With reference to FIG. 1, a flash-spinning process in which this invention is particularly useful will now be discussed. A solution of thermoplastic polymer in an organic low-boiling liquid is delivered at elevated tempera ture and pressure through nozzle into chamber 11. On extrusion through orifice 12, the solvent flashes to create plexifilament 13 which is directed downward by deflection device 14, crosses over the exposed face of annular target plate 15, and proceeds downward to be collected on moving belt 16 as a uniform web 17. Ion gun 18, shown in partial cross-section, is connected to a high-voltage negative direct-current source such that electric current passes from discharge needles 19 across the gaseous gap to electrically conductive and grounded target plate 15. Thereby, plexifilament 13 picks up an electrostatic charge on crossing over target plate 15, and forces of repulsion between elements of plexifilament 13 cause it to expand laterally (e.g., like a fan). Collection belt 16, carried on rollers such as roller 20, is charged to a high positive DC potential whereby oppositely charged plexifilament 13 is attracted to and firmly held upon belt 16. It is apparent that reversed polarities of all applied DC voltages are equally workable.

Additional reference to FIG. 2, taken as indicated at 22 of FIG. 1, clarifies the physical arrangement. Mechanical supports for parts are not shown, since they can interfere with understanding the process involved. Deflection device 14 (FIG. 2) has a round raised central portion tapering down to another flat portion, the taper occurring in the roughly triangular area. The taper defines three lobes. Device 14 is rotated by motor 22. Thus, pleXifilament 13 is not only directed downward by device 14, by also it is oscillated laterally by rotation of the lobes.

, Annular target plate is also rotated independently by motor 23 through a gear system 24. The very small quantity of tiny polymer particles resulting from flashspinning gradually accumulates on the face of tar-get plate 15 until eventually the voltage drop across this electrically insulating film exceeds its dielectric strength, whereupon back-corona develops and neutralizes the electrostatic charge on plexifilament 13. This dielectric breakdown is prevented or greatly postponed if, according to this invention, a liquid comprising a non-volatile conductive substance is wiped onto the face of the target plate at a point removed from the current-receiving area (opposite needles 19 of ion gun 18). Applicator 25 is fed with liquid from reservoir 26 and is held in contact with target plate 15 over the Whole conductive width of its annulus. Suitable applicators are compliant liquid wicking materials such as felts or sponges.

FIG. 3 shows in greater detail a preferred method for wiping liquid 30 onto the target plate 15. The assembly shown is mounted as indicated in FIGS. 1 and 2 so that compliant applicator 25 is adjacent to target plate 15 at a point removed from its current-receiving area. Applicator 25 is held in a bracket 31 which also holds a spring 32 to urge compliant applicator 25 into contact with target plate 15 across the whole width of its annulus. A wicking strand 33 is embedded into applicator 25 at one end and immersed in conductive liquid 30 at the other end, conductive liquid 30 being held in a reservoir 34. Thus, liquid 30 is wicked by strand 33 from reservoir 34 to applicator '25 where it spreads out and is continuously wiped onto rotating target plate 15. It is apparent that size and material of wicking strand 33 must be adjusted to properties of conductive liquid 30 and applicator 25 to provide a suitable rate of wicking.

The method of application illustrated by FIG. 4differs from that of FIG. 3 in that gravity is used to deliver liquid 30 from reservoir 34 through standpipe 41 to applicator 25. A metering orifice 40 in standpipe 41 regulates flow rate, and orifice 40 is preferably adjustable, e.g., a variable-opening valve.

In FIG. 5 is illustrated still another alternative for delivering liquid 3 to applicator 25. A metering pump 50 moves liquid 30 at constant flow rate from reservoir 34, through pipe 41, to applicator 25. This method is particularly effective in the event liquid 30 is of too high viscosity to be wicked as in FIG. 3.

Inasmuch as is possible, all parts of apparatus described in connection with FIGS. 3, 4, and 5 should be constructed of electrically insulating material. Where this is impossible or inconvenient, parts must be positioned 0r shielded to prevent lightning electrical discharge to them from ion gun 18.

It is believed that loss of electrostatic charging is prevented by this invention because the conductive substance, being uniformly distributed over the surface of the target plate provides multitudinous electrically conductive paths through the insulating layer of polymer. Exceeding the dielectric breakdown potential of the accumulated film is thus avoided. For purpose of this invention a conductive substance is one having a volume resistivity of less than about 10 ohm-cm. at normal ambient temperature.

The conductive substance may be either a liquid or solid at normal ambient temperatures. If it is a solid, it is applied as a solution. It should be non-volatile, which, for purpose of this invention, means that it should not evaporate so rapidly at the temperatures to which it is exposed on the target plate that it is lost and therefore ineffectual for the function intended. In a preferred flashspinning process the temperature of the target plate is about 60 C. In this process, at least one of the conductive components of the applied liquid should have a normal atmospheric boiling point of C. or higher. Flash spinning is characterized by release of large volumes of rapidly moving gases as the polymer solution, under high temperature and pressure, passes through the spinning orifice into a much lower pressure region. These gases are directed toward the active portion of the target plate, i.e. the portion which receives current from the ion gun. The conductive substance must not be blown off the target plate surface by these gases. Therefore, the applied liquid, or at least that portion thereof which is sufliciently nonvolatile that it does not evaporate before it is exposed to the high-velocity stream of gas released on flash spinning, should be sufficiently viscous that it is not blown oif by the gas stream. The viscosity required will of course depend upon the rotational velocity of the target plate and on the wetting power of the liquid for the surface to which it is applied; absolute viscosities of at least 20 centipoises at 60 C. are preferred.

The liquid applied should wet both the accumulated polymer particles and the target plate. Otherwise the applied liquid does not form a continuous layer over the target plate but instead retracts into droplets covering only a small part of the area. Without adequate wetting, continuous conductive paths throughout the accumulated film do not result. Liquids which dissolve or plasticize the accumulated polymer facilitate continuous cleaning of the target plate. For applied liquids, which are also liquid at room temperature, the drop angle is an excellent measure of wetting power.

Drop angle decreases as wetting power increases and is determined as follows: A drop of the liquid in question is placed on the surface for which its drop angle is to be determined. Drop angle is the angle between the surface of the drop and the supporting surface at the very edge of the drop, measured within the liquid. The edge of a drop is observed with a microscope equipped with anglemeasuring rotatable crosshairs. Measurements are made at room temperature, i.e., about 25 C. Suitable liquids are found to wet a flat surface of material used to make the target plate, and also a film of the same polymeric composition as the particles which accumulate on the target plate, to an equilibrium drop angle of less than degrees at a rate characterized by attaining a drop angle of 20 degrees in less than 40 seconds. Some elfective agents are solid at C. but liquid at the temperature of application (e.g., 60 C.). For these, adequate wetting power is indicated if the layer of liquid applied to the target plate is seen to remain continuous during operation, without retraction into smaller areas.

It will be apparent from the above discussion that the applied liquid may consist solely of the non-volatile conductive substance, provided it is sufficiently viscous to remain on the target plate but not so viscous as to make application overly difficult, and provided it wets the surface of the target plate and the polymer thereon sufficiently to become uniformly distributed over the surface. Examples of such substances are certain glycerides of fatty acids and phosphate esters of certain long chain fatty alcohols, and numerous oils such as silicones, castor oil, corn oil, salad oil, cocoabutter, and the like. The applied liquid, however, may contain other ingredients. Very viscous or solid conductive substances can be dissolved in an inert, relatively low-boiling liquid to provide a liquid which is of sufficiently low viscosity to permit easy application. If the inert liquid is sufficiently volatile to evaporate before the coated area of the target plate is subjected to the high-velocity stream of gas released on flash-spinning, the liquid in the reservoir can have a viscosity substantially lower than 20 centipoises. The liquid may also contain ingredients to increase the wetting power and/or the viscosity. A minor proportion of a highly conductive substance, e.g. sulfuric acid, may be used in combination with some other conductive substance to provide the desired degree of conductivity. Additional conductive substances which have been found satisfactory include glycerine alcohol sulfates, sorbitol monolaurate, pentaerythritol and potassium iodide.

In a preferred process according to this invention, a blade 29 (FIG. 2) is used to scrape accumulated film from the target plate at a position just prior to the point of application of conductive liquid. Removal of accumulated film by the blade is facilitated if the applied liquid has at least a plasticizing (or swelling) solvent action for the accumulated film. Although not indicated in the figures, a means (e.g., a vacuum nozzle) of removing scraped material must be provided when a scraper blade 29 is used.

Other characteristics for suitable applied liquids must be considered for special circumstances. It may be necessary that they be non-toxic. They must be chemically inert to the wicking strands and compliant applicators selected, and they must wick through these materials efiiciently. It may be necessary to select fluids which, when accidentally accumulated on the fibrous webs, do not result in discoloration or oxidation of these webs during subsequent processing into various sheet-like products.

The following examples illustrate the use of this invention in the flash-spinnig of solutions of high-density polyethylene in trichlorofluoromethane. It is not intended, however, to be limited to this system since it is applicable generally to flash-spinning Where the solidified endless fibrous structure becomes electrostatically charged during passage over an annular rotating target plate.

EXAMPLE I A plexifilament comprising high-density polyethylene (so-called linear polyethylene) is prepared by flash-spinning and is deposited as a random web on a positively charged belt (about 15 kv.) as shown in FIG. 1. Negative electrostatic charge is imparted to the plexifilament as hereinbefore described, the target plate being 7.5 inches (19.05 cm.) in diameter with an exposed metal annulus width of 0.875 inch (2.22 cm.) and being rotated at about 3 r.p.m. Process and apparatus for forming the plexifilament are substantially as described in Example VII of Anderson et al., US. Pat. No. 3,227,794. The flashspinning orifice 12 is 0.030 inch (0.076 cm.) in diameter and 0.025 inch (0.064 cm.) long. A solution of 12.6 parts by weight of polyethylene in 87.4 parts of trichlorofluoromethane is fed at a temperature of 182i0.5 C. and at a rate delivering 35 lb./hr. (15.9 kg./hr.) of polymer to orifice 12, the pressure on the solution being dropped to 1050140 p.s.i.g. (738:0.7 kg./cm. gauge) just before passage through the orifice. The polyethylene has a melt-index of 0.8 measured according to ASTM D123 8-57T-Condition E.

During continuous operation, charging current is monitored via a milliammeter in the ion gun circuit as shown in FIG. 1. The current initially established is 300 microamperes across a needle-tip to target plate gap of 0.625 in. (1.588 cm.). As a high-resistivity film forms on the target plate, the current begins to fluctuate. Fluctuations of :20 microamperes result in erratic web-collection and shortly precede very abrupt further increases in current.

With no liquid wiped onto the target plate, complete loss of electrostatic charging of the plexifilament (current increase to 400 microamperes) occurs in 1.5 hours of operation. Using apparatus as shown in FIG. 4, delivering gm./hr. of liquid to the target plate, three different liquids are employed. Deionized water results in an increase to 410 microamperes in 1.5 hours. Addition of a trace of sodium chloride to the water produces only slight improvement in operating time, with failure in about 3.0 hours. Neither of these liquids exhibits either the necessary viscosity or wetting powers. Surprisingly, on wiping onto the fouled target plate (current of 380 microamperes) an aqueous solution comprising 0.025% by weight of mixed ammonium phosphate esters (predominantly ammonium lauryl phosphate) and 0.5% of ammonium hydroxide, the charging current is reduced to 315 microamperes within 2 hours, and continuous satisfactory operation is restored. This liquid is observed to coat a continuous layer onto the target plate, from which the water quickly evaporates leaving highly conductive ammonium lauryl phosphate uniformly spread over the area. Thus it is seen that applied conductive liquids cannot only prevent fouling but also restore a fouled target plate to satisfactory operating condition.

EXAMPLE II Plexifilaments are prepared as described in Exam le I except that the strands from 22 orifices are collected simultaneously on the moving collection belt. Longitudinal center lines for the orifices are spaced 4.1875 inches (10.636 cm.) apart laterally across the width of the belt. Charging current is 3'40 microamperes. With no liquid wiped onto the target plates, 17.9 failures by fouling per 1000 position-hours occur. Each position operates on the average 2.4 days before failure. A scraper blade is mounted near the top of the target plate to bear uniformly against its surface, but not with enough pressure to wear off the metal surface.

A series of tests is performed in which a mixture of monoand di-lauryl phosphates is wiped onto the surface of the target plate. A gravity-operated feed system as shown in FIG. 4 is used to apply the lauryl phosphates at a rate of 0.2 gm./hr. at about 60 C. There are only 3.0 failures by fouling per 1000 position-hours, or an average trouble-free operation time of 13.9 days per position.

The lauryl phosphate employed has an absolute vis cosity at 60 C. of about 40 centipoises. It wets the steel surface of the target plate to a drop angle of 15 degrees in less than 10 seconds at 25 C., and a polyethylene film to an angle of less than 10 degrees in 11 seconds. Its boiling point is in excess of C., and its volume resistivity is of the order of 10 ohm-cm.

7 EXAMPLE III Separate tests of three conductive liquids are made as described in Example II except that the conductive liquid is applied as shown in FIG. 3. Applicator 25 is a dense wool felt. Wicking strand 33 is a felt of entangled wool fibers having an apparent density of 0.26 g./ cc. and diameter of about inch (0.16 cm.).

Several tests of the mixture of monoand di-lauryl phosphate described in Example II, applied to the target plate of 0.2 gm./hr., provided an average operation time of 5-9 days per position without fouling.

Two single spinning position tests of glycerine containing 1.0% ammonium lauryl sulfate and 1.5% lauryl alcohol (based on total weight of mixture) applied at a rate of 0.18 gm./hr. yielded trouble-free operation for 7-8 days. This liquid has an absolute viscosity at 60 C. of about 35 centipoises, wets both steel and polyethylene surfaces to a drop angle of 20 degrees in less than 20 sec., and has a volume-resistivity of about 10 ohm-cm.

Several tests were made using a mixture of monoand di-fatty acid glycerides (Atmul-80 from Atlas Chemical Co.). This substance is solid at room temperature, but is liquid at 60 C. exhibiting a viscosity of about 31.5 centi poises. It is observed to form a continuous layer on the target plate, the rate of application being 0.2 gm./hr. On the average, it provides 10 to 12 days of continuous operation per position without loss of electrostatic charging. A chemically similar mixture of fatty acid glycerides (Atmus 300) has a viscosity of 28 centipoises at 60 C., wets steel to a drop angle of 15 degrees in less than 10 seconds, wets polyethylene to less than 10 degrees in 11 seconds, and extends operation time before failure similarly. Both of these applied liquids exhibit volume resistivities of less than 10 ohm-cm.

When it is desirable for control of the rate of application to the target plate, these fatty acid glycerides may be mixed with lauryl alcohol (or the like) to reduce viscosity without deleteriously affecting any critical properties of the resultant conductive liquids. Mixtures with up to 30% by weight lauryl alcohol are found similarly effective.

EXAMPLE IV This example, carried out as in Example III, describes a liquid illustrating the vaporization of inert volatile liquid carrier to leave a film comprising finely divided crystalline solids deposited on the target plate. The liquid applied, per 100 grams, contains 6.1 gm. of pentaerythritol, 5.9 gm. of potassium iodide, 0.5 gm. of methylcellulose, 8.3 gm. of isopropanol, and 79.2 gm. of water. The isopropanol is used to promote wetting and the methyl cellulose is used to increase viscosity and provide a continuous film. Wicking strand 33 is a felt of poly(ethylene terephthalate) staple fibers; diameter of the strand is about A inch (0.16 cm.). The solidified material on the target plate has very low volume-resistivity. It is particularly advantageous in that, in addition to providing conductivity, it is very readily scraped from the target plate along wth the accumulating polymeric, electrically insulating material.

The superiority of this liquid led to testing at polymer flow rates of 70 lb./hr./position (31.8 kg./hr./position).

At this flow-rate, fouling of the target plate (charging current of 340 microamperes) with no liquid applied to 5 the target plate occurs in from 0 to 15 minutes. With the liquid of this example applied at 0.5 gm./hr., continuous operation in excess of 4 hours is obtained. When the initial charging current is increased to 540 microamperes, operation for 8 hours results.

What is claimed is:

1. In a process for preparing webs of endless fibrous structures wherein the structures are obtained by flashspinning of a polymer solution, are electrostatically charged by passage between an ion gun and a target 15 plate, and are collected on an oppositely charged or grounded collection device; the improvement, for preventing loss of electrostatic charging of the structures due to polymer build-up on the target plate, which comprises maintaining on the surface of the target plate a nonvolatile conductive substance by applying to the surface a liquid which comprises said substance and which wets the surface and the polymer thereon sufficiently to uniformly distribute said substance thereover and wherein the non-volatile portion of said liquid comprising the conductive substance is sufficiently viscous that it is not blown off of the said surface by the gas stream released during flash spinning.

2. Improvement of claim 1 wherein the target plate is rotated and said liquid is applied by wiping it onto the surface of the target plate continuously as the target plate rotates.

3. Improvement of claim 2 wherein the non-volatile conductive substance is solid at the temperature of the target plate surface and the liquid comprises a solution of said substance.

4. Improvement of claim 2 wherein the non-volatile conductive substance is liquid at the temperature of the target plate surface.

5. Improvement of claim 2 wherein the conductive 4 substance comprises phosphate ester of fatty alcohol.

6. Improvement of claim 2 wherein the conductive substance is potassium iodide.

7. Improvement of claim 2 wherein the conductive substance comprises glyceride of fatty acid.

References Cited UNITED STATES PATENTS 2,272,847 2/1942 Macht 260-963 3,081,519 3/1963 Blades et a1 18-8 3,277,526 10/1966 Hollberg 18-8 3,284,248 11/1966 Rumberger 264-213 3,387,326 6 1968 Hollberg et al 264-24 3,456,156 7/1969 Kilby et a1. 264--22 ROBERT F. WHITE, Primary Examiner I. R. THURLOW, Assistant Examiner U.S. Cl. X.R.