US3489831A - Melt extrusion of thermodegradable matter - Google Patents

Description

Jan. 13., 1.970

A. HOLMES ET AL MELT EXTRL JSION OF THERMODEGRADABLE MATTER Filed Dec. 1, 1967 ATTORNEY 2 Sheets-Sheet l ARTHUR HOLMES NAPOLEON LUCIEN mm: Joszvw MEDERIC GREMFR INVENTORS Jan. 13 1970 A. HOLMES ET AL 3,489,831

MELT EXTRUSION OF THERMODEGRADABLE MATTER Filed Dec. 1, 1967 2 Sheets-Sheet 2 INVENTORS ARTHUK HOLMES NAPOLEON LUCIEN LAIWE JOSEPH MEDERIC (SRENIER BYWM ATTORNEY United States Patent 3,489,831 MELT EXTRUSlON OF THERMODEGRADABLE MATTER Arthur Holmes, Napoleon Lucien Larue, and Joseph Mederic Grenier, Drummondville, Quebec, Canada, as-

signors to Chemcell Limited, Montreal, Quebec, Canada Filed Dec. 1, 1967, Ser. No. 687,291 Int. Cl. D01f 3/00 U.S. 264176 4 Claims ABSTRACT OF THE DISCLOSURE This invention relates to the production of cellulose triacetate fiber and other continuous lengths of easily thermodegradable material and particularly to a method and apparatus for the production of filamentary products from fusible material comprising cellulose triacetate.

Conventional extrusion machines are not suitable for melt spinning unplasticized cellulose triacetate since this polymer degrades and discolours if it is subjected to a temperature higher than its melting point for any appreciable length of time.

Canadian Patent 610,964 dated Dec. 20, 1960, Palmer and Larue and C.P. 647,851 dated Sept. 4, 1962, Killoran and Larue, recognized the desirability of melt spinning cellulose triacetate filaments without the use of solvents. These patents suggest an ingenious apparatus that overcomes degradation of cellulose triacetate powder. The art does not however provide means of obtaining a completely satisfactory uniform fiber product. For example, by using the apparatus described in the art, the extrusion process is very difficult to maintain, particularly at the stage when the melted polymer is reduced to fiber form. The crosssection diameter of the fiber product varies considerably. Another disadvantage is the frequent interruption in the continuity of the process of filament extrusion. Nonuniformity of the filament and even discontinuity in the extrusion results in non-uniformity of the end product and increased operating costs of the extrusion process. For example, in dyeing non-uniform filaments, one may produce a yarn possessing irregular shade and irregular physical propertiesyie. tensile strength. It is, therefore, commerciallyadvantageous to obtain a reasonably regular cross-section of the extruded fiber and continuity of the fiber forming process.

In the past, numerous mechanical modifications, e.g. longer capillaries, larger or smaller orifices, etc. and chemical modifications, e.g. addition of various additives failed to improve the situation.

Having regard to the foregoing, the present invention provides an improvement as to the production of cellulose triacetate fiber from fusible cellulose triacetate powder.

Unexpectedly, it has now been discovered that selecting the particle size of the cellulose triacetate powder greatly improves the homogeneity of the melt, or in other words, it has been discovered that a relationship between the state of division of the acetate powder and its melting characteristics when using a Palmer-Lame Extruder or a modification thereof; due to the small size of the melt pool, finer powders give a more homogeneous melt which improves the forming of fibers.

More particularly, it has been found that in the melt extrusion easily beat degradable material to produce filaments, and the improvement which comprises using the heatdegradable material in the form of powder of which not more than about 5% is retained on a 60 mesh screen. A compression chamber exerting positive pumping action for continuously feeding in the powder state at a definite and constant rate highly heat degradable material to a melt extruder spinneret, said chamber comprising a feeding screw rotating in a tightly fitted sleeve mounted within said chamber, said screw having polished flutes, a flight angle of approximately 25 to 35, a pitch narrow enough to produce a positive pumping action, a fiight thickness sufficient to sustain high pressure, said sleeve having a portion extending beyond the space occupied by the end of the screw so as to form a relaxation room, said room terminating on the melt extruder spinneret, the interior of said sleeve having longitudinal outer grooves with respect to the axis of said sleeve.

According to the present invention, the particle size of the cellulose triacetate powder is critically controlled and is then fed through an improved melt extruder apparatus. More particularly, the cellulose triacetate is generally pulverized so as to obtain a powder of which less than 5% is retained on a 60 mesh screen. In the preferred form of the invention, the cellulose triacetate powder employed should retain 0% on a screen of 100 mesh.

Another advantage of the invention is the reduction in the breakage of filament at extrusion.

Another advantage is the improvement of the continuity of the process of forming fibers from the molten polymer.

Still another advantage is the production of a fiber possessing a more uniform cross-section as well as better physical properties.

A mesh is well known in the art and is defined as the number of openings per linear inch. The following table will show the relationship between the mesh number and the size of the openings.

Mesh No: Size of opening (inch) 50 0.0116 60 0.0097 0.0069 0.0058 200 0.0029 400 0.0015

Sieve Analysis measurement is well known and has previously been described, for instance. in ASTM-C136 39. Although the Sieve Analysis measurement is generally performed for control purposes only; the cellulose triacetate material could be ground through a regular grinder apparatus and the powder resulting therefrom classified by passing it though a sieve of proper dimension.

However, cellulose acetate is generally obtained in spaghetti form A lon cylinders of diameter which are first ground to a broad size distribution. The reduction of the acetate resin to fine size is preferably performed by impact: metal parts in the grinding equipment are made to rotate at high. speed and thus particles of resin are fractured when hit by those parts. The crude product is then separated in coarse and fine particles by air classi fication. The fine fraction is then used for the melt extrusion process while the coarse fraction is recycled to the grinder by further reduction.

The air classification method used to separate fine powders according to their particle size is more practical thanscreen separation. It is based on the principle that powder particles entrained in an air stream along a curvy path will distribute according to their mass. The Pulvacron of Strong-Scott Mfg. and the Air Swept Pulverizer of Schultz-ONeil Co. are examples of Grinder-Classifier that could be used in accordance with this invention.

Other types of grinding equipment with crushing or shearing action as well as other means of particles classi fication may be used though less preferred.

Since cellulose triacetate fine powder is highly explosive, when mixed with air, it is manipulated with reasonable care. To reduce the danger of explosions, the fine powder is ground under inert gas blanket.

The resulting powder is usually fed through a modified Palmer and Larue extruder as described in Canadian Patent 610,964- dated Dec. 20, 1960 or any equivalent apparatus used for the melt extrusion of cellulose triacetate but possessing substantially a modified compression chamber that is a modified screw and sleeve in order to obtain a more efficient positive pumping action with the finely divided cellulose triacetate powder.

In the drawings, which illustrate the embodiments of the invention;

FIGURE 1 is an elevation, partly in section, of the modified portion of the Killoran-Larue Extruder on a smaller scale;

FIGURE 2 is an elevation, partly in section, of an adapter for a new sleeve;

FIGURE 3 is an elevation, partly in section, of the extrusion sleeve;

FIGURE 4 is a cross section along line 44 of FIG- URE 3; and

FIGURE 5 is a side elevation of the special extruding V screw.

The modified portion of the extruder comprises a double flight extruder screw 1 having an increased number of flights 2 and 3, the pitch, e.g. the distance between the two flights 2 and 3, is narrow enough to produce a positive pumping action with the finely divided cellulose acetate. The helix angle 4, i.e the angle between a flight and a perpendicular to the stem 5 may vary from about 25 to 35, advantageously from about 27 to 33, but preferably In the preferred embodiment of the invention, the screw portion is long enough to originate in the hopper 6 and to terminate at substantially V2" from the top of the extrusion jet plate 7. The length of the stem 5 is such that it will properly connect to a shaft which is driven by a motor. The diameter of the screw is such as to tightly fit the inner wall 9 of a sleeve to produce efi'icient compression; said sleeve being preferably slightly conically shaped in the bottom part, i.e. having an internal diameter at A slightly smaller than the bottom diameter 12 and containing vertical grooves 10 to inhibit rotary motion of the mass of powder. Clearance in one preferred embodiment of the invention is in the order of 0.006" to produce an elfective compression room. The upper portion of the external wall of the sleeve is threaded at 13 to fit an adapter 14 which is attached to a hopper 6 of a Palmer Extruder by connecting means 15. The

electrically heated extrusion jet plate 7 is secured to the extrusion sleeve 8 by supporting collars 16 and flange 17.

The construction of the screw is critical. It must be constructed so as to deliver, under positive pressure, a constant flow of a definite debit suflicient to overcome substantial polymer degradation or decomposition which may result if residence time is too long.

' The inside wall of the sleeve should also contain longitudinal grooves, usually longitudinal outer grooves with respect to the axis of said sleeve, to prevent rotation of the compressed powder at the expense of the pump efficiency and the flutes are polished.

By substantial decomposition or degradation is meant that viscosity of the material may be reduced but that the properties of the material are still acceptable for the manufacturing of a filamentary product. In accordance with this invention, it is also preferred to have a thrust on the metering screw of about 800 lbs. to about 1000 lbs. per square inch and the preferred rotation speed of the screw is 4.5 rpm.

The following examples will serve to illustrate the invention. i

EXAMPLE 1 A pressure stabilized cellulose acetate of 61% acetyl value was pulverized in an impact mill. The powdered material was fine enough so that the totality of it would pass a 48-mesh screen, 5% would be retained on a 60- mesh screen and would be retained on a ZOO-mesh screen.

The powder was dried at a temperature of C. under reduced pressure and supplied to the apparatus described above, the jet being maintained at a temperature of 350 C., and the screw being rotated at 4.5 rpm. The filaments emerging from the orifices were drawn away at 225 meters per minute, were of an average filament denier of 15 and a total denier of 480. The resulting yarn had a tenacity of the order of 1.2 grams per denier and an elongation at break of 25% to 35%.

The invention has been described with particular reference to the treatment of pure cellulose triacetate powder. In a similar way materials of this kind, but having included with cellulose triacetate, carriers and pigments e.g., metal oxides such as titanium and iron oxides, metalorganic chelates such as copper phthalocyanines and, other additional compounds having plasticizing action can be treated.

EXAMPLE 2 Cellulose triacetate was pulverized as in Example 1 and then mixed with 0.1% of Chromophtal Green GF. The powder and the pigment were tumbled for a period of 60 minutes to achieve a reasonable degree of pigment dispersion.

The pigmented powder was supplied to the extrusion apparatus, the jet being maintained at a temperature of 350 C. and the screw being rotated at 4.5 r.p.m. The coloured filaments emerging from the orifices were drawn away at a take-up speed of 225 meters per minute, were of an average filament denier of 15 and a total denier of 480. The resulting yarn had a tenacity of the order of 1.2 grams per denier and an elongation at break of 25 to 35%.

Other material easily degradable by heat, more particularly those highly unstable at temperatures above their melting point could be similarly extruded for instance, polyvinyl alcohol though less preferred.

EXAMPLES 3-8 The following examples demonstrate the effect of powder particle size on extrusion stability. The powder particles being used are of cellulose triacetate and were processed as in Example I but varying the particle size. The following results were obtained:

Powder particle Size of fibre Extrusion stability size (0% retained (denier per (pounds extruded Example on mesh N o.) filament) per filament break) EXAMPLE 9 1.5 gms. of molten material 12 gms. per min- 60 see. per mm.=7.5 secs.

This is a rough estimate and may represent a maximum limit since the molten layer probably grows thicker when the machine is stopped on account of the heat accumulated in the system. The expression residence time as found in the specification designates the residence time as defined in the above example and is relative. One skilled in the art would certainly visualize and understand the difiiculties of obtaining absolute value.

The embodiment of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In a process for the melt extrusion of cellulose triacetate to produce filaments, the improvement which comprises using for extrusion heat degradable material in the form of powder of which not more than about 5% is retained on a 60 mesh screen.

2. In a process for the production of filaments of cellulose triacetate by melt extrusion, the improvement of which comprises crushing the cellulose acetate particles so that on Sieve Analysis, of the powder is retained on a screen of at least 60 mesh, and feeding said powder to a melt extruder at such a velocity that the residence time of said particles within said melt extruder is less than 7.5 seconds.

3. The process according to claim 2, wherein on Sieve 6 Analysis, 0% of the powder is retained on a mesh screen.

4. The process according to claim 2, wherein at least one member selected from pigments, carriers and addi tion compounds having plasticizing action is extruded with said cellulose triacetate powder.

References Cited UNITED STATES PATENTS Killodran et a1.

OTHER REFERENCES Characteristics of Fine Particles, Chemical Engineering, June 11, 1962, p. 207, 264-176F.

DONALD J. ARNOLD, Primary Examiner J. H. WOO, Assistant Examiner US. Cl. X.R. 264211

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