Process for preparing extrudable polyimide granules

A process for the preparation of extrudable polyimide granules comprising heating and compacting a powder containing polyimide particles to form a partially densified porous mass and crushing the mass to form granules which may be ram extruded to give uniform articles of high strength.

BACKGROUND OF THE INVENTION 
(1) Field for the Invention 
The present invention relates to a method of making ram extrudable granules 
comprising aromatic heterocyclic polymer particles, such as polyimide 
particles. 
(2) Description of the Prior Art 
Fabrication of useful articles from aromatic heterocyclic polymers, such as 
polyimides, by continuous extrusion processes has proven to be difficult 
to achieve. In fact, aromatic heterocyclic polymers generally are 
intractable and are difficult to fabricate by any continuous process. 
Articles which can be fabricated from aromatic heterocyclic polymers by 
continuous processing normally are lower in density and have properties 
much inferior to those articles that can be obtained by compression 
molding the polymer. 
Compression molding is usually the only method which can be used to form 
thick articles of the intractable aromatic heterocyclic polymers. It would 
be highly desirable to form such polymers into usable articles of high 
density by a continuous processing technique, such as the ram extrusion 
method which is frequently used to continuously process 
polytetrafluoroethylene. In this ram extrusion method, an article of high 
density is formed by forcing particles of a powder through a heated tube 
having a nonvarying cross section by successive strokes of a reciprocating 
ram. If this extrusion method is used to form articles from aromatic 
heterocyclic polymers, the rates for forming the articles are very slow 
and the properties of the articles formed are much inferior to those 
obtained on compression molding the polymers. For example, when a 10 mm 
diameter rod is extruded from a thermoplastic polyimide polymer, the 
surface of the article tends to be rough and discontinuous. The material 
contains large voids and significant color variations, and the flexural 
strength is undesirably low. In addition, the powder is difficult to feed 
because of the low apparent density and the need to vent or exclude a 
large quantity of air, and volatiles given off on heating and compacting 
the polymer. If the extrusion rate is increased, the noted problems 
generally are increased, while even at extremely low extrusion rates, the 
problems are not eliminated and the resulting rod is porous and is low in 
flexural strength. 
SUMMARY OF THE INVENTION 
The present invention provides a method of making extrudable granules from 
aromatic heterocyclic polymers, such as polyimides, which granules can be 
readily continuously ram extruded to give a consolidated article with 
improved properties or properties equivalent to compression molded 
articles of the same polymer. The polymers used in the method of this 
invention include aromatic heterocyclic polymers having, as a recurring 
unit, heterocyclic rings containing at least one nitrogen atom. It is 
particularly preferred that polyimide polymers be used in the method of 
the present invention. 
The polymers which can advantageously be used in the method of the present 
invention are characterized as being basically intractable but which flow 
to at least a slight extent under pressure and at temperatures above the 
polymer's melting point or glass transition temperature. The method 
comprises heating and compacting a powder containing aromatic heterocyclic 
polymer particles to form a partially densified mass and dividing the mass 
to form granules of polymer. During the heating and compacting step of the 
method, the polymer particles are compacted to provide a porous mass 
having an increased bulk density. 
During the heating and compacting stage of the process, the polymer is 
heated to a temperature at which the polymer will soften. In the case of 
thermoplastic polyimides, the temperature of the polymer is raised from 
room temperature to between about 5.degree. C. to about 100.degree. C. 
above the polymer's glass transition temperature. The compacting is done 
under low to moderate pressure, that is, pressures between 5 and about 
1,000 lb/sq. in. The simultaneous heating and compacting can be achieved 
by a number of different means, e.g. pressing of a layer of the polymer 
powder between the heated platens of a hydraulic press, clamping under 
mild pressure in a heated mold, ram extrusion under low back pressure, or 
passing the powder through the nip of a heated two roll compounding mill. 
The mass provided by heating and compacting the polymer is a porous mass 
that can be easily divided by any conventional means such as grinding or 
crushing. The granules provided may be continuously ram extruded at 
relatively high rates with the resulting articles having properties, for 
example, flexural strength and modulus and density, similar to those of 
articles compression molded from the same polymer. 
Optionally, in addition to the aromatic heterocyclic polymer particles, the 
heated and compacted powder may contain up to about 75 weight percent 
filler material. The filler material may be added to improve particular 
properties, such as, for example, flexural strength, friction and wear 
properties, and density or may be added to improve the ability of process 
the powder, or may be added to lower cost. Examples of such additives are 
polytetrafluoroethylene polymer particles, graphite, MoS.sub.2, silicone, 
silica, or glass particles. 
DETAILED DESCRIPTION OF THE INVENTION 
According to the present invention, granules comprising aromatic 
heterocyclic polymers, such as polyimides, are provided. Generally 
speaking, the present invention is directed to providing extrudable 
granules of polymers of the type that are relatively intractable, and 
which are not easily formed into thick articles by any continuous process. 
By ram extrudable it is meant that the granules can be continuously 
consolidated to form an article with density and properties similar to 
those achieved by hot compression molding. The polymers used in the method 
of the present invention are stable at temperatures ranging up to about 
400.degree. C. 
The term "aromatic heterocyclic" polymer is meant to include polymers 
having a recurring structural unit having heterocyclic rings which include 
nitrogen. Such polymers include: for example, polyimides, 
polyamide-imides, polybenzimidazoles, polyphenylquinoxaline, polyparabanic 
acid, poly (phenyl-s-triazine), polybenzimidazopyrollones, polyetherimides 
and polyimide-epoxy polymers of the type of U.S. Pat. No. 3,663,651, and 
mixtures thereof. 
In a preferred form of the present invention, a thermoplastic polyimide 
powder is used to form the granules. The term polyimide polymers is used 
to define polymers having the following recurring structural units: 
##STR1## 
Examples of such thermoplastic polyimide polymers include: Upjohn's 
Polyimide 2080 and Ciba-Geigy's P13N and LSU 679. 
The particularly preferred polyimide is Upjohn PI 2080, which is believed 
to be a copolyimide of the type described in U.S. Pat. No. 3,708,458, the 
polyimide having the following recurring structural unit: 
##STR2## 
wherein 10 to 90 percent of the units, R represents 
##STR3## 
and the remainder of the units, R represents either: 
##STR4## 
It has been determined that it is desirable for the particles in the powder 
to be quite fine. The fine particles allow for smoother ram extrusion of 
the compacted polymer granules and for the achievement of improved 
ultimate strength properties in the extruded article. 
Preferably, the powder includes fine polymer particles having average 
diameters as determined by the Fisher Sub-Sieve Sizer below about 50.mu., 
most preferably in the range about 1.mu. to about 10.mu.. Generally 
speaking, the powders have bulk densities in the range from about 125 to 
about 400 gm/liter. 
In the process of the present invention, the polymer particle is heated and 
compacted to form a partially densified mass. The heating and compacting 
can be carried out on a layer of powder in a hydraulic press having heated 
platens. It should be understood that the heating and compacting of the 
powder may be done by a number of means, involving, that is, use of a 2 
roll compounding mill or even of a ram extruder under a low back pressure. 
The temperature of the platens in a hydraulic press and the compaction 
pressure will depend on the specific type of polymer which is being formed 
into granules, but in general, the platens are heated to a temperature 
near or above the polymer's softening point. 
It should be understood that the compaction pressure is dependent upon the 
type of polymer particle being compacted. It is preferred that the 
compaction pressure be in the range of about 5 and about 5,000 psi. 
It is preferred that the polymer powder include at least about 25 weight 
percent aromatic heterocyclic polymer powder. Fillers may be added to the 
powder in amounts ranging up to 75 weight percent. The fillers may serve 
to modify the properties of an article formed from the granules produced 
by the method of the present invention. The fillers may also modify the 
processing properties of the powder and/or of the granules. The fillers 
include, for example, polytetrafluoroethylene polymer particles, graphite 
particles, molybdenum disulfide particles, silicone particles, silica and 
glass particles. The additives or fillers should be intimately mixed with 
the aromatic heterocyclic polymer powder by a conventional method, such 
as, for example, by powder blending using a high shear mixer. 
In a preferred embodiment of the invention, the polymer powder comprises 
thermoplastic polyimide particles and the powder particles are 
simultaneously heated and compacted at a temperature between about 
610.degree. and about 650.degree. F. and at a pressure of between about 5 
and about 5,000 psi to thereby form a porous mass of polyimide polymer. 
After heating and compaction, the porous mass is allowed to cool. 
Thereafter, the mass is divided into granules which may be used in 
different forming processes, but which are particularly suited for use in 
continuous processing by the ram extrusion method. The dividing of the 
mass can be accomplished by many conventional methods, such as, for 
example, grinding or crushing the mass. The mass may be divided into 
granules having a size which is particularly suited for the further 
processing of the polymer. In the case of ram extrusion, it is preferable 
to divide the mass into granules which are easily handled and fed into the 
extruder, such granules having diameters in the range from about 40 to 
about 400.mu.. It should be understood, however, that the granules 
provided may have a non-uniform diameter and shape.

EXAMPLE 1C 
This example is a comparative example which illustrates the results when 
the polymer powder is not pre-compacted. 
A thermoplastic aromatic heterocyclic polyimide polymer sold by the Upjohn 
Company as Polyimide 2080 was ball milled to provide particles having 
irregular shape and average size of 2 to 5.mu. (measured by Fisher 
Sub-Sieve Sizer). This powder exhibited a bulk density of about 170 g/l. A 
portion of the powder was blended with 2% by weight 
polytetrafluoroethylene powder L169A (from ICI) and was continuously ram 
extruded into a 10 mm diameter rod. The flexural strength of the rod was 
no greater than 11,000 psi. The rod produced in this manner had cyclic 
color variations corresponding to the individual powder charges and 
occasional faults and cracks at the intersection of the charges. 
EXAMPLE 1 
20 grams of the polyimide powder described above in Example 1C was placed 
on heated 81/2".times.11" platens of a hydraulic press and subjected to a 
compaction pressure of 200 lb/sq. in. for 3 minutes. During the 
compaction, the platens were maintained at a temperature of 620.degree. F. 
After compaction, a porous mass of polymer was removed from the press and 
allowed to cool. Thereafter, the mass was ground in a hammer mill to 
produce a granular material of nonuniform particle size and shape. The 
bulk density of the granular produce after grinding is typically greater 
than about 400 gm/l and generally about 450 gm/l. 
The granular product was ram extruded under conditions equivalent to those 
used during the extrusion of the powder mentioned above. The rod produced 
from this powder had an excellent uniform appearance, no faults or voids, 
and exhibited a flexural strength of 20,000 psi. 
EXAMPLE 2 
The polyimide powder described in Example 1C was passed through the 0.010" 
gap between the rolls of a two roll compounding mill. The rolls had been 
heated to a temperature of 650.degree. F. The resulting partially 
compacted mass was hammer milled to form a granular product which was 
subsequently ram extruded to yield a 10 mm diameter rod of excellent 
uniform appearance. The rod exhibited a flexural strength of 16,000 psi. 
EXAMPLE 3 
A granular material was prepared by ram extruding at 620.degree. F. at low 
pressure (100-1,000 psi) a dry blend of Upjohn Polyimide 2080 powder with 
28% by weight polytetrafluoroethylene powder L169A from ICI, and 
subsequently hammer milling the porous extruded mass. The granular product 
was ram extruded under high pressure, 15,000 psi, and maximum temperature 
of 650.degree. F. to yield a uniform color and density 10 mm diameter rod 
which exhibited flexural strength of about 8,200 psi. 
EXAMPLE 3C 
This example is a comparative example which illustrates the results when 
the polyimide powder is not pre-compacted. 
If the blended powder of polyimide and polytetrafluoroethylene described in 
Example 3 is not first ram extruded under low pressure before the high 
pressure rod forming ram extrusion process, the resulting rod varies in 
color and density and contains large faults and voids such that the rod is 
so weak that it often separates when ejected from the extruder. 
While preferred embodiments have been shown and described, various 
modifications and substitutions may be made thereto without departing from 
the spirit and scope of the invention. Accordingly, it is to be understood 
that the present invention has been described by way of illustration and 
not limitation.