Polyimide of 2,2-bis(4-(4-aminophenoxy)phenyl)-hexafluoropropane and process for the preparation of same

A polyimide and a process for its preparation wherein the polyimide is prepared from reaction of an organic tetracarboxylic acid or derivative thereof with a mixture of an aromatic diamine and an amine-terminated fluoroether. The polyimides of the invention are particularly useful in the preparation of flexible foams.

This invention relates to new polyimides and more particularly to closed 
cell polyimide foams which are hydrolytically stable, have improved heat 
resistance, and possess a majority of cells which are closed resulting in 
improved vapor barrier properties. 
It is known, as is described in U.S. Pat. Nos. 3,966,652 and 4,315,076, 
that polyimides can be produced which are hydrolytically stable and which 
have improved fire resistance properties, giving off essentially no smoke 
or toxic fumes when exposed to flame. 
Foams of the type there described are useful in aircraft cabins, space 
vehicles, land and sea transport equipment, as well as other applications 
where relatively non-flammable and negligible smoke-emitting 
characteristics are desirable. 
In the practice of the prior art as described above, such polyimide foams 
are prepared by reacting an alkyl diester of a tetracarboxylic acid with 
one or more aromatic and/or heterocyclic diamines. Typically, a 
tetracarboxylic dianhydride is reacted with methanol or ethanol to form 
the corresponding diester which is then reacted with the amine or amines 
to form the corresponding polyamide-acid/ester which can then be converted 
to a polyimide by further heating. As a general rule, a cellular structure 
is developed, because both water and a lower alkyl alcohol corresponding 
to the alcohol portion of the ester is generated in situ during the 
reaction. These volatile materials produce open cell polyimide foams which 
are flexible and resilient and have fine, homogeneous cellular structure. 
While amines of the type described above represent a significant advance in 
the art, their properties when used in insulation in applications 
involving high humidity are not as great as might be desired. 
Specifically, foams produced in accordance with the foregoing concepts 
possess an open cellular structure and therefore do not have significantly 
high vapor-barrier qualities to facilitate their use in, for example, 
fire-resistant, anti-sweat submarine hull insulation. 
It is known, as described in U.S. Pat. Nos. 4,203,922 and 4,111,906, that 
2,2-bis(4-(4aminophenoxy) phenyl) hexafluoropropane can be reacted with 
tetracarboxylic acid dianhydrides to produce polyimides. However, the use 
of that ether alone is too expensive for use in the preparation of 
polyimide foams. 
It is accordingly an object of the present invention to provide polyimides 
which are capable of being foamed and which can be used in applications in 
high humidity to provide vapor-barrier qualities and high temperature 
resistance. 
It is a more specific object of the invention to provide a polyimide foam 
and a process for the preparation of the same to produce polyimide foams 
having fine homogeneous closed cellular structure which are flexible, 
resilient, and which can provide significantly improved vapor-barrier 
characteristics and high temperature resistance. 
The concepts of the present invention reside in a polyimide foam and a 
process for its preparation in which the polyimide is prepared by reaction 
of an organic tetracarboxylic acid or corresponding anhydride or ester 
derivative thereof with a combination of diamines including at least one 
organic aromatic diamine and an amine terminated fluoro ether. It has been 
found that the combination of diamines described provides foams which have 
a good closed cellular structure, which are flexible and resilient even at 
low temperatures and which provide high vapor-barrier characteristics and 
significantly improved high temperature resistance. 
As the amine-terminated fluoro ether, use is preferably made of an ether 
having the formula: 
##STR1## 
Such amine is chemically known as 2,2-bis(4-(4aminophenoxy) phenyl) 
hexafluoropropane and its preparation is described in the foregoing 
patents. 
As the organic aromatic diamine, use is preferably made of one or more 
aromatic and/or heterocyclic diamines which are themselves known to the 
art. Such aromatic diamines can be represented by the structure: 
EQU H.sub.2 N--R--NH.sub.2 
wherein R is an aromatic group containing 5 to 16 carbon atoms and 
containing up to one hetero atom in the ring, the hetero atom being 
selected from the group consisting of --N--, --O-- and --S--. Also 
included herein are those R groups wherein R is a diphenylene group or a 
diphenylmethane group. Representatives of such diamines include: 
2,6-diaminopyridine 
3,5-diaminopyridine 
3,3'-diaminodiphenyl sulfone 
4,4'-diaminodiphenyl sulfone 
4,4'-diaminodiphenyl sulfide 
3,3'-diaminodiphenyl ether 
4,4'-diaminodiphenyl ether 
meta-phenylene diamine 
para-phenylene diamine 
p,p'-methylene dianiline 
2,6-diamino toluene 
2,4-diamino toluene 
It is sometimes preferred to use two of the foregoing amines, one amine 
being a carbocyclic aromatic diamine and the second amine used in 
combination with it being one of the foregoing aminopyridines. 
In the preferred practice of the invention, the organic tetracarboxylic 
acid in the form of its diester from methanol or ethanol is reacted with 
the combination of diamines to form a polyamide-acid/ester prepolymer 
which can then be foamed and cured, either alone or in the presence of a 
blowing agent, to provide the desired polyimide foam. 
The tetracarboxylic acids or derivatives thereof preferably employed in the 
practice of the invention are those having the general formula: 
##STR2## 
wherein A is a tetravalent organic group and R.sub.1 to R.sub.4, 
inclusive, are each selected from the group consisting of hydrogen and 
lower alkyl, and preferably methyl, ethyl or propyl. The tetravalent 
organic group A is preferably one having one of the following structures: 
##STR3## 
wherein X is one or more of the following: 
##STR4## 
Preferred among the tetracarboxylic acid and derivatives thereof is 
3,3',4,4'-benzophenone tetracarboxylic acid and its corresponding lower 
alkyl (preferably lower dialkyl) esters. 
It is also possible, and sometimes desirable in the preparation of the 
polyamide-acid/ester prepolymer, to include in the reaction mixture one or 
more aliphatic diamines. Such aliphatic diamines are preferably 
alpha-omega diaminoalkanes having the general formula: 
EQU HN.sub.2 --(CH.sub.2).sub.a --NH.sub.2 
wherein a is an integer from 2 to 16. Representative of such diamines 
include 1,3-diaminopropane, 1,4-diaminobutane, 1,5-diaminopentane, 
1,6-diaminohexane, etc. 
In place of the foregoing aliphatic amines, use can also be made of the 
polyamines marketed by Texaco Chemical Company under the trademark 
JEFFAMINE. Those amines can be described as polyoxypropylene amines, and 
have the formula: 
EQU NH.sub.2 CH(CH.sub.3)CH.sub.2 --OCH.sub.2 CH(CH.sub.3)--.sub.x NH.sub.2 
wherein x varies from 1 to about 5. 
In accordance with one preferred embodiment of the invention, the 
polyamide-acid/ester prepolymer is formed by reaction of a dialkyl ester 
of the organic tetracarboxylic acid with the amines described above. The 
relative proportions used in the preparation of the prepolymer can be 
varied to relatively wide limits to provide good foams. In general, it is 
preferred to employ between 0.5 to 1.5 total moles of amine per mole of 
the tetracarboxylic acid dianhydride or ester derivative thereof. The 
aromatic amine generally constitutes, when used, between 10 and 95 mole % 
of the total amount of the acid or ester used. The molar concentration of 
the amine-terminated ether generally ranges from 5 to 90 mole %. 
When the aliphatic amine is employed, it is generally used in an amount 
ranging from 1 to 40 percent based on the mole of acid ester employed. 
In the preparation of the prepolymer, the tetracarboxylic acid derivative, 
usually in the form of the diester, is reacted with the diamines at a 
temperature below the reflux temperature of the reaction mixture. Without 
limiting the invention as to theory, it is believed that the prepolymer is 
formed of a low-molecular weight polyamide-acid/ester which can then be 
heated to complete the polymerization reaction. The prepolymer can thus be 
in the form of a liquid or a solid having a low-molecular weight, so long 
as it is capable of being converted by further reaction to a 
high-molecular weight polyimide polymer. 
When using a lower alkyl ester of the tetracarboxylic acid, the resulting 
alcohol produced in the reaction as well as the water released during the 
reaction can be used as the blowing agent during polymerization to form 
the desired polyimide foams. Alternatively, use can be made of any of a 
variety of blowing agents, such as the solid blowing agents described in 
co-pending application Ser. No. 532,663, filed Sept. 16, 1983, the 
disclosure of which is incorporated herein by reference. 
As there described, the homogeneity of the cellular structure of the 
resulting polyimide foam can be more accurately controlled by controlling 
the particle size of the solid blowing agent, when such a blowing agent is 
employed. It has been found that the particle size of the solid blowing 
agent is preferably less than 200 microns in diameter, with 98 percent of 
the blowing agent particle sizes being less than 150 microns in diameter. 
In the practice of the invention, it is possible to include in the reaction 
mixture various filler and/or reinforcing materials. For example, 
graphite, glass and other synthetic fibers can be added to the composition 
to produce a fiber-reinforced product. It is frequently desirable to 
employ a surfactant, thereby increasing bubble stability and the 
uniformity of the cellular structure, to increase fatigue resistance of 
the foam and make the foam more flexible and resilient. Such surfactants 
are themselves conventional, and form no part of the present invention. 
The temperatures at which the prepolymer is converted to the polyimide 
polymer are generally those temperatures used in the preparation of other 
polyimide polymers. As a general rule, temperatures ranging from 
200.degree. to 400.degree. C. can be used, with heating times ranging from 
5 to 60 minutes or longer. As those skilled in the art will appreciate, 
the time for carrying out the reaction is somewhat dependent on the 
reaction temperature, higher temperatures enabling the use of shorter 
reaction times. It is also possible to heat to a lower temperature in the 
first stage of the reaction and then higher temperatures in the later 
stages. 
Heating can be carried out in a conventional oven if desired. 
Alternatively, the foaming and curing of the prepolymer into a polyimide 
polymer can be effected by means of microwave heating. It has been found 
that the prepolymer can be converted to the corresponding polyimide foam 
by exposing the prepolymer for 1 to 120 minutes to radio frequencies 
within the range 915 to 2450 mHz, with the power output ranging from 1 to 
100 kw. The power output to prepolymer weight ratio generally reads from 
about 0.1 to 10 kw per kg. 
Having described the basic concepts of the invention, reference is now made 
to the following example which is provided by way of illustration, but not 
by way of limitation, of the practice of the invention.

EXAMPLE 
This example illustrates the practice of the present invention in the use 
of the amine-terminated fluoroether in the preparation of polyimide foams. 
A reaction mixture is formulated of 3,3'4,4'-benzophenone tetracarboxylic 
acid dimethyl ester (BTDA), 2,6-diaminopyridine (DAP), p,p'-methylene 
dianiline (MDA) and 2,2-bis(4-(4-aminophenoxy)phenyl)-hexafluoropropane 
(BDAF) in molar % concentrations of 5, 10, 20, 30, 40, and 50 based on the 
number of moles of the BTDA. The molar proportions of each reaction 
mixture are shown below: 
______________________________________ 
SAMPLE BTDA DAP MDA BDAF 
______________________________________ 
A 1.0 0.3 0.65 0.05 
B 1.0 0.3 0.6 0.10 
C 1.0 0.3 0.5 0.20 
D 1.0 0.3 0.4 0.30 
E 1.0 0.3 0.3 0.40 
F 1.0 0.3 0.2 0.50 
______________________________________ 
The resulting reaction mixtures are then heated to 60.degree. C. for 180 
minutes to produce a solid prepolymer. 
Thereafter, the prepolymer is heated at 300.degree. C. to form a series of 
foam polyimides. The foams produced were fine-cell, homogeneous flexible 
foams having resistance to high temperatures. 
It will be understood that various changes and modifications can be made in 
the details of procedure, formulation and use without departing from the 
spirit of the invention, especially as defined in the following claims.