A process is described for producing shaped articles of imidazopyrrolone polymers comprising molding imidazopyrrolone polymer molding power under pressure and at a temperature greater than 475 C. Moderate pressures may be employed. Preferably, prior to molding, a preform is prepared by isostatic compression. The preform may be molded at a relatively low initial pressure and temperature; as the temperature is increased to a value greater than 475 C., the pressure is also increased

Johnson, C. L.

English

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https://ntrs.nasa.gov/search.jsp?R=19740005064 2020-03-17T14:11:30+00:00Z
-United States Patent Office 3,772,418Patented Nov. 13, 1973
3,772,418
MOLDING PROCESS FOR IMIDAZOPYRROLONE
POLYMERS
Charles L. Johnson, Houston, Tex., assignor to the United
States of America as represented by the Administrator
of the National Aeronautics and Space Administration
No Drawing. Filed Oct. 29, 1971, Ser. No. 193,980
Int. Cl. B29f 5/02; C08g 20/20
U.S. CL 264—294 3 Claims
ABSTRACT OF THE DISCLOSURE
A process for producing shaped articles of imidazopyr-
rolone polymers comprising molding imidazopyrrolone
polymer molding powder under pressure and at a tem-
perature greater than 475° C. Moderate pressures may be
employed. Preferably, prior to molding, a preform is pre-
pared by isostatic compression. The preform may be mold-
ed at a relatively low initial pressure and temperature; as
the temperature is increased to a value greater than 475°
C., the pressure is also increased.
ORIGIN OF THE INVENTION
The invention described herein was made by an em-
ployee of the United States Government and may be
manufactured and used by or for the Government for
governmental purposes without the payment of any royal-
ties thereon or therefor.
FIELD OF INVENTION
The present invention relates to a process for molding
polymers. More specifically, the present invention relates
to a process for producing tough, compact, rigid, high
temperature stable, flame resistant, and readily machin-
able shaped articles of imidazopyrrolone polymers.
BACKGROUND OF INVENTION
Various polymers, particularly linear polyamides, poly-
esters, etc., have found wide application in the plastics
industry in the form of films, fibers and molded articles.
There has developed an ever-increasing need in certain
industries, such as the aerospace industry, for polymeric
materials which exhibit particular specific properties. These
properties include a high order of stability at elevated
temperatures, flame resistance, stability to ionizing radia-
tion, strength, toughness, hardness, and machinability.
Imidazopyrrolone polymers exhibiting a "ladder" struc-
ture have been found to exhibit such physical and chem-
ical properties.
Imidazopyrrolone polymers may be prepared by react-
ing a mixture of an aromatic dianhydride or aromatic
tetracarboxylic acid with an aromatic tetraamine having
two pairs of ortho-diamines. One method of preparing
polyimidazopyrrolones involves dissolving an aromatic di-
anhydride in a highly polar solvent, such as dunethy-
acetamide, dimethyl sulfoxide, dimethyl formamide or the
like, and mixing with an aromatic tetraamine dissolved in
a like or similar solvent. The resulting mixture may be
readily converted to an amide-acid-amine intermediate
structure, which can then be cyclized to a fused "ladder"
or partial "ladder" structure.
The preparation of potentially complete imidazopyrro-
lone "ladder" polymers may be illustrated by the reaction
between pyromellitic dianhydride (PMDA) and 1,2,4,5-
tetraamine benzene (TAB) in accordance with the fol-
lowing reaction:
NH,
10
15
20
Numerous imidazopyrrolone polymers and methods of
preparing them are disclosed in U.S. Pats. Nos. 3,518,232
and 3,414,543 and in Polymer Letters, Vol. 3, pp. 977-
984 (1965).
PRIOR ART
Prior art techniques of preparing shaped articles of
imidazopyrrolone polymers involve casting, extruding, or
molding dry powder of a poly (amide-acid-amine) pre-
polymer. The shaped article is then heated to a tempera-
ture adequate to complete cyclization, whereby the imida-
zopyrrolone ladder structure is obtained. Such techniques
are disclosed in U.S. Pats. Nos. 3,518,232 and 3,414,543.
In accordance with prior art techniques, cyclization is
achieved by heating to a temperature of less than 400° C.
It was generally thought that gradual degradation of the.
polymer would occur at temperatures above 450° C. and
that molding process using temperatures higher than 450°
C. would yield only badly charred and weakened material.
It has now been found, surprisingly, that tough, com-
pact, rigid, high temperature stable, flame resistant and
readily machinable molded structures may be obtained by
4Q molding cured (cyclized) imidazopyrrolone molding pow-
ders. Molded articles may be obtained, very rapidly at
moderate pressures, if molding temperatures between
about 450° C. and 600° C. are employed.
45 SUMMARY OF INVENTION
Shaped articles of imidazopyrrolone polymers may be
prepared by molding, under pressure, cured polyimidazo-
pyrrolone molding powder, at a temperature greater than
475° C. Conventional molding techniques and moderate
50 pressure, i.e., as low as 500 p.s.i., may be employed.
Preferably, prior to molding a preform of cured poly-
imidazopyrrolone molding powder is prepared by isostatic
compression.
55 DETAILED DESCRIPTION OF INVENTION
The process of the present invention is useful for pre-
paring shaped articles of any imidazopyrrolone polymer.
In contrast to previous process for preparing shaped arti-
gg cles of polyimidazopyrrolone polymers, which utilized an
amide-acid-amine prepolymer, the process of the present
invention employs cured polyimidazopyrrolone molding '
powder. The use of a cured polymer and a high tem-
perature makes it possible to obtain strong and coherent
65 shaped articles rapidly (in a matter of minutes) of a
moderate pressures. The prior art techniques, employing
3,772,418
uncured molding powder, require lengthy (often a mat-
ter of hours) and complicated molding procedures.
The expression "cured polyimidazopyrrolone," as used
herein, refers to imidazopyrrolone polymers which have
been partially and substantially completely cyclized to a _
fused "ladder" structure.
The molding process of the present invention is con-
ducted at a temperature of at least 475° C. The tem-
perature should not exceed a value at which the imidazo-
pyrrolone polymer will be substantially degraded. Tem-
 10
peratures up to about 600° C. may be conveniently em-
ployed.
Preferably, cured cyclized polyimidazopyrrolone pow-
der is isostaticly compressed to yield a dense compact
preform prior to the final molding operation. The pre-
 15
form is easy to handle and assures even distribution of
the powder particles in the molded cavity. Techniques of
preparing articles of polymer molding powders by iso-
static compression, and the advantages thereof, are well
known in the prior art. See for example JPL Space Pro- 20
grams Summary, No. 37-38, vol. IV, pp. 86-90. While
suitable preforms can be prepared at moderate pressures,
preferably isostatic pressure in excess of 10,000 p.s.i. is
employed.
If the molded cavity can be filled evenly with the poly- 25
mer particles, the preparation of a preform is not neces-
sary. The preform, or molding powder, can be molded
to the desired shape by either conventional compression
molding or by isostatic molding at pressures as low as
500 p.s.i. and temperatures within the range of 475° C. 30
to 600° C. There is no upper limit with respect to the
pressure which may be employed to produce the shaped
articles. While not necessary, pressure as high as 100,000
p.s.i. or higher may be employed if desired.
The preform or polymer molding powder may be 35
heated to a temperature of at least 475° C., under pres-
sure, using conventional molded heaters or by employ-
ing such heating methods as dielectric radio frequency
or microwave.
A preform or molding powder may be placed in a 40
molded cavity and subjected to any convenient initial
pressure and to an initial temperature of less than 475°
C. The temperature of the mold may then be raised to a
value greater than 475° C. As the temperature of the
mold is raised, preferably the pressure is also raised, par- *5
ticularly if an initial pressure of less than 500 p.s.i. is
employed. The polyimidazopyrrolone should not be
heated to temperatures approaching 475° C., unless it
is under pressure.
It is speculated that the use of high temperatures 50
(475° C. to 600° C.) in the preparation of shaped articles
of polyimidazopyrrolone polymers results in a chemical
reaction whereby the polymer powder particles are com-
pletely fused together. This complete fusion in the mold-
ing results in molded articles having strength and heat 55
resistance consistent with the chemical structure of im-
idazopyrrolone. It is believed that conventional moldings,
or powders of imidazopyrrolone polymers, are not fully
cyclized. The high molding temperatures employed in the
process of the present invention results in advancing the 60
imidazopyrrolone polymer of the shaped article to a more
completely cyclized polymer. The more completely cy-
clized polymer has a more aromatic, semi-graphitic and
thermally stable state, than the incompletely cyclized
polymer of the molding powder. The high temperatures 65
employed also probably result in the expulsion of final
traces of volatiles with the result that there is a transitory
plasticization of the powder mass. '.
The process of the present invention can be used to make
filled (composite) moldings with a wide range of phys- '°
ical, chemical and electrical properties. For example,
fillers such as pigments, glass fibers, electrically conduc-
tive carbon black, metal particles, abrasives, dielectrics,
etc. can be blended with the molding powder.
Shaped articles produced by the process of the present
invention can be used in a variety of applications particu-
larly when strength, heat stability, radiation resistance,
toughness, hardness, thermal resistance, flame resistance
and insolubility are of importance. Articles prepared in
accordance with the process of the present invention are
particularly useful as components for aircraft, spacecraft,
nuclear engines, rocket engines, generators, etc.
What is claimed is:
1. A process for producing shaped articles of im-
idazopyrrolone polymers comprising:
(a) providing a partially or incompletely cyclized im-
idazopyrrolone polymer molding powder in a pres-
sure mold,
(b) applying and maintaining a pressure of at least
500 p.s.i. on the mold while
(c) simultaneously increasing the mold temperature to
the range of about 475° C. to about 600° C., whereby
a tough, compact, rigid, high temperature stable,
flame resistant, and readily machinable completely
cyclized molded structure is obtained.
2. A process for producing shaped articles with im-
idazopyrrolone polymer powders comprising:
(a) providing a partially or incompletely cyclized
imidazopyrrolone powder,
(b) forming a preform of the imidazopyrrolone mold-
ing powder by isostatic compression,
(c) molding said preform in a pressure mold at
(i) a pressure of at least 500 p.s.i., while
(ii) simultaneously heating the mold to a tem-
perature of about 475° C. to 600° C., whereby
a tough, compact, rigid, high temperature stable,
flame resistant, and readily machinable com-
pletely cyclized molded structure is obtained.
3. A process for producing shaped articles of imid-
azopyrrolone polymers comprising:
(a) providing a quantity of a partially or incompletely
cyclized imidazopyrrolone polymer powder,
(b) molding the imidazopyrrolone powder under pres-
sure in a pressure mold and at a temperature of less
than 475" C., and
(c) increasing the pressure and raising the temperature
to a value greater than about 475° C., but less than
about 600° C., said pressure, after the temperature
has been raised to a value greater than 475 ° C., being
at least 500 p.s.i.,
whereby a tough, compact, rigid, high temperature stable,
flame resistant, and readily machinable completely cy-
clized molded structure is obtained.
References Cited
UNITED STATES PATENTS
Hughes et al. 260—47 CP
Glass et al. 264—89
Boyer et al. 264—88
Morello 264—331
Paufler 260^7 CP
Bell 260—47 CP
Nowak et al. 264—331
Twilley et al. 260—47 CP
ROBERT F. WHITE, Primary Examiner
J. R. THURLOW, Assistant Examiner
U.S. Cl. X.R. J
260—78 TF; 264—119, 88, 320, 331 ' , - ,
3,657,190
3,497,578
3,523,148
3,573,260
3,414,543
3,518,232
3,662,052
3,549,594
4/1972
2/1970
8/1970
3/1971
12/1968
6/1970
5/1972
12/1970


Molding process for imidazopyrrolone polymers

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