US 2683073 A
Description (OCR text may contain errors)
July 6, 1954 R. L. PIERCE PROCESS FOR PREVENTING NYLON GEL 'FORMATIQN Filed Aug. 22, 1951 ATTORNEY R m w N W V m .m P L 1v o w a 2 0 f Z %,%r 9 3 m7 a .A m m L a .2 o 9 W 0 all. 1i 9 1 a 1 A: w 3 a W \6 la 1 I 3 Z o F[ l l o l 0 %M 9 4 o Z O o Patented July 6, 1954 PROCESS FOR PREVENTING NYLON GEL FORMATION Robert L. Pierce, Seaford, DeL, assignor to E. I. du Pont de Nemours and Company; Wilming ton, Del., a. corporation, of Delaware Application August 22, 1951,-Serial No. 243,163
This invention relates to the melt spinning of superpolyamides, and relates more particularly to a method for preventing nylon gel formation in molten superpolyamides just before extrusion to form filaments, fibers or films.
The basic method of making the fiber-forming polyamides and the fibers therefrom is described in detail in U. S. Patents 2,071,250, 2,071,253, 2,130,948, and 2,190,770 to W. H. Carothers. These fiber-forming polyamides are linear condensation products made by continued heating of biiunctional reactants under polymerizing conditions with removal of volatile material, until a product is obtained suitable for preparing filaments which can be cold drawn into useful fibers showing by X-ray examination orientation along the fiber axis. A valuable property of these superpolyamides is that they can be spun from melt by extruding the molten polymer through suitable orifices and cold drawing the filaments thus obtained. These fiber-forming polyamides are called superpolyamides. They may be made from a diamine and a dibasic acid or a' polymerizable aminoacid.
In the commercial process for producing filaments and fibers from superpolyamides, described by Greenewalt in U. S. Patent 2,217,743, the polymer is melted in a melting chamber and then flows down into a reservoir from which the pump meters a supply to the spinning orifices. In normal operation the reservoir may contain up to two to three hours inventory of the molten polymer, depending upon the speed of spinning and the denier of the yarn spun. It has been found that thermal degradation occurs in the reservoir, particularly at the stagnant polymer-metal boundaries such that there occurs on these metal surfaces a layer of gelled polymer. This gelled polymer where-it adheres to the interior wall builds up an effective insulating layer which adversely afi'ects the rate of heat transfer into the molten mass. When this occurs on the melting surfaces, it eventually results in reducing the rate of melting below that at which the molten polymer is removed from the reservoir. Such an occurrence makes for the production of considerable quantities of uneven and low denier yarn which must be cut to waste. Accordingly, when this happens, the melting and spinning assembly must be removed from service for cleaning.
The reactions in the thermal degradation of superpolyamides are not thoroughly understood. It is believed that the thermal degradation produces a decomposition product which serves to form cross links between amide groups on ad jacent polymer chains. The decomposition reaction proceeds slowly with time finally building up a three-dimensional network of molecules (which I will hereinafter call gelled polymer), and which eventually reaches the stage where it is both insoluble in common polyamide solvents and infusible.
A serious difficulty which arises from the formation of these gelled polymer layers on the interior walls is that from time to time pieces break on and get into the flowing polymer stream where they produce damage to the spinning equipment. These fragments of gelled polymer plug the entrance port to the metering pump and effectively bring the spinning operation to a halt for lack of polymer fiow to the pump. The gelled polymer sometimes enters the pump causing it to jam or, in other cases, causing it to become scored from a partial jamming action. If the fragments are small enough so that they pass through the pump, they collect on top of the filtering medium where they cause the back pressure of the filtering medium to build up to such an extent that the pump must labor against an unduly large head with resulting excessive wear to the parts. This increase in back pressure will also cause the molten polymer to slip back through the pump which results in smaller delivery of polymer per revolution of the pump and which in turn results in a smaller and oif standard spun denier. These high pressures may also deform the spinnerets or other pack parts and thus render them unfit for further use.
The greatest difficulty, however, is caused by gelled polymer which has progressed to the threedimensional structural state but which has not yet reached the stage of being infusible. This kind of gelled polymer exists in the, uppermost portion of the layers which are adhering to the interior walls, and is hence more readily washed into'the stream of flowing polymer. Being still molten or at least heat softened, it passes through the pump and even through the filter medium to show up either as physical discontinuities or as viscosity diiferences in the spun filaments. When these filaments are later cold drawn, these discontinuities and differences cause breaks in the filaments which either cause the whole thread to break or else go through to be counted as quality defects in the final yarn. Those discontinuities and differences which do not show up as broken filaments in the yarn can be the cause of lowered tenacity and decreased elongation when a working load is applied.
In the melt casting of film these gelled polymer discontinuities and differences show up as variations in sheet thickness, as variations in transparency, and as colored specks.
It is an object of this invention to avoid the above difiiculties by preventing gel formation in the reservoir of molten superpolyamide. Another objeet is to avoid accumulation of gelled polymer on the walls, inthe pump, or in the filtering medium. A further object is to improve the uniformity and quality of filaments, fibers and films formed from the molten polymer. Other objects will become apparent from the disclosure and the appended claims.
The difiiculties described above are obviated in accordance with the process of this invention by stirring the molten polymer in the reservoir. The stirring need not be rapid and should not be vigorous enough to incorporate bubbles of gas in the polymer melt. It is desirable to operate the stirring device in the range of 50 revolutions per minute. It is also preferable to have the blades of this agitator so shaped that the outer edges of the blades pass within a reasonably close distance inch) of all portions of the reservoir wall surfaces which contact the molten polymer. It is surprising that a stirrer operating at a slow rate of speed and producing no turbulence in the highly viscous molten polymer would prevent the formation of gelled polymer and its accumulation on wall surfaces or elsewhere.
In the drawing, which illustrates a preferred mode of carrying out the invention, the single figure is a vertical cross-section of a melt spinning apparatus.
With the exception of the means for stirring the reservoir of molten polymer, this apparatus is the same as that shown in U. S. Patent No. 2,217,743, issued to Crawford H. Greenewalt and assigned to the assignee of the present application. Corresponding parts of the apparatus shown in this application and in the patent will be referred to by the same reference characters and the patent may be consulted for a more detailed description.
The apparatus includes a hopper for holding a supply of solid flakes of polyamide a melting unit 32, a funnel-shaped container 34 forming a reservoir for molten polymer, a metering pump 36 adapted to supply polymer at the required rate and pressure for spinning, and a spinning pack 38 comprising the usual'filtering medium 39 and spinneret 60. Gas inlet and outlet pipes 10, 72 and T4 are provided at the middle, top and bottom, respectively, of the hopper 30 for maintaining an inert atmosphere above the melting unit. An opening 40 is provided for introducing the flake polymer into the top of the hopper.
The molten polymer collects as a pool I5 in container 34. In order to stir this reservoir of molten polymer in accordance with this invention, a stirrer is provided having a plurality of blades I! which are each suitably inch wide, shaped to the contour of the interior walls of the reservoir, and positioned so as to be within A; inch of these walls and within approximately inch of the bottom of the reservoir. The blades are attached to a hub l9 which is secured to a drive shaft 2|. The bottom of the shaft is supported by a bearing 23, which is held in place in the bottom outlet 25 of container 34 by a spider providing for unobstructed flow of poly- (approximately mer around the hearing from the container to metering pump 3 6.
The upper end of drive shaft 2| passes out of hopper opening 40 to an electric motor 21 supported above the hopper 30 by legs 29. The motor is provided with a built-in speed reducer, or other suitable means for driving the stirrer at slow speed may be used. The hopper opening 40 may be closed by a cover 42 to exclude air. The shaft 2| will then pass through a hole in the cover which may form an additional bearing. This bearing may be provided with a sturling box 3! of conventional design to prevent leakage of gas out of or air into the system.
The following examples are given to illustrate the improvement in the art of manufacturing polyamide structures which provides for supplying essentially gel-free polymer to the spinning system. The details set forth in the examples, however, are not to be considered as limitations of the invention.
EXAMPLE I The melt spinning apparatus described above was used in the production of superpolyamide filaments. The stirrer was rotated at 23 revolutions per minute. A superpolyamide made from hexamethylene diamine and adipic acid was charged into the melting chamber in flake form through a lock to keep air out of the melting chamber. The flake polymer contacting the melting grid at 290 C. melted and flowed down into the reservoir, where it was slowly stirred. The molten superpolyamide was then picked up by the metering pump, forced through the filtering medium and out through the spinneret orifices to become filaments which were subsequently quenched and wound up. This unit after twenty-seven days continuous operation was torn down, taken apart and examined. There was no accumulation of gelled polymer on the interior walls of the reservoir. A similar unit except that it was not equipped with the stirrer in the melt reservoir after operating the same length of time under the same conditions was similarly taken apart and examined. In this case there was an accumulation of gelled polymer on the interior walls of the reservoir which in places was ft; inch thick.
EXAMPLE II This example demonstrates that the same beneficial results were achieved when steam was used in place of the inert blanketing gas of Example I.
A melting and spinning assembly for the production of nylon yarn as described in connection with the drawing was adapted for steam spinning by jacketing hopper 30 and providing steam at 10-100 p. s. i. g. pressure in this jacket, by passing atmospheric steam in through pipe 14 and blanking off pipes 10 and 12, by leaving the cover 42 off the opening 40 in the hopper, and by allowing polymer flake to run in through this opening as fast as it was melted and spun out the bottom of the unit. The agitator shaft did not require a stufiing box to prevent leakage of the steam out of the hopper since the hopper itself was open to the atmosphere through opening 40. The superpolyamide, made from hexamethylene diamine and adipic acid, progressing downward through the steam atmosphere, was melted at the grid (290 C.) in the absence of air and flowed down into the reservoir Where it was slow- The stirrer was rotated at 23 R. P. M.
1.- IT on. i
1y stirred. The molten superpolyamide, now at equilibrium with one atmosphere of steam, was picked up by the metering pump, forced through the filtering medium and out through the spinneret orifices to become filaments which were subsequently quenched and wound up. This unit after 36 days continuous operation was torn down, taken apart and examined. There was no accumulation of gelled polymer on the interior walls of the reservoir. A similar unit except that it was not equipped with the stirrer in the melt reservoir after operating 28 days under the same conditions was similarly taken apart and examined. In this case there was an accumulation of gelled polymer on the interior walls of the reservoir which in places was T 5 of an inch thick.
EXAMPLE III This example typifiesthe increased operating efficiency which is achieved by the use of the stirrer in the melt reservoir.
Thirteen melting and spinning assemblies were equipped with stirrers in the melt reservoirs as" described in Example 11. The operating performance of these units over a twenty-day period was compared with similar units which were not equipped with stirrers in the melt reservoir. These stirred systems showed 0.17 low-luster occurrences per unit day, an improvement of 40% over the 0.29 low-luster occurrences per unit day for the ordinary systems.
Low luster is observable on the spun yarn as wound up. Low luster is undesirable since it causes variations in yarn properties and trouble in the drawing of the yarn. Consequently it is an off standard occurrence that must be eliminated immediately. Each low-luster occurrence required downgrading the yarn produced and replacing the whole melting and spinning assembly. The assemblie equipped with melt stirrers therefore possessed a considerable eco-- nomic advantage in that a more continuous operation was achieved with a resulting increase in total production and a decrease in secondgrade yarns.
Of additional importance is the fact that the yarn from these spinning assemblies containing stirrers in the melt reservoirs is of much improved quality. For example, in Table I, appended, where the yarn properties of first-grade yarn are compared with those for low-luster yarn, it is seen that the low-luster yarn produced from the systems without stirrers is of a much inferior quality. This low-luster yarn has more drawtwist breaks, more broken filaments, lower tenacity, lower elongation, a higher number of inspection rejects, and a different dye depth than does first-grade yarn.
Table I.Oomparison of first-grade yarn produced by stirred systems with low luster yarn produced by unstirred systems.
For reasons of simplicity the invention has been specifically described in terms of polyhexamethylene adipamide yarn since that is the common nylon yarn of the market. The invention, obviously, embraces other synthetic linear superpolyamide yarns as well as related synthetic filaments. Related synthetic linear polymer yarns such as, for example those derivable from polymerizable mono-amino-carboxylic acids or their amide-forming derivatives and those derived from the reaction of suitable diamines with suitable dicarboxylic acids or amide-forming derivatives of dibasic carboxylic acids are thus included within the purview of this invention.
As many different embodiments of the present invention may be made without departing from the spirit and scope thereof, it is to be understood that the invention is not limited to the specific embodiments disclosed except to the extent defined in the appended claims.
What is claimed is:
1. The process for melt extruding superpolyamides which comprises melting a superpolyamide, collecting up to 2 to 3 hours supply of molten polymer in a reservoir from which air is excluded, stirring the molten polymer to prevent gel formation at a slow rate of 10 to 50 revolutions per minute with a stirrer which passes within a close distance of all surfaces of the reservoir which contact the molten polymer, continuously flowing polymer from the reservoir and forcing the molten polymer through an orifice to form a shaped article.
2. The process of melt spinning superpolyamides which comprises melting a superpolyamide, fiowing the molten polymer into a supply reservoir, slowly stirring the molten polymer in the reservoir with a stirrer which passes within a close distance of all surfaces of the reservoir which contact molten polymer to prevent gel formation, continuously flowing the polymer from the reservoir to a metering pump, pumping the molten polymer through a filtering medium and out through spinneret orifices to form filaments, and cooling the filaments.
3. In the process of melt spinning superpolyamides, including melting a superpolyamide, collecting the molten polymer in a reservoir and pumping a continuous supply of molten polymer from the reservoir through filtering medium and out through spinneret orifices to form filaments, the improvement for preventing gelled polymer from depositing: (1) on the walls of the reservoir, (2) in the pump or its entrance ports, (3) on and in the filtering medium and, (4) at the spinneret, and for reducing forma-- tion of low-luster filaments, which comprises slowly stirring the molten polymer in the reservoir with a stirrer which closely approaches all surfaces of the walls of the reservoir which contact the molten polymer.
4. The process as defined in claim 3 in which the superpolyamide is polyhexamethylene adipamide.
References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,217,743 Greenewalt Oct. 15, 1940 2,295,942 Fields Sept. 15, 1942 2,303,340 Dreyfus Dec. 1, 1942 2,508,462 Marshall May 23, 1950