Resin transfer imidization of polyimide matrix composites

A process is provided for producing high temperature composite components from a thermosetting polyimide resin system, and more particularly to an in-mold technique for impregnating a reinforcement fiber preform with the polyimide resin system, and thereafter imidizing the polyimide resin system in such a way that exposure to the uncured (nonimidized) polyimide resin system is minimized. The process entails an injection technique that ensures complete impregnation and wetting of the reinforcing fabric, and complete removal of the condensation byproducts formed during imidization. The imidized component is then cured by heating the component to a temperature sufficient to remelt the polyimide resin system, and then applying pressure and additional heat to cause the polyimide resin system to cross-link to produce the desired composite component. The process results in a fiber-reinforced polyimide matrix component characterized by a void content of less than about three percent.

This invention relates to methods for processing thermosetting polyimide 
resin systems. More particularly, this invention is directed to a method 
for producing composite articles from PMR-15, a thermosetting polyimide 
resin system, in which the process significantly reduces the labor 
intensity and handling costs for producing the articles, as well as 
minimizes hazards attributable to exposure to the uncured resin system. 
BACKGROUND OF THE INVENTION 
PMR-15 is a commercially-available thermosetting polyimide resin system 
that has found wide use as the polymer matrix for composite components 
that require a service temperature of up to about 290.degree. C. (about 
550.degree. F.). This particular resin system, whose commercial sources 
include SP Culver City Composites, has been used to form structural and 
non-structural components for the aerospace industry, as well as numerous 
other industries. The general composition of PMR-15 is, in weight percent, 
about 30 to 40% methylenedianiline (MDA), about 40 to 50% benzophenone 
tetracarboxylic acid dimethyl ester (BTDE), and about 20 to 30% 
5-norbornene-2,3-dicarboxylic acid monomethyl ester (NE). For use, the 
above mixture is conventionally diluted with a solvent, such as methyl 
alcohol (methanol), at a weight ratio of about 1:1. 
Thermosetting polyimide resin systems of the type exemplified by PMR-15 
have a complex reaction system. These resin systems release large amounts 
of volatiles, such as methanol and water, during the cure process, which 
complicates the processing and manufacture of quality parts. In addition, 
utmost care must be taken in the handling of the uncured resin system 
since it contains incompletely reacted MDA, which is a suspected 
carcinogen, can cause chemically-induced hepatitis in humans, and is a 
known kidney and liver toxin. Accordingly, in the processing of PMR-15 
resin, precautions must be taken to minimize the risk of exposure to 
personnel. 
The fabrication of composite components from this polyimide resin system 
typically involves a prepreg, which is a woven reinforcement fabric that 
has been impregnated with the uncured resin system. Conversion of the 
resin and a suitable reinforcement structure into a prepreg that meets 
user specifications is a specialized process performed at a limited number 
of facilities. Known methods for impregnating the fabric material include 
solvent dilution, hot melt and powder coating techniques. Prepregs are 
typically produced to contain about 32 weight percent resin and about 60 
volume percent fabric (after cure), with carbon and glass fiber fabrics 
being most common. 
Typical production practices employ sheets of the prepreg on a backing 
material. Because they are in the uncured state, the prepregs must be kept 
in cold storage, typically in the form of rolls. The prepreg rolls must 
therefore be thawed before use, which usually requires about four hours. 
After thawing, the prepreg can be rolled out onto a cutting table and a 
portion sufficient to form the desired component is cut from the roll. The 
roll must then be repacked and returned to cold storage, with the out-time 
noted to monitor any deterioration of the prepreg. The portions are then 
cut to form plies which are shaped and oriented with respect to the fabric 
weave of the prepreg. The plies are numbered, stacked in sequence for 
production, and placed in a plastic bag to form a kit, which is then ready 
for transfer to a layup area where the plies are sequentially placed in a 
mold to form the desired composite component. If the kit will not be used 
immediately, it must be returned to cold storage. The trimmings, or offal, 
from the cutting operation must be disposed of in a controlled landfill 
due to the presence of MDA in the material. 
In the layup area, the kit is thawed if necessary, then removed from the 
bag and placed in ordered sequence on a mold surface. As each ply is 
positioned, the backing material must be removed and the ply oriented 
appropriately within the mold cavity according to the part geometry. A 
debulking operation must typically be performed to remove interlaminar air 
pockets, typically after every two to four plies are placed on the mold. 
The layup process is labor intensive, particularly since a number of plies 
are typically required to form a composite component. 
Following the layup process, the component formed by the prepreg plies must 
be cured on the mold through the application of carefully controlled heat 
and pressure. The cure process generally involves three stages: 
imidization, final cure, and post cure. Imidization involves a prescribed 
heat cycle that is applied through the mold to the component, causing 
condensation and other reactions to occur by which the constituent 
monomers of the resin system form an uncross-linked polyimide resin. Upon 
imidization, the monomer MDA is chemically reacted with other monomers to 
form the polyimide, and is therefore no longer the previously-noted health 
hazard. The resulting imidized component may then be removed from the 
mold, since the use of separate imidization and final cure molds improves 
the throughput of the fabrication process. 
Final cure involves heating the imidized component within the cure mold to 
a level where the imidized resin remelts and flows, after which pressure 
is applied to purge entrapped air. While pressure is maintained, the 
temperature of the imidized component is increased to about 315.degree. C. 
(about 600.degree. F.) to cause cross-linking of the polyimide, which 
imparts the desired structural properties for the final component. Post 
cure involves additional heating of the component at about 315.degree. C. 
(about 600.degree. F.) to cause additional cross-linking of the polyimide 
so as to increase its glass transition temperature (T.sub.g), and 
therefore enhance the thermal properties of the component. This process is 
typically accomplished by baking the component in a convection oven 
according to a controlled heating cycle. Final manufacturing processes for 
the resulting composite component include trimming, machining and 
inspection, as required. 
The complexity and labor intensive nature of processing thermosetting 
polyimide resin systems can be readily appreciated from the above. 
Particularly notable disadvantages of this process include the requirement 
for cold storage of the prepregs, waste and disposal of prepreg trimmings, 
and exposure hazards to unreacted MDA in the uncured prepreg. Resin 
transfer molding (RTM) techniques are known by which composite components 
are formed from reinforcing fibers that are impregnated in-mold with lower 
temperature resin systems such as epoxies. However, such techniques are 
not feasible for polyimide resin systems due to the high viscosity of 
thermosetting polyimide resins and the significant amount of condensation 
byproducts formed during imidization of the resins, resulting in 
insufficient fiber impregnation and the formation of voids within the 
composite component. As opposed to a prepreg which is manufactured by 
impregnating woven reinforcement cloth with a resin, a tow is a single 
fiber bundle which is impregnated with resin. Pre-imidized tow has the 
resin within the tow in an imidized state rather than an uncured state. 
Experimentation directed to the use of pre-imidized tow, a single fiber 
bundle impregnated with imidized resin, has indicated complications in the 
fabrication of some components, particularly those with axisymmetric 
structures when using reinforcement fabrics having sheet woven or braided 
architectures. From such experiments, it has been concluded that the resin 
bulk (about forty to about fifty volume percent) will inhibit the 
fabrication of components with through-thickness reinforcement fabric 
preforms, such as those produced by three-dimensional braiding, knitting 
and weaving. The degree of compaction required to consolidate a three 
dimensional processed preform would be substantial and cause significant 
amounts of distortion or crushing of the reinforcement architecture. 
Accordingly, it would be desirable if a less labor-intensive process were 
available by which polyimide matrix composite components could be formed 
with a thermosetting polyimide resin system that is imidized with minimal 
exposure risks to MDA, and the resulting component is capable of retaining 
its structural integrity at temperatures of up to about 290.degree. C. 
SUMMARY OF THE INVENTION 
It is an object of this invention to provide a method for processing a 
thermosetting polyimide resin system, and particularly PMR-15, so as to 
significantly reduce the exposure risk of MDA prior to imidization of the 
resin system. 
It is a further object of this invention that such a process entails an 
in-mold impregnation and imidization technique, by which the uncured resin 
system is injected directly into a mold, impregnates a reinforcing fabric 
within the mold cavity, and is thereafter imidized. 
It is another object of this invention that such a process involves process 
parameters that are tailored to ensure complete impregnation of the 
reinforcing fabric and removal of condensation byproducts formed during 
imidization. 
It is yet another object of this invention that such a process produces a 
polyimide matrix composite component that is capable of retaining its 
structural integrity at temperatures of up to about 290.degree. C. 
It is still another object of this invention that such a process lends 
itself to the manufacture of components using braids or 3-D woven fiber 
architecture, in addition to the more conventional 2-D woven architecture. 
The present invention provides a process for producing high temperature 
composite components from the PMR-15 polyimide resin system, and more 
particularly to a novel technique for impregnating a reinforcement fiber 
preform with this polyimide resin system, and thereafter imidizing the 
PMR-15 polyimide resin system in such a way that exposure to the uncured 
polyimide resin system is minimized. Importantly, the process entails 
unique sequences and steps to ensure complete impregnation and wetting of 
the reinforcing fabric, and complete removal of the condensation 
byproducts and other volatiles which are produced during the process 
steps. 
The process of this invention generally entails positioning a suitable 
dry-fiber (unimpregnated) preform in a mold cavity which is capable of 
withstanding at least moderate injection pressures. Uncured (nonimidized) 
thermosetting PMR-15 polyimide resin monomers that have been dissolved in 
a solvent are then injected into the mold cavity. The PMR-15 resin system 
of interest to this invention contains unreacted MDA in the uncured state. 
The exposure risk of RDA has been previously discussed, but advantageously 
is significantly reduced by the process of this invention. 
The parameters of the injection process, including resin viscosity and 
pressure, must be controlled such that the polyimide resin system 
permeates and wets the fiber preform within the mold cavity. In 
particular, the resin is dissolved in a sufficient amount of solvent to 
suitably reduce its viscosity, while a pressure of not more than about one 
atmosphere is maintained during injection in order to avoid collapse of 
the preform. The mold cavity is then sufficiently heated to evaporate the 
solvent, whose vapors are vented or drawn from the mold. Imidization of 
the polyimide resin monomers is then performed by applying a vacuum to the 
mold cavity so as to remove condensation byproducts produced during 
imidization, and heating the mold cavity. The resulting imidized component 
is then cured by heating the component to a temperature sufficient to 
remelt the polyimide resin, and then applying pressure and additional heat 
to cause the polyimide resin to cross-link to produce the desired 
composite component. The curing step may be performed in a separate 
compression mold, in an autoclave or, if properly adapted, in the mold 
cavity in which the imidization process is conducted. Following cure, the 
composite component can undergo a post cure operation so as to achieve 
further cross-linking of the polyimide resin, and thereby raise its glass 
transition temperature. 
The method of this invention results in a fiber-reinforced polyimide matrix 
component characterized by a void content of less than about three 
percent, and a high temperature service capability of up to about 
290.degree. C. Importantly, the injection process of this invention 
enables the PMR-15 polyimide resin to successfully permeate reinforcement 
preforms having complex architectures, such as two or three-dimensional 
braided, knitted, woven and filament wound, a capability which was 
previously unknown and unexpected. As such, a resulting composite 
component can have desirable mechanical properties that make it 
particularly suited for structural applications in the aerospace industry.

Other objects and advantages of this invention will be better appreciated 
from the following detailed description. 
DETAILED DESCRIPTION OF THE INVENTION 
The process of this invention is a resin transfer imidization technique 
that involves sequences and process steps for impregnating a fiber preform 
with the PMR-15 polyimide resin system containing unreacted MDA, and then 
converting the resin system to an imidized polyimide matrix component, 
while achieving an acceptably low void content in the final cured 
composite component and significantly reducing the exposure risk of 
personnel to MDA prior to imidizing the resin system. To achieve the 
above, the process of this invention is uniquely tailored to the 
processing characteristics associated with such a resin system. Prior to 
this invention, the solvent and condensation reactions characteristic of 
thermosetting polyimide resin systems containing MDA prevented the 
application of resin transfer methodologies to these resin systems. 
The process of this invention begins by placing a dry fiber preform into a 
suitable mold. The fiber preform can be of any suitable material, 
including carbon, glass and quartz fibers. The architecture of the preform 
can be complex, including braided, knitted, woven and filament wound 
preforms, and either two or three-dimensional in nature. The mold must 
have gating and venting ports, as dictated by the geometry of the 
component to be produced, in order to facilitate resin injection and to 
appropriately vent condensation byproducts and volatiles from the mold 
cavity. 
The PMR-15 polyimide resin system is then prepared by diluting uncured 
(nonimidized) PMR-15 monomers with a suitable quantity of solvent. 
Advantageously, PMR-15 exhibits a desirable combination of mechanical 
properties, thermal stability and cost, though it is foreseeable that 
other MDA-containing polyimide resin systems could be used. Because the 
resin system is employed in the uncured state, it contains unreacted MDA 
which, as noted previously, poses an exposure risk. According to this 
invention, the exposure risk to MDA is significantly minimized by the 
process of the invention. Notably, the resin system can be delivered and 
stored in bulk packaging, instead of the large and awkward prepregs 
required by the prior art and often composed of about fifty volume percent 
reinforcement fiber. 
To achieve adequate impregnation of the preform, the resin system must be 
dissolved in about thirty weight percent solvent, and preferably about 
thirty to fifty weight percent solvent, to achieve a viscosity of about 
100 centipoise (100 mpa.s) or lower. Significantly lower dilutions (e.g., 
about five to ten weight percent solvent) generally reduce the ability of 
the resin system to adequately permeate the preform and achieve complete 
wet-out of the reinforcement fibers. Dilutions in excess of fifty weight 
percent solvent require longer periods to achieve adequate evaporation of 
the solvent, which may be economically prohibitive. A preferred solvent 
for this process is methanol, though it is foreseeable that other solvents 
could be used, such as other alcohols or nonreactive solvents. 
The diluted resin system is injected in a controlled manner into the mold 
cavity through an appropriate number of injection ports, such that the 
resin system will permeate the preform within the mold cavity. As with 
prior art resin transfer molding methods, the injection process of this 
invention includes control of the volumetric rate of injection so that 
resin flow does not bypass any portion of the preform and trap air within 
the preform, and the use of a sufficient number of vents in the mold, both 
of which can be ascertained through generally routine experimentation. 
However, unique to the process of this invention is the requirement to 
reduce the viscosity of the resin, as discussed above, to enable the use 
of an injection pressure of not higher than about one atmosphere, 
preferably about 7 kPa to about 100 kPa. According to this invention, the 
latter parameters are necessary to ensure that the resin system will 
successfully permeate the preform, regardless of its architecture, without 
damaging the preform by entraining the preform in the resin flow. 
The ability to successfully impregnate and wet a preform having a complex 
architecture with a thermosetting polyimide resin system was previously 
unknown and unexpected. Specifically, the high viscosity of polyimide 
resins and the high volume of volatile reaction byproducts generated 
during cure have been impediments to the use of polyimide resins in resin 
transfer molding techniques. Prior art resin transfer molding techniques 
have required pressures in excess of one atmosphere to distribute resin 
and collapse voids within the resin to an acceptably small size, with 
pressure being maintained thereafter during cure. In contrast, the process 
of this invention employs two stages. The first stage is the resin 
transfer imidization step described above, in which the resin is 
distributed throughout the preform with the assistance of a solvent, and 
at a pressure of one atmosphere or less. Thereafter, and as will be 
described in more detail below, a final cure is used to remelt and 
consolidate the polyimide resin under pressure so as to allow for further 
redistribution of the resin within the fiber architecture. An important 
feature of this invention is that the dilution of the resin system and its 
injection into the mold cavity constitute the only exposure risk to MDA. 
Once the mold cavity is filled with the diluted resin system, heat is 
applied to the mold cavity in order to evaporate the solvent and return 
the resin system to a suitably less diluted state. In practice, a 
temperature of about 65.degree. C. to about 95.degree. C. is preferred. To 
promote evaporation and removal of the solvent, a vacuum can be applied to 
the mold cavity through a venting port. As the solvent is evaporated, it 
can be condensed and collected in a vessel to more accurately determine 
the amount of solvent that has been removed. After removal of the solvent, 
or at least once the evaporation rate of the solvent has slowed, it will 
be desirable under most circumstances to replace at least part of the 
volume lost to evaporation of the solvent with additional resin. This can 
be done by injecting an additional quantity of the diluted resin system 
into the mold cavity after the heating step, and then reheating the mold 
cavity to evaporate the solvent introduced into the mold cavity by the 
additional quantity of diluted resin. This step can be repeated if 
necessary to achieve adequate fill of the mold cavity. Alternatively, a 
reservoir cavity attached to the mold may be used to automatically 
replenish the volume lost to evaporation of the solvent. 
Imidization of the polyimide resin monomers is then performed by closing 
the injection ports, applying a vacuum to the mold cavity so as to remove 
the methyl alcohol and water condensation byproducts of imidization, and 
heating the mold cavity until a temperature of about 200.degree. C. to 
about 215.degree. C. is attained. The mold cavity is preferably heated in 
a controlled manner over a period of about four hours, more or less 
depending on the size of the part, in order to appropriately eliminate the 
condensation byproducts at a rate which can be accommodated by the mold 
equipment. Though variations are possible, a preferred heating schedule 
for the PMR-15 polyimide resin with methanol as the solvent is as follows. 
The mold is heated slowly from the injection temperature (about room 
temperature up to about 35.degree. C.) up to a temperature of about 
65.degree. C. to about 95.degree. C. over a period of about two to four 
hours. An optimal heating rate will depend on the area of the interface 
between the liquid and gaseous phases in the particular mold. Preferably, 
the heating rate is as high as can be achieved without causing the resin 
to froth and be carried out through the mold vents. A temperature of about 
65.degree. C. to about 95.degree. C. is preferably maintained until most 
(at least about 95 weight percent) of the solvent has evaporated. If a 
second or subsequent injection will be done to achieve adequate fill of 
the mold cavity, the mold should first be cooled to the injection 
temperature as quickly as possible, generally about two to about five 
.degree.C./minute. The injection and evaporation steps can be repeated as 
often as necessary. Once the mold cavity is adequately filled, the mold is 
heated as quickly as possible without frothing the resin (again, about two 
to about five .degree.C./minute) to the final imidization temperature of 
about 200.degree. C. to about 215.degree. C. 
The imidized component must then be cured by heating the component to a 
temperature sufficient to remelt the polyimide resin system, and then 
applying pressure and additional heat to cause the polyimide resin system 
to cross-link to produce the desired composite component. The curing step 
may be performed in a separate compression mold, in an autoclave or, if 
properly adapted, in the mold cavity in which the imidization process is 
conducted. Curing generally requires maintaining a temperature of up to 
about 315.degree. C. while maintaining a pressure of about 1.5 to about 7 
MPa (about 200 to about 1000 psi). As with the imidization process, the 
curing process requires a controlled heating procedure. While variations 
are possible, the basic and preferred cure parameters for the PMR-15 
polyimide resin with methanol as the solvent are as follows. The imidized 
preform is heated to about 230.degree. C. to about 260.degree. C. as 
quickly as possible without causing large temperature gradients within the 
mold cavity. A pressure of about 1.5 to about 7 MPa (about 200 to about 
1000 psi) is then applied over a period of about one to ten minutes, while 
heating continues to a final temperature of about 315.degree. C., which is 
held for about one to two hours. During this stage, the polyimide resin 
cross-links. 
The advantage of using a separate mold is to improve the throughput of the 
molding and fabrication process. Notably, the complexity, finish and 
strength of the mold used for final cure is not required for the 
imidization process. If a compression mold is used for the curing process, 
the imidized component must be carefully placed in the mold cavity while 
aligning the geometric features of the component and mold cavity to assure 
proper fit. The mold is then closed, heat is applied until the imidized 
resin system remelts, and the cure process described above is followed. 
Use of an autoclave may be advantageous for large axisymmetric components 
or if only a small number of components are required. As with the 
compression mold process, the autoclave process requires carefully 
positioning the imidized component on the mold surface while aligning the 
geometric features of the component to the mold surface. For axisymmetric 
components, the core of the injection mold may be suitable for use as the 
autoclave mold. The component is then covered in a conventional manner 
with an appropriate release, breather and bagging materials, and the mold 
and component are then inserted into the autoclave where the above final 
cure pressure and temperature cycles are performed. 
If the injection mold used in the imidization cycle is employed as the cure 
mold, special attention must be given to gate and vent locations, the 
manner in which the gates and vents are closed during compression, and the 
manner in which position control of the mold halves is maintained. 
Notably, final cure pressures are substantially higher than injection 
pressures, and may therefore dictate the design of the injection mold if 
used for both procedures. The use of the injection mold for final cure may 
be advantageous if the anticipated production volume of the component is 
small and the purchase of both an injection mold and a final cure mold is 
unnecessary for production rate requirements. 
Following cure, the composite component can undergo a post cure operation 
so as to achieve further cross-linking of the polyimide resin system, and 
thereby raise its glass transition temperature. A preferred post cure 
cycle involves heating the component to a temperature of about 310.degree. 
C. to about 320.degree. C. The temperature of the component is preferably 
ramped at a rate of about 2.5.degree. C./min to about 250.degree. C., then 
at a rate of about 1.degree. C./min to about 315.degree. C., which is then 
held for about twenty-four hours. The component is then cooled at a rate 
of about 2.5.degree. C./min to about 65.degree. C., at which point the 
component is ready for further processing, including trimming, machining 
and inspection of the component. 
The method of this invention results in a fiber-reinforced polyimide matrix 
component characterized by a void content of less than about three 
percent, and a high temperature service capability of up to about 
290.degree. C. As such, the component has desirable structural properties 
that make it particularly suited for structural applications in the 
aerospace industry. Advantageously, suitable reinforcement fiber preforms 
include those having complex architectures, including two or 
three-dimensional braided, knitted, woven and filament wound. The 
imidization process of this invention ensures that the polyimide resin 
system will successfully permeate such complex preforms, a capability 
which was previously unknown and unexpected. 
In practice, the process of this invention has successfully employed the 
PMR-15 resin system to produce composite panels whose mechanical and 
thermal properties were essentially identical to that of otherwise 
identical composite panels produced by the prior art method employing 
uncured PMR-15 prepregs. Notably, the panels produced according to this 
invention exhibited less loss in mechanical and thermal properties, as 
compared to panels produced by the prior art method, after environmental 
exposure that included thermal oxidative stability and thermal cycling. 
In view of the above, it can be seen that a significant advantage of the 
present invention is that exposure to the uncured polyimide resin system 
is limited to the period in which the resin system is diluted until the 
resin system is injected into the mold cavity. In particular, the 
time-consuming layup of uncured prepreg required by prior art methods is 
completely avoided. As a result, and an important feature of this 
invention, the exposure risk to MDA associated with this invention is 
substantially less than that of the labor intensive procedure of the prior 
art. 
Another advantage of this invention is that the process minimizes shipping 
and storage costs by limiting the cold storage requirement to only the 
nonimidized resin system. The resin system can be stored in bulk packaging 
so as to further reduce the required storage space for the material. In 
contrast, the prior art required cold storage of a nonimidized prepreg, 
about fifty volume percent of which is composed of reinforcement fiber. 
Scrappage of nonimidized material and the requirement for proper disposal 
is also eliminated or at least significantly reduced by the process of 
this invention. Most waste generated by the invention will be nonhazardous 
and significantly less in volume than that previously associated with 
processing of thermosetting polyimide resin prepregs. 
While our invention has been described in terms of a preferred embodiment, 
it is apparent that other forms could be adopted by one skilled in the 
art, such as the use of other thermosetting polyimide resin systems or 
solvents, or by modifying the preferred imidization and cure methods by 
altering temperatures and durations, substituting other processing steps, 
or including additional processing steps. Accordingly, the scope of our 
invention is to be limited only by the following claims.