Composition and process for retarding the premature aging of PMR monomer solutions and PMR prepregs

The polyimides are derived from solutions of at least one low-boiling organic solvent, e.g. isopropanol containing a mixture of polyimide-forming monomers. The monomeric solutions have an extended shelf life at ambient (room) temperatures as high as 80.degree. C. and consist essentially of a mixture of monoalkyl ester-acids, alkyl diester-diacids and aromatic polyamines wherein the alkyl radicals of the ester-acids are derived from lower molecular weight aliphatic secondary alcohols having 3 to 5 carbon atoms per molecule such as isopropanol, secondary butanol, 2-methyl-3-butanol, 2 pentanol or 3-pentanol. The solutions of the polyimide-forming monomers have a substantially improved shelf-life and are particularly useful in the aerospace and aeronautical industry for the preparation of polyimide reinforced fiber composites such as the polyimide cured carbon composites used in jet engines, missiles, and for other high temperature applications.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to stable organic solutions of polyimide-forming 
monomers having improved shelf-life and more specifically to the process 
of manufacturing, shipping, handling, storage and the fabrication layup of 
all types of PMR (polymerization of monomeric reactants) polyimide-forming 
monomeric solutions and the PMR polyimide prepregs derived therefrom 
without adversely affecting subsequent processability at cure temperature 
of the PMR resins and PMR composites. 
2. Description of Related Prior Art 
Polymerization of Monomer Reactants (PMR) to obtain polyimides is an 
important class of ultra high performance composite resins. Polyimide 
graphite fiber reinforced composites are increasingly used in various 
aircraft engine components, which operate at temperatures ranging up to 
371.degree. C. for thousands of hours. For example, PMR-15 is one of the 
best known and most widely used PMR polyimide. PMR-15 attributes include 
relatively easy processing, substantially lower costs, and excellent 
property retention at elevated temperatures, compared to other 
commercially available high temperature resin materials. 
The preparation of polyimides from mixtures of monomeric diamines and 
esters of polycarboxylic acids is disclosed, for example, in U.S. Pat. No. 
3,745,149. Patentee disclosed that polyimides can be processed from a 
mixture of monomeric reactants using lower (primary) alcohols to esterify 
an anhydride endcap and an aromatic dianhydride. These monomeric reactants 
when combined with an aromatic diamine in the molar ratio of N 
diester-diacid/N+1 diamine/2 ester-acid endcap, form a monomeric mixture 
which at high temperature polymerizes to a polyimide. This procedure was 
the evolution of the terminology PMR (polymerization of monomeric 
reactants). The initial concept of using lower (primary) alcohols to 
prepare methyl or ethyl ester-acids remains in use as originally disclosed 
in the art over the last twenty-five years. 
Subsequently, however, few PMR patents have issued that improve over the 
prior art and generally these patents required a new "wrinkle" such as the 
use of monofunctional additives or new formulations using new 
dianhydrides, diamines or endcaps. These prior art patents are still using 
PMR technology based on methyl or ethyl ester-acids or diesters-diacids 
formed from lower (primary) alcohols. There is no prior art specifically 
covering the PMR Extended Shelf Life Technology obtained by the use of 
higher (secondary) ester-acids and higher (secondary) diester-diacids, as 
taught by this invention. Until now, all the prior PMR art remains evolved 
around the use of lower (primary) ester-acids in contrast to the benefits 
obtained by the use of the higher (secondary) ester-acids in PMR Extended 
Shelf Life Technology. 
The major disadvantages of the state-of-the-art PMR technology, as 
practiced commercially today, are the limited shelf life of the monomeric 
solutions at ambient (room) temperatures, the short working outlife time, 
and an extremely high sensitivity toward premature aging at temperatures 
even slightly above room temperature. The disadvantages cause premature 
polymerization during all phases of PMR usage such as in synthesis, 
manufacturing, shipping, handling, storage, and fabrication 
layup/processing. The PMR technology employed today still uses the lower 
(primary) methyl and ethyl diester-diacid and ester-acid of the 
dianhydride and nadic anhydride endcap respectively. This methyl and ethyl 
ester technology is PMR's inherent weakness in that imidization proceeds 
rapidly at about room temperature, thereby quickly aging all PMR types of 
polyimides such that aged solutions and prepregs expire with limited shelf 
life within one to three weeks at room temperature thereby being 
unprocessable with autoclave fabrication techniques. The engineering 
solution, as opposed to the chemical solution, to premature aging of PMR 
solutions and prepregs has been through rigorous handling requirements via 
strict manufacturing temperature control, overnight air shipment in dry 
ice, freezer storage of received PMR materials and stringent quality 
control governing allowed outlife usage time and freezer storage time; all 
of which significantly adds to the final cost of PMR composites. 
Another disadvantage of the state-of-the-art PMR technology is the use of 
toxic lower (primary) methanol and ethanol for esterification and as the 
solvent for PMR monomer solution preparation and PMR prepreg 
manufacturing. These primary alcohols create highly toxic volatiles for 
both the manufacturer and user to control. In comparison, the PMR Extended 
Shelf Life Technology of this invention only uses the higher (secondary) 
and much less toxic isopropyl alcohol for the esterification and solvent 
in the PMR monomer solution preparation and PMR prepreg manufacturing. The 
evidence of reduced toxicity and increased safety are a lower odor 
threshold, well before you reach a much higher allowed threshold limit 
value, an increased autoignition temperature and narrower flammability 
limits along with less serious medical problems of overexposure in the use 
of isopropanol, for example, in comparison to the use of methanol or 
ethanol. 
The PMR extended shelf life technology of this invention is based on a 
chemical solution to significantly retard aging of PMR solutions and PMR 
prepregs, rather than on an engineering solution of rigid temperature 
control as evolved and still practiced in present PMR technology. The 
chemical solution to these problems is the use of the higher (secondary) 
C.sub.3 to C.sub.5 alcohols, e.g. isopropyl for esterification of the 
anhydride endcaps and dianhydride monomers. 
SUMMARY OF THE INVENTION 
This invention relates to novel compositions of matter and to the process 
of using higher (secondary) C.sub.3 to C.sub.5 ester-acids of 
monoanhydrides and higher (secondary) C.sub.3 to C.sub.5 diester-diacids 
of numerous dianhydrides, both formed from C.sub.3 to C.sub.5 secondary 
aliphatic alcohols, in preparing the polyimide-forming solution of 
monomers. One of the preferred compositions and processes relates to the 
use of the higher (secondary) isopropyl ester-acid of nadic anhydride and 
the isopropyl diester-diacids of numerous commercially available 
dianhydrides, e.g. BTDA, 6FDA, PMDA, ODPA, BPDA, etc. Preferably, the 
isopropyl ester -acids and isopropyl diester -diacids are mixed in 
solution with C.sub.3 to C.sub.5 secondary aliphatic alcohols and aromatic 
polyamines such as the diamines to form PMR monomer solutions, which 
subsequently form addition cured PMR resins at higher cure temperatures. 
The polyimides of this invention are derived from solutions of low-boiling 
organic solvents and a mixture of polyimide-forming monomers. The 
solutions of the polyimide-forming monomers are characterized as having an 
improved or extended shelf-life at ambient (room) temperatures, i.e. 
stable solutions at temperatures ranging up to about 80.degree. C. and 
comprise effective amounts of (a) at least one mono-alkyl ester-acid 
having the formula: 
##STR1## 
wherein R.sub.2 is a lower secondary alkyl radical of 3 to 5 carbon atoms, 
and R.sub.1 is a divalent radical selected from the Group consisting of 
alkyl, substituted alkyl, aryl and substituted aryl radicals, and 
(b) at least one diester-diacid or an isomer thereof having a formula 
selected from the Group consisting of 
##STR2## 
wherein R.sub.4 is a tetravalent radical selected from the Group 
consisting of naphthalene, benzene, and biphenyl radicals, R.sub.3 is the 
same or a different lower secondary alkyl radical of 3 to 5 carbon atoms, 
and X is a divalent radical selected from the Group consisting of 
##STR3## 
(c) at least one aromatic polyamine selected from the Group consisting of 
aromatic diamines, aromatic triamines, aromatic tetraamines and mixtures 
thereof in any proportion. 
The organic solutions of polyimide-forming monomers of this invention can 
be heated to temperatures ranging from about 250.degree. C. to 400.degree. 
C. to obtain crosslinked polyimide resins having average molecular weights 
in excess of 10,000. These polyimide resins can be formed into various 
shapes and sizes in an autoclave or molding equipment e.g. polyimide 
impregnated carbon fibers for use in high temperature applications. 
Accordingly, it is an object of this invention to improve the room 
temperature storage stability of PMR monomer solutions and PMR prepregs 
without adversely affecting the processability of PMR polyimide 
composites. 
It is another object of this invention to provide organic solutions of 
polyimide-forming monomers and a process of preparing said monomeric 
solutions for use in preparing polyimide resins and polyimide prepregs. 
It is still another object of this invention to provide a mixture of 
polyimide-forming monomers that retards the ambient (room) temperature 
reactivity of PMR solutions and PMR prepreg materials. This improvement 
provides a wide safety margin against mishandling of PMR solutions and 
prepregs by significantly retarding the premature aging and the expiration 
of the PMR solutions shelf life. 
It is a further object of this invention to reduce the solvent toxicity, 
limit the solvent flammability, provide higher autoignition temperatures, 
and lower the odor threshold of the polyimide-forming monomeric solutions 
compared to the state-of-the-art and to improve other health issues by 
using polyimide-forming monomeric solutions that are less toxic. 
These and other objects of this invention will become apparent from a 
further and more detailed description of the invention as follows.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The PMR extended shelf life technology of this invention is directed to a 
composition of matter comprising a polyimide-forming monomeric solution 
consisting essentially of, for example, at least one aromatic polyamine 
and a C.sub.3 to C.sub.5 secondary alkyl ester such as an isopropyl ester 
of nadic anhydride and at least one dianhydride preferably selected from 
the Group consisting of 3,3',4,4'-benzophenone tetracarboxylic dianhydride 
(BTDA); 1,1,1,3,3,3-hexafluoroisopropylidene bisphthalic acid dianhydride 
(HFDA or 6FDA); 1,2,4,5-pyromellitic dianhydride (PMDA); 
3,3',4,4'-oxydiphthalic dianhydride (OPDA) and 
3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA) converted to the 
corresponding diester-diacid and to the processes for preparing the 
ester-acid (and other mono ester-acid endcaps) and the diester-diacids of 
various other aromatic dianhydrides such as BTDA, 6FDA, PMDA, ODPA, and 
BPDA. 
More specifically, the monomeric solutions of the polyimide-forming 
monomers having an extended shelf life can be illustrated by using the 
C.sub.3 to C.sub.5 secondary aliphatic alcohols such as isopropanol, 
secondary butanol, 2-methyl-3-butanol, 2-pentanol or 3-pentanol; the 
preferred being isopropanol to form the esters and diesters of this 
invention as follows: 
##STR4## 
As a specific example, the mono ester-acids and diester-diacids in 
combination with at least one aromatic polyamine are mixed in solution 
with a C.sub.3 to C.sub.5 secondary aliphatic alcohol as the solvent, e.g. 
10% to 80% and preferably 30% to 60% by weight of isopropyl alcohol, and 
heated to temperatures ranging from about 500.degree. F. to 750.degree. F. 
to form the cured corresponding polyimides, as illustrated: 
##STR5## 
In the above example, the R.sub.2 and R.sub.3 are either the same or 
different secondary alkyl radicals derived from C.sub.3 to C.sub.5 lower 
secondary aliphatic alcohols, and R.sub.4 is a tetravalent aryl radical. 
The original PMR (Polymerization of Monomer Reactants) technology was 
developed in the early 1970's as a means of producing large void free 
polyimide fiber composites. The principal resin in the PMR family, PMR-15, 
has been commercially available since the late 1970s and is regarded as 
the industry standard for aircraft engine applications for long term use 
at temperatures ranging up to 500.degree. F. The PMR-15 components are 
currently being used in both military and commercial aircraft engines. 
More recent adaptations of the PMR approach to high temperature polyimides 
can be found in such resin systems as PMR-II (a 1977 IR.cndot.100Award 
Winner), AFR-700, TRW-800, and RP-46 (a 1992 R&D 100 Award Winner). 
Presently, the PMR process uses methyl or ethyl ester-acid monomers. 
However, work at the NASA Lewis Research Center clearly found that resin 
solutions and prepregs made with PMR resins have limited shelf life at 
room temperature. Research at NASA Lewis has determined that this is due 
to the premature formation of low molecular weight imides. Formation of 
these aging products adversely affects the handling and autoclave 
processability of PMR resin solutions and the prepregs. Presently, this 
aging process can be retarded only by storage of these materials at 
freezer temperatures. These PMR resin solutions and the prepreg therefrom 
must also be packed in dry ice and shipped overnight in order to minimize 
the formation of the aging products. This procedure results in a 
substantial cost for the handling and shipping of these materials. 
However, recent research efforts have led to the development of PMR resins 
that have an improved shelf life, both as PMR monomer solutions and as PMR 
prepregs. It was found that this new extended shelf life technology was 
based upon the use of only aliphatic secondary alcohols having the 3 to 5 
carbon atoms such as isopropyl alcohol rather than the methyl or ethyl 
ester-acids in PMR monomer formulations. Kinetic studies performed at the 
NASA Lewis Research Center discovered that the rate determining step in 
the formation of polyimides, as well as the imide-forming solution and 
prepreg aging products, via the PMR process, is the conversion of the 
methyl or ethyl ester-acids into anhydrides. The use of the bulkier 
isopropyl ester-acids, however, significantly retards the rate of 
anhydride formation which, in turn, limits the formation of undesired 
solution and prepreg aging products. The secondary isopropyl ester-acids, 
for example, were also shown to be the only secondary ester-acids that 
vastly improved the shelf life without unduly further complicating the 
composite processability. The use of this new chemistry to existing PMR 
technology increases the room temperature storage life of PMR solutions 
and prepregs by over an order of magnitude. Thus, the PMR extended shelf 
life technology eliminates the need for dry ice shipping and freezer 
storage of PMR materials. The PMR fiber composites prepared with extended 
shelf life prepregs retain the excellent thermo-oxidative stability and 
thermo-mechanical strengths equivalent to those made from the current 
commercially available PMR prepregs. 
There are a variety of experimental new and previously developed PMR-type 
polyimides that compete with each other on the basis of material cost 
versus desired use temperature such as the nonfluorinated polyimides 
(DuPont KIII's, PMR-15, RP46) at moderate cost for 500 to 600.degree. F. 
long term use, to the fluorinated polyimides (PMR-II, VCAP, AFR700, DuPont 
Avimid N) at high cost for 700.degree. F. shorter term use temperatures, 
to the difficult-to-process nonfluorinated polyimides (TRW-800) at 
moderate cost for 800.degree. F. very short term applications. The PMR 
extended shelf life technology of this invention was not developed to 
replace or compete with any of these PMR type materials; rather the 
development was to enhance the utility of all current and future PMR 
polyimides by allowing a significantly longer shelf life at lower storage 
and manufacturing temperatures (freezer to hot melt manufacturing 
temperatures) in order to prevent premature PMR aging during long term 
storage and manufacturing. Without the PMR extended shelf life technology 
of this invention, the current state-of-the-art PMR polyimide products 
will age at least 10.times. faster over a wide range of storage 
temperatures (freezer to room temperature) and manufacturing temperatures 
(hot melt prepreging up to 80.degree. C.)as shown by both formation of 
soluble aging products (seen by high pressure liquid chromotography) and 
by formation of precipitates and/or phase separations (highly visible 
events well past the PMR solution shelf life manufacturers desire). 
As more specifically shown in the following Table I, the PMR extended 
shelflife technology of this invention provides the following benefits: 
(a) 10-30.times. increase in room temperature shelf life of PMR solutions 
making PMR materials more friendly to use, (b) lowers odor threshold to a 
much safer limit, the opposite of current state-of-the-art methanol based 
PMR technology, (c) reduced cost and complexity of transporting and 
storing of PMR materials by allowing room temperature 
shipping/handling/storage while current state-of-the-art PMR technology 
requires dry ice shipping and freezer storage, (d) negligible sensitivity 
to handling/shipping mistakes due to the vastly retarded aging at elevated 
temperatures and (e) increased applicability for hot melt prepreg 
manufacturing due to significant reduction in polymerization at hot melt 
temperatures (up to 80.degree. C. (176.degree. F.) for short times). 
TABLE 1 
______________________________________ 
PMR Extended 
Shelf Life 
Current PMR Technology, 
Technology, Isopropyl esters Magnitude of 
Methyl esters with Isopro- Improvement, 
with Methanol panol solvent X Fold 
______________________________________ 
Cost, non- 
fluorinated PMR 
e.g., PMR-15, Moderate Moderate No Change 
RP-46 
fluorinated PMR 
e.g. PMRII, VCAP, High High No Change 
AFR700, 
Some AVIMID N 
Room Temperature 
PMR 
Solution Stability, 
Time to a Visible 
Event* 
Nonfluorinated 
PMR, 
i.e. PMR-15 3 weeks 100 weeks &gt;30X 
fluorinated PMR 
containing p- 
phenylene diamine 
and nadic ester 
endcap 
i.e. PMRII, 2-7 days &gt;60 days 10X Minimum 
PMRII-50, to &gt;30X 
AFR-700 
Some newer &gt;3 days 42 days &gt;14X 
Developmental non- 
Toxic PMR-BAX 
formulations 
After 2 hr. @ 
methanol isopropanol 
isopropanol 
80.degree. C. solution solution prepreg** 
(176.degree. F.), 
PMR solution 
and prepreg 
stability 
PMR-15 
Diamine consumed 45% 3% 5% 9-15X 
Diester-diacid 25% 2% 3-4% 8-12X 
consumed 
endcap & diamine 89% 7% 17% 5-13X 
aging product 
formed 
Prepreg handling 
excellent excellent No Change 
characteristics, 
manufacturing 
transpor- dry ice room tempera- lower shipping 
tation required ture allowed costs 
outlife dries overnight months to dry &gt;30X 
drape fine fine No Change 
(unless dry) 
tack fine (unless excessive Excessive 
dry) (unless dried) 
Solvent toxicity and methanol isopropanol 2X 
safety TLV (time 200 ppm 400 ppm 
weighted avg.) 
odor threshold 5900 ppm 40 ppm &gt;148X 
odor threshold-TLV 29.5 (unsafe) 0.1 295X, 
(10X safety much safer 
margin) 
autoignition temp. 385.degree. C. 399.degree. C. 14.degree. C. 
Flammability 6.7 to 36% 2.2 to 12% narrower, 
limits, (lower but lower 
to upper,%) 
vapor pressure 97 mm 33 mm 3X, less 
boiling point 65.degree. C. 82.degree. C. 17.degree. C., 
evaporation 
______________________________________ 
*Time to a visible event = precipitation or phase separation. Not all 
PMR's do this as their aging products may only be seen by high pressure 
liquid chromatography because the aging products are still soluble or kep 
in solution by adding other solvents (which complicates composite 
processability) 
**PMR solutions age faster than PMR prepregs at room temperature due to 
increased mobility, but at elevated temperature prepreg ages faster as 
increased mobility and higher monomer concentration increases aging rate. 
The PMR extended shelf life technology of this invention is founded on the 
use of C.sub.3 -C.sub.5 secondary alcohols in preparing the ester-acids 
and the use of C.sub.3 -C.sub.5 secondary alcohol solvents which is an 
improvement over state-of-the-art methyl ester-acids and methanol solvent. 
Specifically, the improved shelf life reduces or eliminates the need for 
low temperature transportation and storage while providing a safer prepreg 
that is still well below the safety allowed Threshold Limit Value (TLV) 
when the isopropanol is smelled at its odor threshold. The opposite is the 
situation for the current use of the methanol or ethanol based PMR 
technology. 
The lack of aging for extended storage life PMR materials also means that 
variation in the batch-to-batch manufacturing of PMR resins can be 
significantly lessened which would provide benefits in consistent 
processability (the material is identical each time before processing) 
resulting in reduced scrap rates in comparison to the current PMR 
technology. The current PMR technology low scrap rate and consistent 
processability are attained by the composites industry only by use of low 
temperature shipping, handling, and storage specifications during 
manufacture and processing of PMR prepregs. These specifications include 
rigid quality control via high pressure liquid chromatography for the 
formation of the premature imide aging products which the new PMR extended 
shelf life technology retards from forming. 
The application of the extended shelf life technology of this invention 
applies to all of the non-toxic and current PMR polyimide monomeric 
solutions and prepregs, because this technology is general to retarding 
the rate determining step (anhydride formation) in the low temperature 
aging of all PMR materials. The extended shelf life technology can be used 
for all types of polyimides formed via the PMR process that has been 
developed over the past quarter of a century for aerospace and 
aeronautical PMR resin and PMR composite applications such as currently in 
jet engine, missile and other high temperature composite applications. 
This technology does not expand on the applications, rather it enhances 
the PMR material available for these applications. 
Some applications not currently optimized or available with PMR technology 
are applications requiring longer shelf life or higher manufacturing 
temperatures such as stable PMR material repair kits with long term 
expiration dates now possible and hot melt PMR prepregs that now would not 
age during the hot melt manufacturing process (done typically for short 
finite times up to 80.degree. C. (176.degree. F.). Both of these 
applications now are more feasible due to the proven long term shelf life 
at storage times and temperatures for which the current methyl or ethyl 
ester PMR technology does not meet the requirements. Additionally, the 
general nature of the extended shelf life technology of this invention 
also makes it applicable to the newer PMR resins now under development. 
The following are some specific examples of tetracarboxylic acid 
dianhydrides suitable for practicing this invention including 
2,3,3',4'-benzophenonetetracarboxylic acid dianhydride 
3,3',4,4'- benzophenonetetracarboxylic acid dianhydride 
2,2',b 3,3'- benzophenonetetracarboxylic acid dianhydride 
2,3,3',4'- biphenyltetracarboxylic acid dianhydride 
3,3',4,4'- biphenyltetracarboxylic acid dianhydride 
2,2',3,3'- biphenyltetracarboxylic acid dianhydride 
4,4'- isopropylidenediphthalic anhydride 
3,3'- isopropylidenediphthalic anhydride 
4,4'- oxydiphthalic anhydride 
4,4'- sulfonyldiphthalic anhydride 
3,3'- oxydiphthalic anhydride 
4,4'- methylenediphthalic anhydride 
4,4'- thiodiphthalic anhydride 
4,4'- ethylidenediphthalic anhydride 
hexafloroisopropylidene bisphthalic anhydride (6FDA), 
phenyltrifluoroethylidene bisphthalic anhydride (3FDA), 
2,3,6,7- naphthalenetetracarboxylic acid dianhydride 
1,2,5,6- naphthalenetetracarboxylic acid dianhydride 
benzene-1,2,3,4,- tetracarboxylic acid dianhydride 
benzene-1,2,4,5-tetracarboxylic acid dianhydride 
pryazine-2,3,5,6- tetracarboxylic acid dianhydride 
thiophene-2,3,4,5- tetracarboxylic acid dianhydride, and the indicated 
esters thereof. 
These anhydrides and esters thereof and methods for their preparation are 
disclosed in U.S. Pat. No. 3,745,149 and U.S. Pat. No. 3,856,752, the 
disclosures of which are incorporated herein by reference. 
In preparing the polyimide-forming solutions of this invention, various 
polyfunctional aromatic amines, including the diamines, triamines and 
tetraamines and mixtures thereof are used with the alkyl ester-acids and 
diester-diacids. The preferred polyfunctional amines include the diamines, 
e.g. aromatic diamines containing at least one benzene ring and preferably 
two benzene rings including: 
para-phenylenediamine 
meta-phenylenediamine 
4,4'- diamino-diphenylpropane 
4,4'- diamino-diphenylmethane 
4,4'- benzidine 
4,4'- diamino-diphenyl sulfide 
4,4'- diamino-diphenyl sulfone 
3,3'- diamino-diphenyl sulfone 
1,5- diamino-naphthalene 
bisaniline-m-xylidene (BAX) 
bisaniline-p-xylidene (BAX) 
bisaniline-p-benzyl carbonyl (COBAX) 
3,3'- diaminobenzophenone 
4,4'- diaminobenzophenone 
3,3'- diaminodiphenylether 
3,4'- diaminodiphenylether 
4,4'- diaminodiphenylether 
4,4'- diaminodiphenylmethane 
3,3'- dimethoxy benzidine 
2,2'- dimethylbenzidine 
3,3'- dimethyl benzidine and triamines such as 
1,3,5- triaminobenzene 
2,4,6- triamino-s-triazine 
4,4',4"- triaminotriphenylmethane 
4,4',4"- triaminotriphenylcarbinol 
Monoamine endcaps used to replace the alkyl acid-ester endcaps include, for 
example, mono amines such as 3- or 4-amninophenyl acetylene (APA), 3 or 
4-penylethynylaniline (PEA) and 3 or 4- aminostyrene (PAS) having the 
formulae: 
##STR6## 
This embodiment of the invention reverses the molar ratio, N 
diester-diacid, N+1 polyamine and 2 alkyl ester acid endcap to molar ratio 
N polyamine, N+1 diester-diacid and 2 monoamine endcaps. 
This embodiment uses an endcap that contains an amine group rather than an 
anhydride group to be esterified, thereby eliminating the use of the 
isopropyl ester of nadic anhydride. Here the embodiment is to use the 
reverse ratio as N moles aromatic diamines, N+1 moles isopropyl 
diester-diacids of the dianhydride monomer and from about 0.8 to 2.2 moles 
of the monoamine endcap, but 2 moles is generally preferred as a 
monofunctional crosslinkable primary aromatic amine. 
Another embodiment of this invention is the use of an unreactive 
non-crosslinkable endcap, such as aniline or isopropyl ester of phthalic 
anhydride, or slight excess of isopropyl diester-diacid of the dianhydride 
to control at N&gt;20 for the preparation of high molecular weight linear 
condensation polyimides. Another embodiment is the use of less than 2 
moles (e.g. only one) of isopropyl nadic ester as endcaps, leaving the 
excess diamine as the other endcap. An example of this is AFR700. It 
consists of one mole of isopropyl nadic ester, N moles of isopropyl 
diester-diacid of 6FDA, and N+1 moles of p-phenylenediamine; the +1 mole 
serving as the second endcap. For purposes of this invention, some of the 
preferred aromatic diamines include the C.sub.6 -C.sub.20 arylene diamines 
such as p-phenylenediamine, m-phenylenediamine and bis(p-aminophenyl) 
methane, bis aniline-p-xylidene (BAX) and the like. 
The following examples illustrate the polyimide-forming solution of 
monomers which have an improved or extended shelf life. 
EXAMPLE 1 
______________________________________ 
Preparation of isopropyl ester-acid of nadic anhydride 
Yields 
Isopropyl ester- 
Nadic Heat Heat Temp. diacid of Melting 
anhydride time Reached Nadic Anhydride Points 
______________________________________ 
20 g 78 hrs. -- 80.9% 88-89 1/2.degree. C. 
100 g 49 hrs. -- 86.9 87-89.degree. C. 
853 gms 48 hrs. 89.degree. C. 100% 76-89.degree. C. 
685 gms 48 hrs. 89.degree. C. 92.2% 80-85.degree. C. 
886 gms 24 hrs. 89.degree. C. 97.6% 72-88 1/2.degree. C. 
______________________________________ 
In the reaction, the temperature reached 89.degree. C. even though 
isopropanol boils at 82.degree. C. As the alcohol is consumed the solids 
content increases and the boiling point elevates from 82 initially to read 
89.degree. C. by reaction end. The nadic anhydride all dissolves quickly 
but takes extended time to react. The reaction takes a minimum of 12 hours 
after dissolution to reach completion, 24 hours is safer, hence a time 
range of 12-24 hours is suggested. 
Combined 886.2 g. Nadic anhydride (m.p. 161-162.degree. C.) with 
.apprxeq.996 g iPrOH (ACS reagent grade, b.p. 82.1.degree. C.-82.4.degree. 
C.) in a 2 liter round-bottom flask with a stir bar. Refluxed at 
89.degree. C. for 24 hours. A HPLC of the solution at 24 hours showed that 
the reaction was complete, allow to cool to room temperature and reaction 
solidified. Heated flask to dissolve product. Evaporated as much 
isopropanol off as possible on the evaporator. Solution resolidified on 
the evaporator. Used heptane to get the crystals from the flask to a 4 
liter beaker. Washed crystals with hexane, filtered, and placed in two 
recrystallization dishes. Placed both dishes in a warm oven to dry. 
Recrystallized entire product from hexane (.about.100 g./800 ml). Filtered 
those solutions which were cloudy through a Mr. Coffee filter. Collect 
additional product by partial evaporation, cool and filter. Continued to 
isolate remaining product from wash hexane repeated evaporation, cooling, 
filtering and washing. Placed all recrystallization crops in the oven to 
dry. Filtered remaining product and placed in a warm oven to dry. HPLC of 
one of the crops (#1) showed about 1.2% diacid. 
Weights, m.p. and LC results of all dishes: 
______________________________________ 
Crops Product Weight 
Melting Point Analysis 
______________________________________ 
#1 415.75 g m.p. 86-88.degree. C. 
1.20% diacid 
#2 474.38 g m.p. 86-88.degree. C. 1.42% diacid 
#3 208.71 g m.p. 87.5-88.5.degree. C. 2.08% diacid 
#4 58.43 g m.p. 82-86.degree. C. 5.13% diacid 
#5 11.79 g m.p. 80-86.degree. C. 4.88% diacid 
#6 12.53 g m.p. 72-78.degree. C. 6.68% diacid 
total = 1181.59 g 
______________________________________ 
Theoretical Yield = 5.3983 mol (224.259 g/mol) = 1210.617 g 
% Recrystallized Yield = 1181.59 g/1210.617 g X 100% = 97.602% 
EXAMPLE 2 
Preparation of isopropyl diester-diacids of BTDA 
##STR7## 
wherein R.sub.3 is derived from isopropyl alcohol. 
Dry the BTDA overnight in a vacuum oven at least at 120.degree. C., to 
insure it is not hydrolyzed by moisture in the air. In a 250 ml. round 
bottom flask with a magnetic stir bar, heat to reflux temperature a 
mixture of 26.85 gms (0.083 moles) of 3,3',4,4'-benzophenone 
tetracarboxylic dianhydride (BTDA) and 46.89 gms of isopropanol (an amount 
calculated to arrive at 36.87 gms of isopropanol and 36.87 gms isopropyl 
diester-diacid of BTDA at reactions completion. The dissolution of the 
BTDA takes 6 to 12 hours in the refluxing isopropanol. Continue heating 
for no more than 2 additional hours (on large reactions shut the heat off 
when dissolution occurs as the reaction will continue as the large 
reaction cools over up to 1 hours). The isopropyl diester-diacid BTDA 
solution is used in situ in further PMR formulation; adding an appropriate 
diamine(s) and endcap(s) in an appropriate molar ratio (N/N+1 diamine/2 
end caps). 
EXAMPLE 3 
Preparation of isopropyl diester-diacid of hexafluoroisopropylidine 
bisphthalic acid dianhydride (6FDA) Dry the 6F overnight in a vacuum oven 
at least at 120.degree. C. to insure any diacids are converted back to 
anhydrides before proceeding. As in example 3, esterify 47.48 gms (0.107 
mole) of 6FDA (previously dried) in 73.18 gms of isopropanol at reflux 
temperature; the amounts being calculated to prepare 60.33 gms isopropyl 
diester-diacid of 6FDA in 60.33 gm of isopropanol=a 50 weight percent 
solution. The dissolution of 6FDA takes 2 to 4 hours in refluxing 
isopropanol (whereas in methanol the same reaction takes only 1 hour or 
less). Continue heating for no more than 2 hours after dissolution occurs, 
as before, to prevent forming tri and tetra esters during prolonged 
esterifications. The isopropyl diester-diacid 6FDA solution is also used 
in situ in further PMR formulations by adding appropriate diamine(s) and 
endcap(s) in an appropriate molar ratio N/(N+1) diamine/2 isopropyl 
ester-acid of nadic anhydride. 
EXAMPLE 4 
Simultaneous preparation of in situ ester mixtures of BTDA and NA in a 
molar ratio for N BTDA/N+1 diamine/2 isopropyl nadic ester end cap where 
N=2 for 3 diamine to be added later. Use previously dried BTDA and NA. 
Combine 4.92 gm (0.03 mole) of NA, 9.67 gm (0.03 mole) BTDA in 25.41 gm 
isopropanol to achieve 50% solids when reacted, heat to reflux temperature 
and solution occurs 2-4 hours (instead of 6 hrs. for BTDA and 1 hour for 
NA individually). HPLC shows both are esterified in 5 hours in refluxing 
isopropanol, instead of 6-12 hours for BTDA and 12-24 hours for NA 
individually. Increasing the solids content to 75% (again 4.92 gms NA, 
9.67 gm BTDA but only 12.07 gm isopropanol) results in a faster 
dissolution time of 1 hour at 75% solids (instead of 2-4 hours at 50% 
solids) but the same dissolution time as BTDA or NA done individually at 
75% solids. However, the important point is complete coesterification at 
75% solids was in less than 3 hours versus 5 hours when coesterified at 
50% solids in refluxing isopropanol. 
EXAMPLE 5 
Simultaneous preparation of in situ ester mixtures of 6FDA and NA in a 
molar ratio of N 6FDA/N+1 diamine/2 isopropyl nadic ester where N=9 (for 
10 diamine is to be added later). Again start with previously dried 6FDA 
and NA. Combine 19.99 gm (0.045 mole) 6FDA and 1.64 gm (0.01 mole) of NA 
in 33.65 gm isopropanol to achieve 50% solids when reacted, heat to reflux 
temperature and solution occurs in 3 hours (instead of 2-4 hours for 6FDA 
or 1 hour for NA individually). HPLC shows both are also esterfied in 5 
hours in refluxing isopropanol, instead of 2-4 hours for 6FDA and 12-24 
hours for NA individually. Increasing the solids content to 75% (9.99 gm, 
0.0225 mole, 6FDA, 0.8202 gm, 0.005 mole NA and 7.6119 gm isopropanol) 
results in an almost identical dissolution time 21/2 hours vs. 3 hours, 
both individually or combined (except NA alone at 75%=1 hours dissolved). 
However the important point is complete coesterification (shown by HPLC) 
at 75% solids was in 31/2 hours vs. 5 hours for coesterification at 50% 
solids. In a similar fashion other dianhydrides and nadic anhydride (or 
other anhydride endcaps) can be in situ coesterified to isopropyl esters, 
but each needs to be individually carefully studied by the user as 
esterification times will vary depending on temperature, traces of water, 
reaction size, upheat rates and solids content. 
PREATION OF POLYIMIDE PREPREGS 
The following are the standard methods of preparing isopropanol based 
PMR-15 at N=2.08 iPrBTDE/3.08 MDA and 2 iPrNe as in calculations, the PMR 
monomer solution of which was coated by hand into unidirectional Celoon 
6000 graphite fiber wound on a drum. Alternatively, the PMR monomer 
solution can be coated on woven graphite (or glass) cloth instead of on 
unidirectional drum wound. The calculations are the same. How much resin 
is needed to produce a composite of X % fiber with 100-X % resin? In this 
case 61% fiber and 39% resin. The range can go from about 50 to 80% fiber 
(50 to 20% resin). The curing is via autoclave or compression molding at a 
final temperature of 600.degree. F., final pressure is no more than 200 
psi for autoclave but compression is done at 500 to 2000 psi in a matched 
metal mold. Both procedures can use what is called unidirectional prepreg 
tape (all fiber in one direction) or cloth woven material with fiber in 2 
directions. For PMR II-50 and VCAP75 examples, all the same calculation 
methodology applies but final cure temperature is 700.degree. F. 
Calculation of iPr BTDE/MDA/iPrNE PREPREG DATA 
12 turns/inch.times.12 inches.times.0.4747 g/turn=68.3568 g fiber on drum. 
68.35 g. fiber/61%=X/39% =gms resin desired. 
##EQU1## 
______________________________________ 
Monomer Moles Molar ratio 
Mole Wt. 
______________________________________ 
iPrBTDE 
= (.03) (2.0836) 
(442.426) 
= 27.655 g 
MDA = (.03) (3.0836) (198.270) = 18.342 g 
iPrOH to main- 
tain 50% 
solids = 
31.797 
iPrNE = (.03) (2) (224.259) = 13.445 g 
Prepare iPrBTDE as per example 3 using 
BTDA = (.03) (2.0836) 
(322.233) 
= 20.142 g 
iPrOH = 2X(27.655 -20.142 = 35.168 g 
to create 27.65 g iPrBTDE in 27.655 gm iPrOH 
Add wt. of MDA 18.342 g 
Add wt. of iPrNE 13.455 g 
Add iPrOH to maintain 50% solids = 31.797 gms. 
______________________________________ 
Remove twice the weight of desired molding powder (2.6 g). 
Use remaining 50% solids PMR solution to prepare PMR prepreg. 
Combined BTDA (20.142 g), and iPrOH (35.168 g) in a 200 ml round bottom 
flask with stir bar. Heated at reflux until dissolved (51/2 hr) then for 
one hour to ensure complete reaction as in Example 2. Removed from heat 
and added 18.342 g MDA, 13.445 g iPrNE. After the mono and diester acids 
and diamine are combined, solvent(s) are added to maintain 50% solids 
which can include other alcohols, even primary alcohols such as methanol 
or ethanol, and non-alcoholic solvents as long as these solvents are low 
boiling, e.g. bp. below 100.degree. C. In this example, 37.797 gms 
isopropanol is added as the preferred solvent to maintain 50% solids. 
Dissolved with stirring and weighed the flask to determine the solution 
weight and removed 2.6 g for molding powder. Brush coat PMR solution 
evenly unto 12" of drum wound graphite fibers, rinsing the flask with 
iPrOH. Prepreg was allowed to dry overnight and taken off the drum and cut 
into 3".times.8" pieces. Molding powder solution was heated until it 
formed a gummy residue. It was then placed in an oven at 400.degree. C. 
for 1 hour to remove all volatiles. Laminate was made from prepreg by way 
of compression (example 6) or autoclave molding techniques (examples 7-8). 
EXAMPLE 6 
Assemble 12 3.times.8 inch plies into a stack and preheat them in a tray 
with glass 3.times.8 inch glass cloth top and bottom to 400.degree. F. for 
1 hour to imidize and remove solvents. Place ply stack in a 3.times.8 inch 
matched metal die, heat to 450.degree. F., apply 1000 psi and continue to 
600.degree. F. Keep at 600.degree. F. 2 hours in press, cool, remove 
finished laminate. 
STANDARD COMPRESSION MOLDING CYCLE 
iPr/iPr PMR-15 in 100% iPrOH sample calculations needed to determine resin 
flow (bleed) during compression processing using prepregs stored six 
months at room temperature. 
Before staging: 
##EQU2## 
After staging: 
##EQU3## 
After Processing at 600.degree. F./2 hr.: 
##EQU4## 
Preparation for Autoclave Heat Pressing 
Freecoat 3".times.8" plates and frame--let dry. Place 81/2".times.81/2" 
nonporous (Teflon) over bottom of large frame. Cut 3".times.8" sheets of: 
4 glass cloth, 1 nonporous, 2 porous. Weigh these 3".times.8" pieces 
separately except glass cloth weigh 2 at a time. Keep pieces for top and 
bottom of laminate separate. Place 3".times.8" frame coated side down then 
layers in this order: 2 glass, 1 porous and 12 plies (weigh these 
together), 1 porous, 2 glass, 1 nonporous. Put plate on top coated side 
down. Put two glass cloths 81/2".times.81/2" on top of plate. Place high 
temperature adhesive around edge of large frame. Place Kapton film on top. 
Cut the excess Kapton away from frame. Place top part of frame on and 
place clamps on 3 to a side. Place thermocouple in hole on side of frame. 
Place frame in press. Attach vacuum tube and plug in thermocouple. Pull a 
vacuum on the frame. Set stops on a piece of glass cloth on frame. Lower 
the press. 
Autoclaving--Heat Press 
Get temperature up to .about.125.degree. F., rate of 5.degree. F./min., at 
15 mmHg and then hold at that temperature for 30 minutes (by shutting off 
temperature control). Turn temperature control on again (set at 
250.degree. F.). Record the temperature at 3 minute intervals (trying to 
keep rate at 5.degree. F./min.). As the temperature nears 250.degree. F. 
begin to move the control up in 20-30.degree. F. increments. At 
300.degree. F., close the valve on the water vacuum to increase vacuum to 
.about.72 mmHg. Heat to 400.degree. F. and hold for 1 hour. After 1 hour 
set the pressure to 0, hit the open button, remove the stops and place a 3 
inch.times.8 inch metal plate on the prepreg. Close. Set temperature at 
.about.460.degree. F. When the temperature reaches .about.450.degree. F., 
apply 270 psi pressure. Increase the temperature by 50.degree. F. 
increments until 600.degree. F. Hold at 600.degree. F. for 2 hours. After 
2 hours, shut off heat and vacuum pump. Let cool down. Remove laminate and 
weigh. 
PMR II-50 monomer solution and prepreg preparation. 
______________________________________ 
#STR8## 
Monomer Moles Molar Ratio Mole Weights 
______________________________________ 
iPr6FDE =(.04458 mole) 
(9)(mole wt.) 
= 203.833 gm 
PPDA =(.04458 mole) (10)(mole wt.) = 48.194 gm 
iPrNE =(.00458 mole) (2)(mole wt.) = 19.998 gm 
______________________________________ 
6FDA=(9) (0.04458) (444.246)=178.179 gm in 274 gm isopropanol--dissolved in 
5 hours--heat 1 hour more and cool as in example 3. Add 48.194gm PPDA and 
19.988 gm iPrNE. Slightly warm to dissolve and when in solution coat via a 
paint brush a T650-35 graphite cloth (301/2 inch.times.52 inch). Let air 
dry and cut into 84 - 4.times.4 inch plies, using the edge scraps for 
characterization via Rheology, DSC, TGA, TMA, etc. The prepregs can be 
stored at room temperature up to 4 years and can be still autoclave 
processed satisfactorily. 
The final process temperature time is about 700.degree. F./1 hr. compared 
to PMR-15 at 600.degree. F./2 hr. 
PMR II-50 IN AUTOCLAVE MOLDING 
iPr/iPr PMRII-50 IN 100% iPrOH calculations needed to determine resin flow 
(bleed) during autoclave processing using prepreg stored twelve months at 
room temperature. 
Before staging: 
##EQU5## 
After Autoclave Processing: 
______________________________________ 
Flow into bleeder 
______________________________________ 
Top = 12.59 g - 11.80 = 
0.79 
Bottom = 12.14 g - 11.84 = 0.30 
Nonporous - still 1.99 g 
1.09 total flow 
______________________________________ 
Laminate weight=47.60 g 
Wt. of bleed=1.09 g 
% resin flow=2.29% 
weight loss=8.51 g 
% weight loss=15.167 g 
VCAP-75 MONOMER SOLUTION AND PREPRREG PREATION 
The isopropyl nadic ester (iPrNE) was changed for a different endcap, 
4-aminostyrene. This changes molar ratio from N/N+1/ 2 for PMR II-50 to 
N+1/N/2 where N=14 for VCAP-75 (due to endcap is an amine instead of 
ester-acid). For same size graphite cloth as for PMRII-50, 225 gm resin 
for 375 gm cloth. 
225/7874 (VCAP mole wt)=0.02858 moles resin desired 
iPr6FDE=(0.02858) (N+1=10) (mole wt.=564.440)=241.8317 gm 
PPDA=(0.02858) (N=9) (mole wt.=108.14)=43.2948 gm 
4 amino styrene=(0.02858) (2) (mole wt.)=6.807 gm 
Esterify (10) (444.246) (0.02858)=190.335 gm of 6FDA in 293 gm isopropanol. 
(Use 6FDA after previously drying 24 hours at 130.degree. C. in vacuum). 
The solution is 241 gm iPr6FDE in 241 gm isopropanol. Cool. Add 43.29 gin 
PPDA and 6.807 gm aminostyrene. Brush by hand the solution unto a 301/2 
inches.times.52 inches piece of T650-35 graphite cloth and air dry 
overnight at room temperature. Cut into 4.times.4 inches plys which are 
used to compression and autoclave process laminate after up to 50 months 
of room temperature storage using similar 700.degree. F. cycles as for PMR 
II-50. 
As disclosed herein, the PMR extended shelf life technology of this 
invention under scores the importance of PMR resins to the aerospace 
industry and the far-reaching benefits of this extended shelf life 
technology to current and future PMR systems. This invention represents a 
significant advancement in simplifying the manufacturing, shipping, 
handling, storage, and fabrication of all types of PMR polyimide solutions 
and prepregs, rather than for only a specific PMR material, by providing 
reduced aging, lower shipping/storage costs, improved solvent safety, and 
consistent processability while eliminating batch-to-batch variability. A 
greater than an order of magnitude improvement in shelf life without 
adversely affecting subsequent composite processability is a major 
accomplishment that vastly improves on the state-of the-art PMR technology 
as first discovered over 25 years ago and yet to be improved upon until 
now with the introduction of this new PMR extended shelf life technology. 
Its application as a general solution to the premature shelf life aging of 
all PMR materials insures that the PMR extended shelf life technology will 
be continually applied to future PMR resins under development including 
the environmentally friendly, non-toxic PMR-15 replacement resin the 
aerospace industry is currently desperately searching for. 
While this invention has been described by a number of specific examples it 
is obvious that there are other variation and modification that can be 
made without departing form the spirit and scope of the invention as set 
forth in the appended claims.