US2403476A - Extrusion apparatus - Google Patents

Description

July 9, 1946. *K[ BERRY ET AL EXTRUSION APPARATUS F iled Aug. 8, 1944 ATT 51 K913125112 1 5 INVENTORS I Joseph R-D0W12i1gg Patented July 9, 1946 UNITED STATES PATENT OFFICE Ex'rnUsIoN APPARATUS Kenneth L. Berry, Hockessin, and Joseph R. Downing, Wilmington, Del., assignors to E. I. du Pont de Nemours & Company, Wilmington, Del., a corporation of Delaware Application August 8, 1944, Serial No. 548,536

7 Claims.

cially designed extrusion die known as a spinneret,

the procedure in the latter case usually being referred to as "melt-spinning. The .present invention is also concerned with extrusion dies or spinnerets used in the solution spinning of fibers.

Diillculty has been experienced heretofore in the extrusion of thermosplastic resins because of adhesion of the hot resins to the extrusion die and, occasionally, because of die corrosion from small amounts of heat decomposition products from the resins. Occurrence of either of these results in operational difllculties and the production of products having a rough surface. Similar difliculties have been encountered inthe meltspinning of fibers where the sticking of resins in or around the spinning orifice prevents the easy initiation of spinning, or, during spinning, interrupts the operation. In the solution spinning of fibers, particularly into coagulating baths as in the case of viscose and proteins, formation of adherent precipitates in the spinning orifice and corrosion of spinnerets by coagulating baths have been major difllculties.

Attempts to solve the problems mentioned above have usually been in the direction of providing special alloys for die and spinneret construction. Corrosion problems have been lessened but the problem of fouling by sticking has not been alleviated and materials and fabrication costs have usually been increased. Further efforts to achieve an optimum balance of characteristics have involved the use of glass or the coating of metal extrusion dies and spinnerets with organic materials. These devices eliminated to some extent the use of noble metals but have not been generally applicable because of limitations imposed by solubility, form stability, adhesion, and/or heat resistance of the structural materials.

An object of the present invention is to provide extrusion dies, both of conventional design and of the spinneret type, for general use which are economically and readily manufactured by molding, stamping, machining, or punching; to which hot resins will not adhere; which are unaffected chemically or physically by any known resins, spinning dope, or coagulating medium; and which are useable over a wider temperature range than is commonly encountered in the commercial fabrication of organic plastic materials by extrusion through orifices. Other objects will be apparent from the description of the invention given hereinafter.

The above objects are accomplished according to the present invention by the use of an extrusion apparatus comprising a die having a surface consisting essentially of a tetrafiuoroethylene polymer.

The present invention resides partly in the discovery that tetrafluoro'ethylene polymer is peculiarly and unexpectedly adapted as the surface material of extrusion dies in that thermoplastic resins, melt-spinnable poLvmers, fiber-forming solutions, and the likedo not adhere to surfaces of this material nor do they corrode such surfaces. Further, tetrafluoroethylene polymer is sufilciently form-stable and heat resistant to be well suited for the purpose. The dies may be .made of metal as has beenccnventional heretofore, and coated with a tetrafluoroethylene polymer or the die may be made entirely of the polymer. When a metal die is coated, it is only necessary to coat those surfaces which will come in contact with the material being extruded and, in

the case of spinnerets used in melt-spinning processes, it is suflicient if merely the external surface of the spinneret is coated.

Referring to the drawing forming a part of the present specification and illustrating preferred embodiments of the present invention:

Fig. 1 is a sectional view of the delivery end of a more or less conventionally designed extrusion apparatus in which certain surfaces of the extrusion die are coated with tetrafluoroethylene polymer;

Fig. 2 is a similar section of a slightly different extrusion apparatus in which the extrusion die is made wholly of tetrafiuoroethylene polymer; Fig. 3 is a section of the delivery end of a meltspinning extrusion apparatus in which one surface of the spinneret is coated with tetrafluoroethylene polymer; and

Fig. 4 is a view similar to Fig. 3 in which the spinneret is made. wholly of tetrafluoroethylene polymer.

Referring to Fig. 1, the reference numeral 1 indicates generally the delivery end of a more or less conventional extrusion apparatus provided with the chamber 2 for a heat transfer liquid whereby the plastic 3 may be'maintained in a 3 highly plastic or fluid condition .up to the extrusion die. The extrusion die 4, usually made of steel, is provided with a coating 5 of tetrafluoroethylene polymer over those surfaces which may be exposed to the plastic to be extruded therethrough.

The method of applying the coating 5 to the extrusion die 4 may vary considerably although it is preferred to dip the extrusion die in the molten polymer. The thickness of the coating is not critical inasmuch as merely rubbing thoroughlyv the metal surface to be coated is quite eil'ective, the coating resulting from this treatment clearly being only microscopic in. thickness and not necessarily even in the form of a continuous film. Generally, a coating of appreciable thickness will be employed since it is more durable.

Extrusion die 4 is provided with the chamber 6 which could be used for a heating liquid but, preferably, is used for a cooling liquid. If it were attempted to cool and set or harden a thermoplastic resin in a conventional metal extrusion die, the plastic would immediately "freeze" in the die and necessitate shutting down of the extrusion operation to clean out the obstruction in the die. However, in a die having its surfaces coated with tetrafluoroethylene polymer, cooling liquid may be run into the chamber 6 and the plastic'being extruded may thus be set and hardened without any freezing in the extrusion die. That is, the plastic emerges from the extrusion die without any difficulty even though it is set or hardened within the die. This has not been possible with any extrusion dies heretofore known.

In Fig. 2, the delivery end I of the extrusion apparatus is as shown in Fig. 1 but the extrusion orifice 1 in this case is a solid block of tetrafluoroethylene polymer machined to proper size and held in place by the bolts 8 and the annular ring 9.

Fig. 3 shows the delivery end III of a conventional melt-spinning extrusion apparatus having a metal spinneret I I which is provided with the coating I! On the delivery side of the spinneret. In this modification the molten polymer l3 does contact the metal surface of the spinneret but it has been found that, if the external face of the spinneret is coated with the tetrafluoroethylene polymer, this is sufficient to stop effectively the accumulation of polymer on the external surface and eliminate the frequent interruptions heretofore encountered in melt-spinning operations due to this cause.

Fig. 4 shows the delivery end ill of an extrusion apparatus similar to that shown in Fig. 3 except that the spinneret i6 is made from a solid block of tetrafluoroethylene polymer machined to the required dimensions.

The invention is further illustrated by the following examples:

v Example .I

Polyethylene is heated andextruded through aconventional extrusion die. Frequent adhesion of the hot resin to the die surface imparts a rough surface to the extruded object. The conventional die is then replaced with a die lined with polytetrafluoroethylene having the same finished dimensions. An identical polyethylene lot is extruded under the same conditions as formerly. There is no adhesion of the polyethylene to the die and the extruded shape is characterized by an exceptionally smooth surface.

4 Example II A block of polytetrafluoroethylene is machined to the-same dimensions as the conventional extrusion die of Example I and is used in a similar polyethylene extrusion. There is no adhesion of the polyethylene to the die and as a result the extruded object has a smooth surface.

Example III The extrusion die of Example II is employed in the extrusion of a cellulose acetate composition. The operation is characterized by a complete lack of adhesion.

Example IV stopping the pumping, spinning is resumed immediately on discontinuing the interference. This spinneret is cleaned by immersion in concentrated nitric acid without affecting the polytetrailuoroethylene coating or its bond to the metal. It was discovered also that the polymer to metal bond is not impaired by the molten nylon or the high temperature encountered.

As a control, the same type of nylon is spun under similar conditions through a conventional stainless steel spinneret. At frequent intervals, the spinning, molten filament touches the external face of the spinneret and the melt accumulates and drips, This accumulation is wiped away mechanically but a residue is left which aids the repetition of the drip. After several such interruptions of spinning continuity it is necessary to shut down the operation toclean the spinneret by chemical means.

Example V The external face of a stainless steel nylon spinneret is rubbed vigorously with a piece of,

Example VI A diameter disc of polytetrafiuoroethylene, thick and containing holes punched therein is employed for the spinning of viscose into an acid coagulating bath. Viscose does not adhere to the spinneret nor is there any corrosion of the spinneret by the viscose or the coagulating bath.

Example VII A platinum alloy spinneret is coated with a tetrafluoroethylene/ethylene interpolymer containing 79% tetrafluoroethylene by dipping the spinneret in a 20% dispersion of microscopic particles of the polymer in diisobutyl adipate, baking the spinneret at 360 C. for four minutes, and quenching in water. The coating and its bond t ethylene polymer.

the metal is unaffected .by exposure to hot concentrated aqueous alkali or acid.

As indicated previously, dies and spinnerets of this invention can be constructed wholly or partially of a tetrafluoroethylene polymer with the relative amounts of construction materials varying widely. The only important condition is that certain surfaces, as described, be tetrafluoroethylene polymer. Where coatings of this material are employed, they may vary widely in thickness from the microscopic amount applied when the metal surfaces are rubbed with polymer, to coatings of considerable thickness applied by melt. The effects achieved by merely rubbing the metal surface with the polymer are surprising in view of the extremely minute amount of material deposited and the fact that the deposit is not necessarily in the form of a continuous film The tetrafluoroethylene polymer may range in grade from low molecular weight, hard, waxy, high melting materials to the highest molecular weight, film-forming polymer. The preparation and properties of suitable polymers are disclosed in Plunkett U. S. Patent 2,230,654.

Although pure tetrafluoroethylene polymers are preferred in the practice of this invention, material modified as it is for other applications can be used. The polymers can be mixed with other substances in various amounts. are finely divided non-metallic elements such as carbon; inorganic compounds such as silica, fluorspar, mica, and fuller's earth; mineral fibers such as glass and asbestos; and metal flakes, granules, and strands. Resistance heating wires can be embedded directly in the tetrafluoro- The polymer may be pure polytetrafiuoroethylene or a copolymer prepared by copolymerization of tetrafluoroethylene with small amounts of another polymerizable organic compound, such as ethylene.

An advantage of the present invention is that it provides a novel extrusion die which is relatively simple and economical to make and yet it overcome the constant difliculty heretofore en- Suitable fillers countered due to adherence of the materia passing through the extrusion die to the die. A further advantage is that the tetrafluoroethylene polymer surface of the extrusion dies of this invention are not corroded by heat decomposition products of the resins being extruded and, furthermore, they possess to a satisfactory degree the properties of form-stability, adhesion to metals, and heat resistance. A particular advantage of the invention is that it provides an improved spinneret through which resins, spinning dope, and the like may be forced without difliculty in the initiation of the spinning or interruption in the spinning operation due to accumulations of the resin or the like on the external surface of the spinneret.

As-many apparently widely different embodiments of this 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 thereof except as defined in the appended claims.

We claim:

1. An extrusion die having a surface to be contacted by the material to be extruded, consisting essentially of a tetrafluoroethylene polymer.

2. An extrusion die having all the surfaces to be contacted .by the material to be extruded, consisting essentially of a tetrafluoroethylene polymer.

3. An extrusion die consisting essentially of a tetrafluoroethylene polymer.

4. A metal extrusion die having all the surfaces to be contacted by the material to be extruded, coated with a tetrafluoroethylene polymer.

5. A spinneret whose external face consists essentially of a tetrafluoroethylene polymer.

6. A metal spinneret whose external face is coated with a tetrafluoroethylene polymer.

7. A spinneret consisting essentially of a tetrafluoroethylene polymer.

KENNETH L. BERRY. JOSEPH R. DOWNING.

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