US3443899A - Process for making graphitic-type fibers - Google Patents

Description

y 1969 o. E. ACCOUNTIUS 3,443,899-

PROCESS FOR MAKING GRAPHITlC-TYPE. FIBERS Filed July 22, 1966 INVENTOR. .OA /1// E ACCOUA/Wl/S ,mg w, w m;

,4 TTOPNEY US. Cl. 23209.1 4 Claims ABSTRACT OF THE DISCLOSURE A process for making graphitic-like fibers by extruding parapolyphenylene into a fibrous shape and heating said para-polyphenylene fibers for a sufiicient time at elevated temperatures to cause pyrolysis thereof.

This invention relates to fibers. More particularly, the invention relates to graphitic-like fibers and their method of manufacture.

Graphite or carbon fibers are of particular interest in todays technology. The fibers are of interest particularly as high-strength reinforcing elements in materials to be used in high temperature environments. The present technology for making carbonaceous fibers involves the pyrolysis of polyester threads. One of the fibers most often chosen is rayon. The rayon thread which has been previously formed in subjected to graphitization temperatures in the order of 2500-2600 C. Other fiber materials, such as Orlon, have been subjected to pyrolysis to produce carbonaceous fiber. In addition to the requirement of the extremely high temperatures for pyrolysis to obtain graphitization, the diameter of the fibers can only be controlled through the weaving of the material used. Each length of synthetic fiber is made up of a multitude of single strands intertwined to form the material. Addition ally, the materials utilized, generally, upon pyrolysis, give off gas which produces discontinuities in the finished material.

It is an object of this invention to provide an easier method for the manufacture of graphitic-type fibers.

Another object of this invention is to provide graphitictype fibers having aligned crystallinity.

Still another object of this invention is to provide graphitic-type fibers having superior tensile strength, elastic modulus, and electrical and thermal conductivity in an axial direction.

The above and other objects of the invention will be more apparent from the following description and drawings in which:

FIG. 1 is an exploded view of a typical die for extruding the fibers of the invention.

FIG. 2 is a cross-sectional view of the punch through which the polyphenylene is extruded.

Briefly, this invention relates to the extrusion and subsequent pyrolysis of para-polyphenylene into carbonaceous fibers having aligned crystallinity. In copending application Ser. No. 492,315, filed Oct. 1, 1965, there is disclosed a method of making a graphitic-like material due to the compaction of para-polyphenylene with the subsequent pyrolysis of the compacted mass. The present invention extends the concept of the copending application to the novel area of producing fibers. It was found that through the extrusion of the para-polyphenylene in a die the para-polyphenylene polymers align axially in the produced fiber that is extruded. Upon pyrolysis the residual structure has a graphitic, crystalline structure with an ab axis of the crystallites parallel to the fiber axis.

The alignment of the crystallites together with the anisotropy in the chemical and physical properties of the graphite crystal is such that carbon fibers and filaments can be prepared with superior tensile strength, elastic ted States atent O modulus, and electrical and thermal conductivity in the axial direction for the first time.

The para-polyphenylene, in accord with this invention, is extruded into thin, continuous fibers. The fibers are subsequently pyrolyzed to form the continuous carbon or graphitic-like filament. Extrusion may be accomplished by compressing the powder polyphenylene in a two-punch steel die. One punch in the die is pierced by a small hole. Pressures of from 10,000 to 100,000 p.s.i. within the die cavity can be used to extrude the powder material through the hole. Extrusion at room temperature is possible. However, the application of heat accelerates the extrusion process. Temperatures from to 550 F. can be successfully used to apply heat to the die. Generally, it is preferred to extrude at temperatures from 485515 F.

The extruded fibers will vary in diameter according to the size of the extrusion hole. For example, fibers M inch in diameter and 3 mils in diameter have been extruded in lengths up to three feet. The fibers are straight and have a smooth surface. Additionally, the fibers are anisotropic, that is, the para-polyphenylene molecules are axially aligned in the fibers. After extrusion to form the fibers, they are then pyrolyzed in an inert atmosphere at temperatures from 8001400 C. and above, if desired. Pyrolysis times as short as ten minutes have given satisfactory results.

The tensile strength measurements on the fibers made indicate a minimum strength of 20,000 p.s.i. The elastic modulus is above 50 10 p.s.i. It is believed that the invention will be better understood from the following detailed description and specific examples:

EXAMPLE I The extrusion die, as shown in FIGS. 1 and 2, used to form the filaments from the crude polyphenylene in these examples was comprised of a tool steel cylinder 11 2 /2 inches in diameter by 2 inches long, having a /2 inch axially aligned hole 13 therethrough to accommodate the polyphenylene. At the bottom of the die was an extrusion punch 15, as particularly seen in FIG. 2. The punch was provided with a 1 /2 inch diameter shoulder 17 /2 inch thick which thus overlapped the die body /2 inch. The part 19 of the punch extended into the die body /2 inch. This extrusion punch was drilled with the desired orifice 21, either inch or for some examples 3 mils. The outlet end was chamfered 23 at a 30 angle to the axis for a depth of 5 inch. A heater 25 constructed of a Chromel resistance wire encased in an Inductoseal case surrounded the punch body. Leads 29 were connected to an electrical source. A small hole 27 capable of accepting a thermocouple was drilled into the top of the die cavity halfway from the outer edge to measure the temperature of the die. Using the heater, the effective temperature on the extrusion behavior in the polyphenylene was determined. Extrusions were attempted at ascending high temperatures. In order to accomplish the above, the die was loaded with polyphenylene and mounted in a hydraulic press. Approximately 50,000 p.s.i. pressure was applied while slowly increasing the die temperature. There was no unusual observable phenomenon until the die temperature was at approximately 250 F., at which point the extrusion seemed more facile. However, when the temperature was brought to approximately 500 F.i10 extrusion became much easier. It was possible to then extrude M inch filaments in 3 or 4 inch lengths.

EXAMPLE II The punch described in Example I was used, however, with the extrusion hole having a 3 mil diameter. At approximately 500 F. and 50,000 p.s.i., it was possible to extrude 3 mil fibers in lengths of about 30 inches. The extrusion proceeded at the rate of a few inches per minute, but was continuous. It was found that the extrusion could be stopped and restarted even if the die was allowed to cool to room temperature, provided that the approximate conditions of 50,000 p.s.i. and 500 F. were attained. It is to be noted that while extrusions occur easily over a wide range of temperature, 475520 F., the temperatures are that of the die body and not the polyphenylene used. However, it would appear that the polyphenylene was brought to equilibrium with the die upon residence for a period of time. This occurred by bringing the die to temperature rather slowly, that is at a few degrees per minute. The appearance of the extruded fibers was very good. The fibers were hard, smooth, uniform in diameter, and relatively straight. The green 3 mil fibers were quite fragile but could be handled with care. Attempts to pick the fibers up even in short lengths with metal tweezers usually caused breaking of the fibers indicating a definite absence of any plastic deformation in the extruded fibers.

EXAMPLE III The procedure of Example II was used, except that a 5 weight percent of finely powdered graphite was admitted to the polyphenylene before the polyphenylene was added to the die. The graphite was prepared with a particle size less than 50 microns so that it would pass through the 3 mil extrusion orifice. The added graphite provided lubrication and made the extrusion more facile.

EXAMPLE IV The fibers prepared in Example II above pyrolyzed readily at 1000 C. in an electrical resistance furnace and in times as short as ten minutes. This clearly indicates that it would be possible to have a continuous process of extrusion and pyrolysis so that the green fibers need not betransported or handled separate from the die to a furnace.

Although the invention has been described and illustrated in detail, it is to be clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of this invention being limited only by the terms of the appended claims.

I claim: 1. The method of makingcarbon fibers with a graphitic crystalline structure comprising:

extruding powdered para-polyphenylene into continuous fibers through a die having an opening corresponding to the desired circumference of said fibers, and heating in an inert atmosphere said extruded para-polyphenylene fibers to a temperature at which pyrolysis occurs, and for a sufiicient time for pyrolysis to produce the desired fiber. 2. The method of claim 1 additionally comprising: heating said extrusion die to temperatures of 100 to 550 F. 3. The method of claim 2 wherein said die is heated between 485 and 515 F.

4. The method of claim 1 wherein the extrusion pressures are from 10,000 to 100,000 p.s.i.

References Cited 1963, pages 454-458.

EDWARD J. MEROS, Primary Examiner.

U.S. Cl. X.R. 23-2094

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