US 3600483 A
Description (OCR text may contain errors)
Aug. 17, 1971 w DAVIS ErAL 3 ,600,483
PROCESS OF FLASH SPINNING AND COLLECTING PLIuXll"ll|/\.Ml-lN'[ TO FORM ROD-SHAPED BACK-WINDABLI') BAT'l' Original Filed Sept. 50, 1964 l3 FIG.2 H
1;; o o O o o :11 i O O O O w O O O O O J y/111 1 J m! (P o o o 6 7 9 I4 (I I2 I 9 |3 I m .2 ll) 36 l INVENTORS THOMAS WADE DAVIS ROBERT JOHN GILARDI ATTORNEY United States Patent Ofice U.S. Cl. 264-53 6 Claims ABSTRACT OF THE DISCLOSURE Flash-spun plexifilament is collected by allowing the network to expand in an elongated passageway adjacent the extrusion orifice and impinge on a yieldable surface whereby the network folds upon itself to form a rodshaped back-windable batt. The passageway is at a pressure lower than that of the solution prior to extrusion and preferably above atmospheric. The cross-section of the passageway governs the size and shape of the crosssection of the expanded network.
This application is a divisional of application S.N. 400,353, filed Sept. 30, 1964, now U.S. Pat. No. 3,413,185.
This invention is concerned with a back-windable batt of continuous strand material and with a process and apparatus for achieving such a batt.
INTRODUCTION In the commercial production of textile yarns various methods are known for collecting yarns at speeds above 3000 meters/minute in a flat package, cake, or batt which is adapted to be easily unwound provided certain prescribed patterns have been observed in collecting the strand. This type of package typically contains helically deposited coils or loops in contrast to straight-wound packages such as those wound on cones or bobbins. Straightwound packages are not satisfactory for collecting strands at Very high speed because traverse difficulties and anal ogous problems are created by the motion of the collecting device. Strands which have heretofore been collected in the form of a fiat package, cake or batt have generally consisted of multifilament yarns in which the single filaments are separated from one another, the filaments being generally larger than 8 microns in thickness.
In U.S. Pat. 3,081,519 of Blades and White a fibrillated type of strand is described which consists of a three-dimensional integral plexus of synthetic organic, crystalline, polymeric fibrous elements, the fibrous elements being in the form of film-fibrils less than 4 microns thick. The film-fibrils within these strands have great attraction for one another, presumably because of electrostatics or because of their planar structure and high surface area. Consequently it has been found particularly difiicult to obtain a package from which such a strand can be removed continously without entanglement.
Accordingly the purpose of the present invention is to provide a back-windable package of a fibrillated strand wherein the strand is comprised of an integral network of film-fibrils. -It is an object to provide such a package which is in a form suitable for shipping, particularly in a form which can be pulled oiT continuously as a fiuffy tow and converted into cigarette filters or other items where bulk is desired. Another object is to provide a process and apparatus for packaging a strand-produced at speeds greater than 3000 meters/minute. In particular it is an object to provide such a process and apparatus for 3,000,483 Patented Aug. 17, 1971 continuous formation of a fibrillated strand, continuous collection of the strand in the form of a batt in a collecting zone, and continuous removal of the resulting batt from the collecting zone.
SUMMARY OF THE INVENTION The above objectives are accomplished by providing a rod-shaped batt of 1ongitudinally-collapsed, continuous strand material, the strand material comprising a threedimensional integral network of film-fibrils of crystalline oriented synthetic polymer, the film-fibrils having an electron diffraction angle of less than and an average film thickness of less than 4 microns. The batt has recoverable strand ends at each of its opposite ends and has a density of about 1 to 15 lbs./ft. The collapsed network is composed of tiny folded film-fibrils or film-fibril composites. The total package cross-sectional area is advantageously between one and four times that of the network. The strand material in the batt lacks any sort of winding, pattern or twisted or piddled configuration; is. the strand material has simply collapsed upon itself-somewhat like the action that occurs in the bellows of an accordion.
The apparatus of the invention comprises, in combination, a spinneret having an orifice for producing a strand of filamentary material and collecting means for receiving the filamentary material upon its issuance from the spinneret. The collecting means comprises a tubular-shaped conduit extending out from the orifice to define an elongated passageway therein, a first end of the passageway communicating with the orifice whereby filamentary material is discharged under elevated pressure from the orifice into the conduit. A second end of the passageway in re mote from the orifice. Means is provided in the passageway for restraining the forward motion of the batt. For example, the cross-sectional area of the passageway may diminish, i.e. narrow or gradually taper, from the first end to the second end. This enables the freshly-spun filamentary material to accumulate by collecting upon itself. Also the conduit includes means for maintaining the pressure within the passageway at a level between the spinning pressure and atmospheric pressure. In a preferred embodiment the means for maintaining the pressure within the passageway will comprise simply a series of spaced apart perforations in the conduit in the vicinity of the first passageway end. In any case, the pressure in the passageway, which will be somewhat above atmospheric, causes the collected batt to slowly extrude from the downstream opening of the passageway. The rod-shaped package so produced will thus have essentially the same cross-sectional configuration as the opening.
The rod-shaped batt of longitudinally-collapsed fibrillated strand material is made by spinning a solution of organic polymer through the spinneret orifice, the solution upstream of the orifice being under pressure at a temperature above the boiling point of the solvent. The solution passes through the orifice into an enclosed elongated passageway or collecting zone which is at substantially lower pressure but which is nevertheless at greater than atmospheric pressure. A fibrillated continuous strand comprising a three-dimensional network is formed at the orifice exit. The strand expands and impinges at essentially a right angle against a yieldable collecting surface zone. In this way it is caused to collapse longitudinally, egg. in the direction of spinning. The lateral or cross-sectional dimensions of the accumulated batt are governed by the size and shape of the walls which define the passageway. These walls may comprise a generally cylindrical or tubular shaped conduit. The cross-sectional area of the passageway may be equal to or greater than the maximum cross-sectional area of the strand network but, in either case, should be small enough to prevent folding of the gross strand in zig-zag arrangement. The gaseous solvent which separates from the polymer at the orifice is allowed to expand in the collecting passageway partially by pushing the collecting surface and filamentary material away from the Orifice and partially by escape through holes in the walls defining the passageway. The collapsed fibrillated network piles up continuously and is forced continuously from the collecting passageway by means of gas pressure created therein so as to extrude a continuous rod-shaped batt into the surrounding atmosphere.
The invention will be further described with reference to the drawings wherein:
FIG. 1 is a drawing of a rod-shaped batt of strand material comprising a longitudinally-collapsed integral threedimensional film-fibril network.
FIG. 2 is a drawing in perspective, partially cross-section, of a spinneret and strand collector and showing extrusion of a rod-shaped batt.
FIG. 3 shows a spinneret and collecting chamber, partially in cross-section, the tubular conduit being generally rectangular in cross-section, the top wall being hinged at its upstream end.
FIG. 4 is a drawing, partially in cross-section, of a spinneret and strand collector, in this case the tubular conduit is a perforated cylinder provided with an obstruction in the nature of an end closure.
FIG. 5 is a drawing of a spinneret suitable for use in the apparatus of FIGS. 2, 3, or 4.
BACK-WINDABLE BATT The product of this invention, herein referred to as a batt, is similar in some respects to the cocoon spun by the silkworm in that the strand may be removed continuously and is composed of very tiny fibrous elements. In other ways the batt differs greatly from a cocoon. For example, the batt is made in the form of a long rod or stick and the strand can be removed from the end of the package, actually either end, without rolling the rod or developing torque in the strand.
Considering FIG. 1 in more detail the product comprises a long rod-shaped batt 1 with recoverable ends 2 from a single strand. The strand is composed of a three dimensional integral network of film-fibril elements 3. The film-fibril elements are connected at random intervals along and across the strand and are less than four microns thick.
The strand is a plexifilament as described in U.S. Pat. 3,081,519 to Blades et al. It is preferred from synthetic filament-forming polymers or polymer mixtures which are capable of having appreciable crystallinity and a high rate of crystallization. A preferred class of polymers is the crystalline, non-polar group consisting mainly of crystalline polyhydrocarbons, e.g. polyethylene, polypropylene, and copolymers of ethylene and propylene. Common textile additives such as dyes, pigments, antioxidants, delusterants, antistatic agents, reinforcing particles, adhesion promoters, removable particles, ion exchange materials, and U.V. stabilizers may be mixed with the polymer solution prior to extrusion to provide strands containing such.
Referring again to FIG. 1, because of the nature of the strand collector the batt is usually concave on one end 4 and convex on the other end 5, the convex end being the first spun end. It is usually easier to remove the strand continuously from the concave end, but removal is actually possible from both ends. An interesting feature of this strand package is the ease with which it may be divided into smaller packages. By simply bending the rod vigorously it can be divided into two parts, each with two recoverable strands ends. These ends remain intact during the package breaking operations, but may be cut like the umbilical cord after separation of the two package parts. Although the rod-shaped package is extruded continuous- 1y from the apparatus and can be made in interminable length, it is conveniently broken into approximately 6 foot lengths for packaging in side-by-side relationship in boxes for shipping. It is @150 possible, however, to ship the 111.9.-
- 4 terial in continuous form by coiling as is done with telephone cable or electric wiring.
The density of the tow package may be regulated y means to be described further hereinafter in connection with the process details. The density of the product should be, however, between about 1 and 15 lbs./ft.
The strand which can be removed from either end of the package comes off in the form of an integral network. The network normally takes the form of a cone during removal; the apex being pointed in the direction of strand travel during such removal. The periphery of the network at the base of the cone is generally circular, but within the circular periphery are multitudinous film-fibril elements which are being simultaneously withdrawn in random fashion from all parts within the circular area. The base of the cone may oscillate somewhat within the boundaries of the package during removal but there is preferably no appreciable zig-zag folding of the entire strand in the package. If the strand is initially removed with little or not compaction it is in a fluffy, high bulk, three-dimensional form, frequently with a density as little as a feW hundredths of a pound per cubic ft. In this form it is essentially indistinguishable from a bulky plexifilamentary strand which has been simply spun into the atmosphere without the aid of a collecting device.
To preserve the plexifilamentary multifibrous network character, the cross-sectional area of the package should not be more than about four times the cross-sectional area of the base of the cone, i.e., less than four times the crosssectional area of the integral film-fibril network at any point in the package. In the preferred product, the crosssectional area of the package is less than 1.5 times that of the film-fibril network. This means that in removing the strand from an end of the package, at any given time filmfibrils are being lifted from essentially all parts of a g nerally circular area, the size of that area being at least one-forth, but preferably at least two-thirds, the crosssectional area of the package. Whereas the lateral dimensions of the strand network and package may be nearly equal, the latter will have a length of the order of /25 to that of the strand before packing.
A desirable feature of the package is the ease with which it may be converted to a fluffy high bulk tow. The simple process of pulling the strand from the package causes it to bloom, i.e. to form a soft bulky tow which is satisfactory for textile uses and for preparing cigarette filters. Many other uses, including amusement devices, will also be apparent from the package because of its unique ability to give seemingly infinite lengths of a bulky strand from a small size package. The easy blooming of the tow is understandable if one considers the method of film-fibril deposit in the package. Essentially the network has never been compacted transversely; it has simply been collapsed longitudinally to remove the bulk of the air that normally fills the spaces between film-fibrils in the work. The film-fibrils are folded individually or as composites, but in general the total strand does not fold.
Although the specific description is limited here to rodshaped packages of circular and rectangular cross-section, it will be evident that a variety of cross-sectional configurations may be formed to suit the intended use. In general, shapes such as rectangular which can be boxed with a minimum loss in space are preferred.
Considering now the plexifilamentary strand itself, the strand is formed by extruding a solution of a fiber-forming polymer in a liquid which is a non-solvent for the polymer below its normal boiling point, at a temperature above .the normal boiling point of the liquid, and at autogenous pressures or greater into the atmosphere or other medium of lower temperature and substantially low pressure. The vaporizing liquid within the extrudate forms bubbles, breaks through confining walls, and cools the extrudate, causing solid polymer to form therefrom. The resulting multi-fibrous yarn-like strand has an internal fine structure or morphology which may be char t r zed as a three-dimensional integral network or plexus consisting of a multitude of essentially longitudinally extended interconnecting random length fibrous elements, hereafter referred to as film-fibrils, which have the form of thin ribbons of a thickness less than 4 microns. The film-fibril elements, often found as aggregates, intermittently unite and separate at irregular intervals called tie points in various places throughout the Width, length and thickness of the strand to form an integral three-dimensional plexus. The film-fibrils are often rolled or folded about the principal film-fibril axis, giving the appearance of a fibrous material when examined without magnification. The strand comprising a three-dimensional network of filnrfibrll elements is referred to as a plexifilament. The plexifilaments are unitary or integral in nature, meaning the strands are one piece of polymer, are continuous in nature, and the elements which constitute the strand are cohesively interconnected. Minor physical treatments of the continuous strand such as shaking, washing, or textile processing will not cause appreciable amounts of the film-like elements to separate from the strand.
For the purpose of simplifying the visualization of the fibrillated plexifilameut strands, one may suppose that all the morphological elements of the plexifilament are derived from bubbles in the viscous solution which form rapidly as the pressure is reduced during the initial stage of conversion of fluid polymer to strand material. The bubbles then grow and rupture in various ways to form the multifibrous network. The extreme thinness of the pellicular material imparts desirable aesthetic properties such as softness and suppleness to plexifilaments and enables them to be easily discernible from multi-fibrous strands or coarsely porous fibers of the prior art.
The strands are continuous in nature and can be pr duced in essentially endless lengths. The whole strands can have deniers as low as or as high as 100,000 or even higher. The highly fibrillated strand has the appearance of sliver or tow from extremely fine fibers. The filmfibrils, however, are connected in a network, there being few if any unconnected fibril ends.
The strands of this invention generally have tenacities of at least 1.0 g.p.d. and, when drawn, may have tenacities as high as 23.0 g.p.d. The strands are twisted 8 t.p.i. prior to making the measurement.
All of the strands are characterized morphologically by a three-dimensional network of film-fibril elements. These networks may exist in various forms, but in all cases the film-fibrils are extremely thin. On the average the filmfibril thickness is less than 4 microns thick. In the preferred products the film-fibrils are less than two microns thick and may indeed have a thickness of less than 1 icron. The film-fibril elements are normally at least five times as wide as they are thick, the actual Width being between about 1 micron and about 1000 microns.
The film-fibril elements in plexifilaments are found in the form of fibril composites which are laminates, aggregates or bundles within the gross strand. Because these fibrils in the strand. However, for convenience, the averjoin other bundles, it is difficult to count individual filmfibrils in the strand. However, for convenience, the average number of fibril composites in a 0.1 mm. thick crosssectional cut of the strand is used as a measure of the degree of fibrillation. The number of these fibril composites per 1,000 denier in a 0.1 mm. length of strand is hereafter referred to as the free fibril count. It is recognized that number of additional film-fibrils which can be pulled away from the fibril composites with slight tension will be many times the number found already free, but film-fibrils which adhere to each other are not counted as separate fibrils in the standard test.
The predominantly longitudinal orientation of the filmfibrils of all plexifilamentary strands is readily apparent from the fact that all such strands are much more resistant to tearing or breaking transversely than to splitting lengthwise. The general coextensive alignment of the fibrous elements in the direction parallel to the strand axis is easily discernible to the naked eye: for most plexifilamentary species.
The plexifilamentary strands of the invention are made of crystalline polymer. It has been found that the pellicular material in the asspun strand when consisting of a crystalline polymer is substantially oriented as a measured by electron diffraction, i.e., it has electron diffraction orientation angles smaller than It is believed that the high strength of the plexifilamentary strand as spun is closely related to the crystalline orientation within the film-like ribbon and in the structural arrangement of the fibrils themselves in the strand. In the preferred crystalline oriented products of the invention, the film fibrils have electron diffraction angles of less than 55. The orientation of the crystallites in the film-fibrils is in the general direction of the film-fibril axis.
X-ray diffraction patterns which are obtained using the Whole strand instead of just film-fibrils show a substantial amount of orientation in the strand as spun. The X-ray diffraction orientation angles are less than 55 in the preferred embodiments of the invention. The substantial orientation which is exhibited by the gross strands, indicate that not only are crystallites oriented along the fibrils, but the fibrils are themselves oriented in the general direction of the strand.
The plexifilament strands have a surface area greater than 2 m. /g., as measured by nitrogen adsorption methods. Due to the extremely high polymer/air interfacial area the strands have marked light scattering ability and high covering power.
An important characteristic of the strands of this invention is the fibrillar texture of the gross strand as observed with the polarizing microscope.
In order to observe fibrillar texture, a specimen is prepared as follows: a short length of strand is frozen in liquid nitrogen and a segment which is 15 millimeters long is cut from the frozen strand. The segment is placed on its side in immersion oil on a microscopic slide, and the slide is placed in a vacuum chamber and pumped down to remove tra ped air. After removing the slide from the vacuum chamber, the specimen is observed in a polarizing microscope using about 45 X magnification. A cover glass is seen in the microscope is of a longitudinal segment of the whole strand. A first order red plate is used in the microscope: and the Nicols prisms are crossed at 90 to one another.
A striking color view of the sample is seen in the polarizing microscope. In the strands of this invention long streaks of uniform color run parallel to the strand axis. Although there are a variety of colors, each color extends for long periods along the length of the strand. An interpretation of the polarized light patterns may be found in Fiber Microsc py, by A. N. J. Heyn, Interscience Publishers, 1954, pp. 288352. Monochromatic streaks in color photomicrographs taken with polarized light are derived from areas of equal optical path difference and in general will be due to equal orientation and equal thickness in the strand. These photographs demonstrate therefore that the strands have a high degree of organization, and the highly organized areas extend for considerable distances along the length of the strand. The strands are characterized as fibrillar if at least half of the material making up the strand appears as monochromatic streaks when observed in the polarizing microscope. The monochromatic streaks are oriented in the direction of the strand axis and have an actual (unmagnified) length of at least 0.2 mm. The monochromatic areas are considered as streaks when they have a length at least 10 times the width.
Characterization methods for the plexifilamentary strand are further described in US. 3,081,519 referred to above. These methods and other descriptive matter of the patent are incorporated herein by reference.
The melt index of the polymer is the flow in g./ 10 min., as determined by ASTM Method D1238-57T, Condition E, and is inversely related to molecular weight. By linear polyethylene is meant polyethylene having a density of 0.94 to 0.98 g./cm. but preferably having a density of 0.95 to 0.97 g./cm. The polymers are of at least film-forming molecular weight.
SPINNERET AND STRAND COLLECTOR The apparatus of the invention will be described by reference to FIG. 2, which represents one embodiment. As shown therein, the spinneret 6 is provided with an orifice 7 through which a solution 8 of a synthetic organic polymer is extruded by means of high pressure derived from the solvent vapor by pressure at temperatures above the boiling point, or the pressure may be exerted by combinations of solvent vapor pressure with mechanical pressure, pressure of inert gases, or other pressurizing means.
Surrounding the spinneret orifice 7 and extending axially from it is a tubular shaped conduit 10 defining an elongated collecting passageway 9. As will be seen the conduit 10 is in the nature of an elongated conical element of generally circular cross-section, the diameter and crosssectional area gradually diminishing toward the downstream end thereof. Conduit .10 is open at the downstream end 11 to emit the batt product 1. The wall of the conduit 10 is perforated with numerous holes 12, particularly in the vicinity of the upstream end thereof. In operation the solution is forced through the orifice 7 into the collecting passageway 9, whereupon the solvent evaporates suddenly, forming a three-dimensional network .13, which collects on the surface 14 of the previously formed batt 1. Part of the expanding gas escapes through holes 12. The remaining gas, which will usually be at a pressure of .1 to 100 p.s.i.g., acts to compact the previously formed batt and forces the collected material continuously out the opening 11. The conduit 10 has an interior diameter at its upstream end approximating the outside diameter of the spinneret. It can thus be fixedly mounted upon the spinneret and secured to it by any convenient means such as a clamp, not shown.
The process is self-controlling when sufiicient hole area is provided in the collecting passageway for release of the maximum amount of solvent vapor generated. Thus when pressure in passageway 9 is not sufilcient to move the batt, the impacting yarn builds up and covers holes, thereby reducing the open area for vapor escape. Pressure increases by this mechanism until it is sufiicient to move the batt. When pressure in the passageway exceeds equilibrium, the collected batt of filamentary material accelerates and uncovers more holes, thereby increasing the open area for vapor escape. Pressure thereby decreases until equilibrium is obtained. Although the collected batt may move intermittently, it will generally proceed at a constant rate of speed once pressure equilibrium has been achieved.
The pressure in the passageway may, of course, be regulated further by pressure control valves in a gas escape port or bleed-off line in communication with the passageway. Alternatively the frictional characteristics of the collecting tube, e.g. in terms of taper or other obstructions, can also be varied to affect the pressure and thus the rate of extrusion and batt density. In any case the pressure in the passageway should be kept at a level sufficient to cause the rod-like batt to extrude continuously from the passageway at a rate between and of the strand formation rate. The operator will have no difficulty in adjusting one or more variables so as to cause the extrusion process to proceed smoothly and efiiciently at a desired rate.
It is apparent that a vapor collector can be installed surrounding the perforated chamber for recovery of solvent.
The package density can be varied by adjusting the size of the outlet end of the collecting conduit and the degree to which the cross-sectional area of the conduit decreases as that end is approached. By varying one or both of these factors, the amount of pressure needed in passageway 9 to move the batt will be altered. The shape and size of the package can be varied Widely depending upon the geometry and dimensions of the conduit and opening.
One function of the collecting device is the provision of a pressurized chamber between the spinneret and the collected batt with means for bleeding off or venting excess vapor at a rate consistent with the rate at which solution is spun through the orifice. The system develops sufficient pressure in operation to eject the batt completely out of the tubular conduit if holes or other means for venting the tube are not used. A further function of the collecting device is to enable control of the forward travel of the batt. In this respect the tubular conduit 10 of FIG. 2 uses friction developed by the batt in contact with the inner peripheral wall of the tapered tube to balance the force developed by vapor pressure; however, other devices which also restrain forward motion will be apparent.
The spinneret orifice diameter must be small enough to permit continuous replenishment of the solution supply upstream of the spinneret at constant pressure and must be large enough to maintain a constant and moderate pressure (cg. .1 to 100 p.s.i.g.) in the collecting passageway under the conditions of temperature, pressure and solution concentration used for the particular solution being spun. To accomplish this objective the total area of the holes in the collecting zone must be kept at a level which is consistent with the spinneret orifice size.
In designing the apparatus, the various dimensions should be selected to cause the expanded flash-spun strand to impact the surface of the already collected batt at a point where the network has first reached its maximum diameter. If the surface of the already collected batt is too close to the spinneret, the velocity at impact causes tearing of the web which results in yarn breaks during backwinding. In addition the strand tends to fold and arrange itself transversely with respect to the axis of the package. If the point of formation is too far from the spinneret, the yarn will tend to be deposited parallel to the axis of the package or will be blown completely out of the collector. The parallel type of yarn arrangement cannot be backwound without excessive yarn breakage.
When the dimensions of the collecting conduit and the gas escape ports have been appropriately selected for the polymer flow rate, spinneret, and solution concentration being used, batt formation will occur at the point where the yarn reaches its maximum diameter. In this case, the yarn will be longitudinally collapsed without excessive gross folding and can be readily backwound by pulling on the free end.
Under a given set of operating conditions to permit impact of the strand at a point farther away from the spinneret, the taper of the collecting tube may be reduced, .giving thereby less resistance to the collected cake. Alternatively the size of the opening 11 in the collecting tube may be decreased giving higher pressure in the tube. This also causes the rod-like package to extrude at higher speed.
Another embodiment of the apparatus of the invention is shown in FIG. 3, shown partially in cross-section. Here means are provided to enable adjustment of the taper of the collecting passage during the collecting operation. The spinneret 6 is enclosed by a collecting conduit, shown generally as 21, which is rectangular in cross-section. The spinneret orifice passage 7 is oriented horizontally as is the collecting conduit. The upper or top wall 22 of the horizontally mounted conduit fits closely between the adjacent side walls 23 and 24 but is not stationarily attached. It is pivotally mounted at its upstream end by a hinge 15, the hinged top wall 22 being in close-fitting relationship with the two adjacent side walls 23 and 24.
The hinged top 22 and the opposite side 25 are perforated throughout their lengths. The other two sides 23 and 24 are not perforated. The hinged side is forced against the batt by means of weights 26. In operation the collecting conduit has the shape of a truncated wedge. The unperforated sides 23 and 24 of the collecting chamber are sutficiently large to provide for sealing the chamber regardless of the positions of the hinged side 22. A rodlike batt with rectangular cross-section is obtained from this apparatus.
The embodiment of the apparatus shown in FIG. 4 is made of a perforated round tubular element 31, nearly a right circular cylinder. Near the downstream end thereof is a closure plate 32 pivotally mounted and secured by hinge 33. The plate 32 is adapted to engage and rest upon the batt 1 as it extrudes from the opening 11. A weight 34 is afiixed to arm 35 which extends from plate 32. The weight assists in creating the friction needed to control the speed of the extrusion and hence the pressure in the passageway. An extension 36 of tubular element 31 prevents the weight of closure plate 32 and associated parts from breaking the batt 1. In starting up the process, the downstream end 11 is closed by plate 32. It remains closed until suflicient filamentary material accumulates, whereupon the batt begins to extrude and lift. the plate 32 to the position shown.
FIG. is a drawing of a suitable spinneret for use in the apparatus of the invention. As shown the spinneret may be provided with a preflashing or pressure let-down zone to facilitate a high rate of bubble nucleation. Solution enters the spinneret through constriction 42 and passes to the let-down chamber 43 and finally is extruded through spinning orifice 7 into the surrounding atmosphere. This spinneret, of course, must be designed for a specific throughput rate. As described in aforementioned US. Pat. 3,081,519, wide variations can be made in the spinneret design.
PROCESS ELEMENTS Full details for the flash-spinning process are described in aforementioned US. Pat. 3,081,519. In addition, however, there are further process features which are related to obtaining a rod-like package within a specified density range, having good back-windability and being free of knots, slubs, bubbles, or other defects in the continuous network strugtures.
With respect to previously known process elements, solvents for use in forming the high temperature, high pressure polymer solutions required for forming the plexifilaments of the invention should preferably have the following characteristics: (a) a boiling point at least 25 below the melting point of the polymer used; (b) they should be substantially unreactive with the polymer during extrusion; (c) they should dissolve less than 1% of the high polymeric material at or below its normal boiling point; and (d) the solvent should form a solution which will undergo rapid phase separation (i.e., in less than .01 second) upon extrusion forming a non-gel polymer phase, i.e., a polymer phase containing insuflicient residual solvent to plasticize the structure. In these requirements, the flash spinning process employed in the present invention differs radically from conventional solution spinning techniques since in the latter the spinning solvent is invariably a solvent for the polymer below the normal boiling point, usually even at room temperatures.
Among those liquids which may be advantageously utilized in the flash-spinning process (the actual choice of course depending upon the particular polymer used) are aromatic hydrocarbons such as benzene, toluene; aliphatic hydrocarbons such as butane, pentane, hexane, heptane, octane, and their isomers and homologs; alicyclic hydrocarbons such as cyclohexane; unsaturated hydrocarbons; halogenated hydrocarbons such as methylene chloride, carbon tetrachloride, chloroform, ethyl chloride, methyl chloride; alcohols; esters; ethers; ketones; nitriles; amides;
10 fluorocarbons; sulfur dioxide; carbon disulfide; nitromethane; water; and mixtures of the above liquids.
The flashing otf of solvent during the spinning process of this invention is similar in some respects to the flash evaporation of solvent in flash distillation procedures. The rapid and substantial reduction in pressure upon the confined polymer solution when the orifice is reached results in the production of bubbles within the still fluid polymer followed by expansion of the bubbles and evaporative cooling of the polymer to form pellicular material which ruptures and deforms with resultant production of the characteristic integral plexus. The initial heat content of the spinning solution will alfect the final morphology. If the initial heat content is too small, a closed cell morphology will result and if too high, a sintered product will be produced. It is surprising that despite the violent nature of the process, indefinitely continuous strands may be obtained.
The preferred fibrillated species which is suitable for use in the back-windable package, is composed of linear polyethylene. It is advantageously flash-spun from spinning solutions of 10 to 14% by weight polymer in trichlorofluoromethane. The temperature upstream of the spinneret should be above the critical temperature minus 45 C. for the solvent, but preferably is even higher, being preferably above the critical temperature minus 30 C. In addition, for purpose of the present invention the pressure of the solution upstream of the spinneret should be far above the normal vapor pressure of the solvent at the temperature mentioned, the additional pressure being exerted by mechanical means, such as by a reciprocating pump. The pressure should be kept above the two-liquidphase pressure boundary, this being the pressure for a given solution below which two liquid phases can exist. For example, a 14% solution of linear polyethylene of melt index 0.57 in trichlorofiuoromethane which is spun at C. requires a pressure of at least about 1285 p.s.i.g. to exist as a homogeneous single-phase solution upstream of the spinneret. The vapor pressure of the solvent at 185 C. is 515 p.s.i.g. The additional pressure to obtain 1285 p.s.i.g. must be supplied mechanically. The two-liquid-phase pressure boundary moves to higher pressure with increasing temperature, with increasing melt index (decreasing polymer molecular weight), or with decreasing solution concentration. US application Ser. No. 308,845 of Anderson and Romano filed Sept. 13, 1963 provides a further explanation of this phenomenon and hence the disclosure thereof is incorporated herein by reference.
A further requirement for consistently obtaining a highly fibrillated uniform product is that the solution pressure should be dropped to a pressure which is below the two-liquid phase pressure boundary, just before passing through the final spinneret orifice. For a 14% solution of linear polyethylene in trichlorofiuoromethane this let-down pressure should be between 700 and about 1100 p.s.i.a.
'In polymer-solvent systems where two liquid phases form readily, the drop in pressure in the let-down zone causes two liquid phases to form, with tiny droplets of one phase being carried in the second phase. The tiny droplets apparently act as bubble nuclei and promote an extremely high degree of fibrillation in the threadline when it emerges into the surrounding air at atmospheric pressure. Maximum fibrillation is of particular importance for achieving the greatest degree of opacity, bulk and other desirable fiber properties. Arrangement of the pressures in the system so that the pressure in the flow line upstream of the construction is above the two-phase pressure limit, assures the attainment of such high degrees of fibrillation. As an added safeguard for a commercial installation employing typical available high pressure equipment, a pressure drop from at least 25' p.s.i. above the two-liquid phase pressure to at least 25 p.s.i. below that pressure is desirably provided.
A spinneret which is suitable for providing the necessary pressure let-down is shown in FIG. described above. It is, of course, quite satisfactory to use other pressure let-down systems such as are provided by automatic valves and pressure controlling mechanisms.
The solution which flashes from the final spinneret orifice passes to the collecting passageway which is maintained at pressures substantially below that of the pressure let-down chamber. In order to provide compressive force for the collected strand material this pressure should be above .1 p.s.i.g. On the other hand it should be kept below about 100 p.s.i.g. to promote rapid evaporation of solvent and to avoid complete expulsion of the collected batt from the conduit. Preferably the pressure is between 0.1 and 5.0 p.s.i.g. The pressure in the collecting chamber is measured by means of a conventional pressure gauge.
The solvent which is used in the flash-spinning process must be capable of rapidly separating from the polymer when cooled. As explained in foregoing paragraphs it is also necessary to add sufiicient heat to the solution upstream of the spinneret to provide all of the calories needed for vaporization of the solvent upon flash extrusion so that additional heat is not needed in the collecting chamber. A solvent which is particularly satisfactory for this purpose with linear polyethylene is trichlorofiuoromethane.
As the plexifilamentary strand forms at the spinneret it expands into a net-work of increasing diameter and then increases no more. Optimum operation of the process occurs when the collecting surface is placed at the point where the strand first reaches its full diameter. When placed closer than this the strand tends to deposit in zigzag form or coils. When placed further away the strand loses velocity and again deposits in unsatisfactory form being laid in either longitudinal or transverse zigzag pattern.
When the collecting surface is presented an optimum distance from the spinneret, the network collapses longitudinally. The optimum distance between spinneret and collecting surface is generally between 0.3 and 2.0 inches.
The plexifilamentary strand which is formed at the spinneret may be provided within a broad range of deniers, by adjusting solution temperature, pressure, concentration, and orifice dimensions. Deniers greater than 170 are satisfactory for most purposes. Frequently it is advantageous for these, e.g. upon removal from the rodshaped batt, to be plied to obtain cigarette filter tows of 15,000 denier. Alternatively a heavy tow may be spun directly from a large spinneret orifice.
The following examples illustrate specific embodiments of the invention. All parts and percentages are by weight unless otherwise indicated.
EXAMPLE I A solution of linear polyethylene in trichlorofluoromethane was prepared continuously by pumping approximately 204 kg./hr. of solvent and 33.1 kg./hr. of molten polymer at a fixed ratio into a screw mixer as described in the Anderson and Romano US. application Ser. No. 308,845. The polymer which was used had a melt index of 0.5 and a density of 0.95 g./cm. A 14% solution was delivered continuously to an automatic pressure let-down valve at a temperature of 185 C. and a pressure of 1800 p.s.i.g. The solution passed through an automatic valve to a pressure let-down chamber of 17.2 cm capacity maintained at a pressure of 775 p.s.i.g. The final orifice in the spinneret was round and had a diameter of .117 mm. The length of the orifice passage through the spinneret plate was .127 mm. The spinneret face was flat, there being no flare on the outside edge of the orifice and no countersink on the inside. The spinneret orifice passage was oriented horizontally.
A plexifilamentary strand with denier 660 as backwound or 795 at 50 g. tension was spun from the orifice at the rate of 33.1 kg./hr. The strand was spun directly into a strand collector of the type shown in FIG. 4. The collector was round in cross-section with inside diameter of 4.45 cm. It was 33 cm. in length. The collector was attached snugly to the spinneret. The upstream end was perforated along the first 13.96 cm. with 0.16 cm. diameter holes. The holes were arranged in a square pattern, the centers being .238 cm. apart. These holes were polished to avoid snagging. The remainder of the collector was not perforated.
After the spinning operation reached equilibrium, the collecting operation was started by closing the hinged plate at the downstream end of the strand collector. The force of a ten-pound weight urged the plate against the collector end. As the batt was formed the plate was pushed up to a horizontal position and the batt continuously extruded thereunder. The gas pressure in the passageway was on the order of 1-2 p.s.i.g.
The batt so formed was round in cross-section, 4.84 cm. diameter. Sections were cut from the extruded rod as it formed. These sections were about 1.22 meters long. The rods had a density of 7.24 lbs./ft. (.116- g./cm. The consolidation ratio was 2000:l, i.e., each meter of the rod-like batt contained approximately 2000 meters of strand. The strand was 660 denier. The strand ends during removal from the rod-like batts formed a conical network with a diameter of about 4.7 cm. at the base of the cone.
The strand ends in the rods of fibrous batt were easily recovered and a soft bulky tow could be unwound from each end, the tow being essentially free of knots or slubs. Twenty-three of these rods were mounted in parallel on a creel to permit strand removal, the concave ends of each being aimed in the same direction. The twenty-three strand ends were pulled off simultaneously from the concave ends of the rods and gathered together to form a tow with a denier of 15,400.
Samples of the tow, about 2.3 cm. in diameter, were passed between a pair of 7.5 cm. (in diameter) rolls with a nip load of 4.4 kg./cm. of tow width. A coherent uniform tape of 15,400 denier was prepared which could be rolled up on a reel similar to movie film. This tape was subsequently used to prepare cigarette filters after rebulking by passage through an air jet.
EMMPLE II A plexifilamentary strand with denier 350 was spun by a technique similar to that of Example I. The strand discharged from the orifice at the rate of 20.4 kg./hr. The strand was spun directly into the strand collector shown in FIG. 2. The collector was round in cross-section, 3.2 cm. diameter at its upstream end and approximately 2.5 cm. at its downstream end. It was 27.3 cm. in length. The collector was attached snugly to the spinneret. The collector was perforated with 0.16 cm. diameter holes along the entire length. The holes were staggered, the centers being 0.47 cm. apart. These holes were polished to avoid snagging.
After the spinning operation reached equilibrium, the collecting operation was started by blocking the open end of the collector. An equilibrium was reached between the friction of the rod against the sides of the collector and the equilibrium gas pressure in the collector.
A continuous rod of fibrous batt was extruded from the collector. The batt was round in cross-section (2.5 cm. in diameter). The rods had a density of about 10 lbs/ft. (0.16 g./cm. The consolidation ratio was approximately 2000:l, i.e., each meter of the rod-like batt contained approximately 2000 meters of strand. The strand was 350 denier.
The collector tube was provided with a longitudinal seam to enable the diameter to be reduced by an externally applied force. This diameter was adjusted so that batt formation occured at approximately 2.5 cm. from the spinneret. The strand network as it impinged on the surface of the batt had a diameter of about 2.5 cm.
The strand ends in the rods of fibrous batt were easily recovered and a soft bulky tow could be unwound from each end, the tow being essentially free of knots or slubs. Forty of these rods were mounted in parallel on a creel to permit strand removal, the concave ends of each being aimed in the same direction. The forty strand ends were pulled off simultaneously from the concave ends of the rods and gathered together to form a tow with a denier of 14,100.
EXAMPLE III A plexifilamentary strand with denior 410 was spun in a manner similar to Example I, being discharged from the orifice at the rate of 24.1 kg./hr. The strand was spun directly into the strand collector shown in FIG. 3, hav ing a hinged top. The collector was square in cross-section, the dimensions at its upstream end being 3.5 cm. by 3.5 cm. It was 31.8 cm. long. The collector was attached snugly to the spinneret. The hinged top wall and bottom wall of the collector were each perforated along the first 15.0 cm. There were 140 holes, each 0.16 cm. in diameter. The holes in the bottom and top walls were polished to avoid snagging. The two side walls of the collector were not perforated.
After the spinning operation reached equilibrium the collecting operation was started by closing the hinged side of the strand collector. After the collector was filled with strand material a five-pound weight was attached to the downstream end of the gate.
A continuous rod of fibrous batt was extruded from the collector. The batt was rectangular in cross-section (2.5 cm. x 3.5 cm.).
The application of the five-pound weight to the gate caused the surface of the collected batt to be formed constantly at approximately 2.5 cm. from the spinneret.
The strand ends in the rods of fibrous batt were easily recovered and a soft bulky tow could be unwound from each end, the tow being essentially free of knots or slubs.
What is claimed is:
1. In a process for producing continuous strand material comprising a three-dimensional integral network of film-fibrils of crystalline oriented synthetic polymer, the film-fibrils thereof having an electron diffraction angle of less than 90 and an average film thickness of less than 4 microns, which process comprises flash spinning through an orifice, a composition consisting essentially of a solution of a polymer in a liquid, said polymer being selected from the group consisting of polypropylene, polyethylene and copolymers of ethylene and propylene and said liquid being a nonsolvent for the polymer below its normal boiling point and selected from the group consisting of aromatic, aliphatic, alicyclic and halogenated hydrocarbons and mixtures of such liquids, said composition being under pressure and at a temperature above the boiling point of the solvent prior to extrusion, and collecting the strand material so formed, the improvement comprising causing the strand material upon formation at said orifice to enter axially of an elongated passageway maintained at a pressure lower than that upstream of the extrusion orifice whereby the strand expands..into a network of increased diameter, and impinging the expanded strand at a generally right angle upon a yieldable collecting surface within said passageway and located at a distance between 0.3 and 2.0 inches from the orifice whereby the strand material is collapsed longitudinally upon itself in the form of a batt whose cr0ss-sectional area is governed by the size and shape of the walls which define the passageway, the cross-sectional area of said passageway being less than 4 times the crosssectional area of the expanded strand.
2. Process according to claim 1 wherein the crosssectional area of said passageway is 1 to 4 times that of the strand material.
3. Process according to claim 1 wherein the pressure in said passageway is maintained at a level between that of the spinning pressure and atmospheric pressure.
4. Process according to claim 3 wherein said passageway pressure is maintained at a level to cause the collapsed strand material to extrude continuously out of said passageway at a rate between and of the strand formation rate.
5. Process according to claim 1 wherein said solution comprises linear polyethylene poiymer dissolved in trichlorofiuoromethane.
6. Process according to claim 5 wherein the pressure in said passageway is maintained between 0.1 and 5 p.s.i.g.
References Cited UNITED STATES PATENTS 3,026,272 3/1962 Rubens et al. 264-53 3,081,519 3/1963 Blades et a1. 3,148,101 9/1964 Allman et al 156--167 DONALD J. ARNOLD, Primary Examiner H. MINTZ, Assistant Examiner US. Cl. X.Bl. 264-167, 205