A low temperature processable, thermoplastic polyimide and methods for making and using the same. The polyimide has repeating polymer units of the formula ##STR1## wherein n is 2 to about 20 and the molecular weight of the polymer chain is about 5,000 to about 50,000. The polymer is made by reacting 4,4'-(hexafluoroisopropylidene)bis(o-phthalic anhydride) with a diamine having the formula H.sub.2 N(CH.sub.2).sub.n NH.sub.2 wherein n is 2 to about 20 to form a polyamic acid. The polyamic acid is imidized to form the polyimide described above. The polyimide is particularly adapted to use as a hot melt adhesive by placing it between two articles and applying heat and pressure.

CROSS-REFERENCE TO RELATED APPLICATIONS 
Attention is directed to commonly assigned copending application, A Low 
Temperature Processable, Moisture Resistant, Polyimide and Composite, D. 
Scola and R. Pater, Ser. No. 685,937 filed on even date herewith, which 
discloses material similar to that used in the present application, the 
disclosure of which is incorporated by reference. 
TECHNICAL FIELD 
The field of art to which this discovery relates is polyimide particularly 
adapted to use as hot melt adhesives and methods of making the same. 
BACKGROUND ART 
Modern aerospace technologies have placed increasing demands on polymer 
technology. Polymeric materials used in this environment should exhibit a 
variety of physical properties such as low temperature processability, 
strength, moisture resistance and solvent resistance. Although polymeric 
compounds exist that exhibit one or more of the above physical properties, 
these materials generally do not have the desired combination necessary 
for many applications. 
For instance, the popular commercial epoxies provide good strength as 
adhesives yet they are very susceptible to moisture which causes swelling. 
Swelling results in dimensional changes which may cause application 
problems where tight tolerances are required. In addition, swelling 
results in the loss of mechanical properties such as strength. Another 
class of polymers, polysulfones, exhibit good thermoplastic processing 
characteristics and good strength yet they are not very resistant to many 
solvents including moisture. By contrast polyimides generally exhibit poor 
processability by typical thermoplastic techniques but provide good 
resistance against a number of solvents. However, even polyimides 
generally absorb moisture resulting in swelling and loss of mechanical 
properties. 
Accordingly, there is a continual search in the art for polymeric compounds 
exhibiting improved physical properties. 
DISCLOSURE OF INVENTION 
This discovery is directed to a low temperature processable, thermoplastic 
polyimide material. The polymeric compound exhibiting these physical 
characteristics has repeating polymer units of the formula 
##STR2## 
wherein n is 2 to about 20 and the molecular weight of the polymer chain 
is about 5,000 to about 50,000. 
Another aspect of this invention is a method of making such a thermoplastic 
polyimide by reacting 4,4'-(hexafluoroisopropylidene)bis(o-phthalic 
anhydride) with a diamine having the formula H.sub.2 N(CH.sub.2).sub.n 
NH.sub.2 where n is 2 to about 20. These compounds react to form the 
intermediate polyamic acid. Next, the polyamic acid is imidized to form 
the polyimide described above. 
Yet another aspect of this invention is the use of a polyimide as a hot 
melt adhesive. Two articles can be securely bonded together at 
temperatures below about 190.degree. C. A polyimide having such a 
repeating polymer unit and molecular weight as described above is placed 
between the two articles to be bonded and pressure and heat are supplied 
to bond the articles firmly together. 
This discovery provides a synthetic polymer with significantly improved 
physical properties for a variety of applications including the aerospace 
industries. By incorporating a hexafluoroisopropylidene moiety into a 
polyimide, a polymer has been developed that exhibits for example, easy 
low temperature processing. These improved characteristics and others are 
important for the applications for which these resins have their most 
popular uses. 
Other features and advantages will be apparent from the specification and 
claims which describe an embodiment of the invention. 
BEST MODE FOR CARRYING OUT THE INVENTION 
4,4'-(hexafluoroisopropylidene)bis(o-phthalic anhydride) hereinafter 
referred to as 6F dianhydride has been available commercially and can also 
be made as set forth in U.S. Pat. No. 3,310,593 and Belgium Pat. No. 
649,366, the disclosures of which are incorporated by reference. Briefly, 
the 6F dianhydride can be synthesized by a multistep process. Ortho-xylene 
available from Aldrich Chemical Company and hexafluoroacetone 
sesquihydrate available from I.C.N. Pharmaceuticals, Inc. react in the 
presence of the Friedel-Crafts catalyst, hydrogen fluoride available from 
Matheson Gas Products, Inc. to form 
4,4'-(hexafluoroisopropylidene)bis(o-xylene). Upon isolation this compound 
can be oxidized with nitric acid or potassium permanganate to form the 
corresponding tetra acid. The tetra acid can be dehydrated to form the 
corresponding dianhydride, 4,4'-(hexafluoroisopropylidene)bis(o-phthalic 
anhydride) commonly referred to as 6F dianhydride. 
The alkane diamines utilized in this invention have the formula H.sub.2 
N(CH.sub.2).sub.n NH.sub.2 where n is 2 to about 20 and are commercially 
available generally from Aldrich Chemical Company under the names 
.alpha.,.omega.-diamino-alkanes. It is preferred that n is about 10 to 
about 14 and especially preferred that n is 12 because it is believed that 
adhesion and the glass transition temperature (Tg) both decrease as the 
alkyl content of the polymer unit increases. The inclusion of one or more 
aliphatic or unsaturated diphatic diamines other than disclosed, e.g. 1, 
2, diamino propane, 1, 2 diamino 2-methylpropane to the extent that they 
do not detract substantially from the desirable results obtained with the 
stated reactants are contemplated in the formation of these prepolymers 
and polymers. 
The polyimide resin of this disclosure hereinafter referred to as poly 6F 
diimide has repeating units of the formula 
##STR3## 
wherein n is 2 to about 20. It is preferred that n is about 10 to about 14 
and especially preferred that n is 12. The molecular weight of the resin, 
which can also be referred to as the prepolymer, is about 5,000 to 50,000. 
Also, depending on the solvents, initiators and inhibitors used, the 
polymer chains may have various functional end groups, such as a solvent 
molecule. 
After isolation of the above resin, further processing (exposing the 
polymer to heat and pressure for instance, as a hot melt adhesive), 
increases the molecular weight of the polyimide while retaining the same 
repeating polymer unit. The molecular weight of the polymer chain is such 
that the polyimide absorbs below about 0.20 percent (%) by weight moisture 
at room temperature. Percent by weight moisture refers to moisture 
absorbed (gms) divided by initial dry polymer weight multiplied by one 
hundred. At this molecular weight the polyimide has a Tg of about 
150.degree. C. It is difficult to determine a molecular weight number as 
the polyimide is not soluble in many conventional solvents used in 
molecular weight determination procedures, for example molecular weight 
measurement by an intrinsic viscosity measurement. It is believed that the 
molecular weight is greater than 50,000 as that is typically the minimum 
necessary to achieve minimum moisture absorption of about 0.1 to 2% 
weight. It is also believed that the molecular weight is less than 200,000 
as molecular weights above 200,000 are difficult to achieve. It is also 
believed that the polymer exhibits liquid crystal orientation. Liquid 
crystals are crystals in which the polymer units are arranged in parallel 
arrays to form an ordered pattern of molecules which exhibit 
crystalline-like properties, e.g. high strength, stiffness. 
Polyamic acid can be synthesized by mixing a solution of the 6F dianhydride 
with a solution of the diamine having the formula H.sub.2 
N(CH.sub.2).sub.n NH.sub.2 where n is about 2 to about 20 to produce the 
polyamic acid intermediate by an amidization process. Empirically this can 
be illustrated as: 
##STR4## 
The polyamic acid can be imidized to form the polyimide resin of this 
disclosure. Imidization refers to the cyclic condensation (dehydration) of 
the amide and acid group to an imide by for example the addition of heat. 
Empirically this can be illustrated as: 
##STR5## 
Thus, 6F dianhydride is dissolved in a 1 part chloroform to 3 parts acetone 
solution. The diamine is dissolved in a chloroform solution. Other 
suitable solvent(s) may be substituted in varying proportions for the 
above solvents. The two solutions are then mixed together, preferably, by 
adding the 6F dianhydride solution over time, about 25 to 30 minutes to 
the diamine solution at temperatures conventional in the art including 
room temperature. Upon mixing, the monomers amidize to form polyamic acids 
of particular molecular weights. 
It will be known to those skilled in the art to vary the process parameters 
to obtain polymer chains having the desired physical properties. These 
process parameters include addition time, temperature, pressure, solvents, 
monomer proportions, concentrations, initiators, inhibitors, etc. For 
instance if the two monomers are combined in a 6F dianhydride to diamine 
molar proportion of 1.0 to 1.05, a higher molecular weight occurs than if 
they are combined in 1 to 1 molar ratios. Conversely a 6F dianhydride to 
diamine molar proportion of 1.20 to 1.0 results in polymers of lower 
molecular weights than one to one ratios. Depending on the choice of 
specific process parameters the process specifications may have to be 
adjusted but these changes are conventional in the art. 
The polyamic acid intermediate formed in the chloroform-acetone solution 
precipitates out as a white solid which can be filtered and air dried. 
This polyamic acid intermediate is dissolved in a suitable solvent such as 
.beta.-methoxyethanol (Cellosolve.TM. solvent, Union Carbide) and refluxed 
for about two hours. The poly 6F diimide product can be isolated by, for 
example, aqueous precipitation followed by filtration and drying. The 
resultant polyimide has a molecular weight of about 5,000 to about 50,000.

EXAMPLE 1 
A solution of 6F dianhydride (113.6 grams (g), 0.25 mole) in a 
chloroform-acetone 250 milliliters (ml)/750 ml was added dropwise to a 
solution of 1,12 diamino dodecane (47.5 g, 0.238 mole) in chloroform (200 
ml) at room temperature over a period of one-half hour. The polyamic acid 
intermediate, which precipitated out, was filtered and air dried yielded a 
white solid (159 g). The polyamic acid intermediate was dissolved in 200 
ml cellosolve and heated to reflux for two hours. After cooling, water was 
added to precipitate the polymer. An orange-brown solid, 
poly(N,N'-dodecamethylene)-4-(hexafluoroisopropylidene diphthalimide) 
hereinafter referred to as 6F-1,12-DDA was filtered from the solution and 
dried to yield about 155 g (97-99% yield) 6F-1,12-DDA. 
Poly 6F diimides have a variety of uses including their use as adhesives 
and coatings. The polymer may be utilized in its neat form or, in 
conjunction with other fillers, additives and compounds that impart the 
desired properties and economics to the product. For example, it can be 
used in a preform laminate for a thermoforming structure. The resin can be 
melted to form tough films analogous to Kapton.TM. or Mylar.TM. films 
(DuPont de Nemours, E. I., Company) for use in similar applications. 
This resin is also an effective hot melt adhesive. It need only be placed 
between two articles under pressures of about 25 pounds per square inch 
(psi) to about 200 psi to bind them together when exposed to temperatures 
less than about 250.degree. C., preferably less than 190.degree. C., and 
more preferably about 170.degree. C. over a period of less than about two 
hours, preferably about five minutes to about twenty minutes. After this 
processing the resin is further polymerized to the higher molecular weight 
form described earlier. The minimum thickness of polymer required to 
obtain good bonding is six mils. Typically the resin is applied to the 
article to be bonded (adherend) after being dissolved to about 25-50% by 
weight in a suitable solvent such as chloroform or cellosolve. Virtually 
any articles can be secured together including for instance, the bonding 
of panels to door compartments in helicopters. Other typical aerospace 
applications include the bonding of an airfoil skin to the body frame, and 
airfoil skins to aerodynamic structures such as propellers. 
EXAMPLE 2 
61.35 g of the polyimide prepared in the previous example was dissolved to 
107.6 g of chloroform, and further diluted with 16.4 g methylethyl ketone 
yielding a 49% solids solution. Two aluminum strips (adherends) one inch 
by four inches by 0.25 inch were etched with chromic acid and immediately 
thereafter, a half inch section of each was coated with enough polyimide 
resin solution to form a thickness of ten mils. The solvent was evaporated 
off at room temperature and then further dried at 50.degree. C. under 
vacuum conditions for one hour. Next, the strips were joined. Following 
this the adherends are placed in a press preheated to 177.degree. C. for 
15 minutes at 25 psi yielding an adhesive joint of about 6 mils in 
thickness. After cooling the adhesive joint resulted in the tensile lap 
shear strengths described in Table I. 
Poly 6F diimide exhibits a combination of desirable qualities unavailable 
in other polymer systems. The following text, data and explanations 
illustrate these properties and compare them to the properties of other 
typical commercial polymeric materials. The poly 6F diimide utilized in 
the following data is 6F-1,12-DDA polyimide, a polymer having repeating 
units of the formula 
##STR6## 
and a molecular weight of about 5,000-50,000. 
This polyimide can be processed to the higher molecular weight polymer 
quickly typically at temperatures and pressures of about 190.degree. C. 
and about 25 psi. Other polymers used in these applications usually 
require higher temperatures, pressures, and longer process times. For 
instance, epoxy processes at about 210.degree. C., and about 100 psi in 
about 2-3 hours. 
6F-1,12-DDA polyimide resin exhibits easy low temperature processing to the 
high molecular weight form. However, it does not achieve this at the 
expense of mechanical properties. Overall the resin has equivalent or 
better adhesive properties when compared to other typical commercial 
resins. For example, Table I illustrates the resin's tensile lap shear 
strength in comparison to that of other commercial polymers in its 
application as a hot melt adhesive. Tensile lap shear strength is a 
measure of the adhesive strength of the adherend-adhesive interface or a 
measure of the cohesive strength of the adhesive, depending on where 
failure occurs. The footnotes are important as they illustrate that 
although strenths may be roughly equivalent the polyimide resin is the 
resin of choice because of the combination of other properties that it 
exhibits. 
TABLE I 
______________________________________ 
Adhesive Properties of Hot Melt 
6F-1,12-DDA Resin and Other Adhesives 
Tensile Lap 
Shear Strength.sup.1 
RT 82.degree. C. 
100.degree. C. 
psi psi psi 
______________________________________ 
6F-1,12-DDA 7694.sup.2 5086.sup.2 
6F-1,12-DDA 5221.sup.2 4240.sup. 
Epoxy .TM. 4000-6000.sup.3 
(Ciba-Geigy 
Corp.) 
Acrylate .TM. 
3000-6000.sup.4 
(Loctite Corp.) 
P1700 .TM. 
(Union Carbide 
Corp.) 
Polysulfone 3500.sup. 2700 
______________________________________ 
.sup.1 Chromic acid etched aluminum adherends. 
.sup.2 Processed at 177.degree. C., 15 minutes. 
.sup.3 Required 2-3 hours processing. 
.sup.4 Poor moisture resistance. 
Table II illustrates the moisture absorption properties of 6F-1,12-DDA 
polyimide in the high molecular weight form in comparison with other 
commercial materials. It is clear that this polymer makes a significant 
advancement in the art as the other polymers absorb at least three times 
as much moisture. As was explained in the Background Art section the 
absorption of moisture is a significant problem as it results in swelling 
and the loss of mechanical properties. 
TABLE II 
______________________________________ 
Moisture Absorption Properties 
of High Molecular Weight 6F-1,12-DDA 
Compared with Commercial Products 
Wt %.sup.1 Moisture Absorbed at 
Room Temperature Moisture 
Exposure Time 
Resins 24 hrs. 48 hrs. 
______________________________________ 
6F-1,12-DDA 0.19 0.27 
Epoxy 
3501-6 .TM. (Hercules 
1.20 
Inc).sup.2 
5208 .TM. (U.S. Polymeric).sup.3 
1.50 
Polyimide PMR-15 .TM. (NASA).sup.4 
0.60 
______________________________________ 
.sup.1 Moisture absorbed (gms) divided by initial dry weight multiplied b 
one hundred. 
.sup.2 Value from Augl, J. M. Moisture Sorption and Diffusion in Hercules 
35016 Epoxy Resin, Naval Service Weapons Center White Oak Laboratory 
Technical Report (NSCW/WOL TR) 7939, March 30, 1979. 
.sup.3 Value from Augl, J. M. and Bergen, A. W. The Effect of Moisture on 
Carbon Fiber Reinforced Epoxy Resin Composites, NSWC/WOL TR 767, Septembe 
23, 1975. 
.sup.4 Actual test by inventor. 
Typical polymers for the above-described applications may come in contact 
with other solvents besides water. If a polymer degrades upon exposure to 
a particular solvent, that polymer system is precluded from use in 
applications where the solvents are present. Table II shows the effect of 
a variety of solvents on high molecular weight 6F-1,12 DDA and a 
polysulfone P1700, a typical commercial polymer used in the application 
described above. Clearly the polyimide is a superior material as it is 
less sensitive to many of the solvents tested. 
TABLE III 
______________________________________ 
Solvent Effects on High Molecular 
Weight 6F-1,12-DDA and P1700 
P1700 
Solvent Polysulfone 
6F-1,12 DDA 
______________________________________ 
Jet Propulsion swells no charge 
Fuel JP-5 
n-hexane no change no change 
isopropanol no change no change 
mineral oil swells no change 
Delco Supreme II .TM. 
swells no change 
brake fluid 
(General Motors Co.) 
Transmission swells no change 
Fluid 
(Monarch Co.) 
chloroform dissolves swells & dis- 
integrates 
(still solid) 
Toluene dissolves swells & dis- 
integrates 
(still solid) 
Methylethylketone 
dissolves swells 
N--methylpyrroldinone 
dissolves swells & dis- 
integrates 
o-dichlorobenzene 
dissolves swells & dis- 
integrates 
(still solid) 
______________________________________ 
The properties and characteristics of the polyimide relate to its structure 
and composition. Those skilled in the art will understand that the 
above-described physical properties can vary depending on molecular 
weight, molecular weight distribution, functional end groups, etc. These 
properties can be tailored to the particular use. 
This discovery advances the field of polymeric technology by providing a 
polyimide that has a unique combination of desired properties. This 
polyimide can be processed quickly and at reduced temperatures and 
pressures in comparison to most other commercial resins. In addition, its 
substantial moisture and solvent resistance make it suitable for a wide 
variety of applications as an adhesive where solvents in either gaseous or 
liquid form are present. 
Significantly, poly 6F diimides attain these characteristics without 
sacrificing mechanical properties. Thus, in the high molecular weight form 
its strength is comparable if not better than many other polymers that are 
typically used for the same applications. By incorporating a 
hexafluoroisopropylidene moiety into the polymer backbone a polyimide has 
been developed, for use as coatings and adhesives that exhibit desirable 
characteristics required for many applications. 
Although this invention has been shown and described with respect to 
detailed embodiments thereof, it will be understood by those skilled in 
the art that various changes in form and detail thereof may be made 
without departing from the spirit of the claimed invention.