Note: Descriptions are shown in the official language in which they were submitted.
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PO~.Yl~IDJ~S
The present invention relates to polyimides and, more
specifically, to novel polyimide foams which are rigid, struc-
turally strong, and intumescent.
U.S. Patent No. 3,9G6,652 issued June 29, 1976, to
Gagliani et al for ~..T~IOD ~ND ~KING FOAM~D COPOLYI~IIDES ~D
P~ODUCT OBT~I~ED TilF,}~FROM discloses copolyimide foams having
a number of desirable properties; for example, they are struc-
turally stable at elevated temperatures yet remain flexible
and resilient at cryogenic tempcratures.
The polyimide foams disclose~ in the Gagliani et al
patent are improvements on those described in U.S. Patent ~lo.
3,726,834 issued April 10, 1973, to Acle, Jr. for Tl~ L~STIC
COPOLYI~IID~S .
The primary object of the present invention resides
in the provision o:E novel copolyimide foams having ccrtain
desirable properties which are present to a significantly
greater extent than they are, i at all, in the foams disclosed
in Patents Nos. 3,726,334 and 3,96G,652 and to novel methods
for making those novel foams.
Related and also important, but more specific, objects
of the invention reside in the provision of foams in accord
with the preceding object:
j which are structurally strong to the extent that
they can be used in floor, wall, and ceiling panels, fire
doors, electrical boxes, and other applications where struc-
tural strcngth is a requisite;
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which are intumescent or fire-containing;
which have a high degree of rigidity;
which have various combinations of the foxegoing
attributes.
Like those descri~ed in Patents Nos. 3,726,834 and
3,966,652, the novel foams of the present invention are pre-
pared from precursors which are solid state solutions of a
Cl-C3 alkyl ester of 3,3', 4,4'-benzophenonetetracarboxylic
acid or mixture of such esters and two or more aromatic diamines
which are fre~e of aliphatic moieties. At least one of the
diamines must be meta-substituted, and any diamines which are
not meta-substituted must be para-substituted. Also, the
imide-forming functionalities (the amino and carboxylic moieties~
should be present in substantially equimolar amounts.
Exemplary of the diamines which may be employed are:
3,3'-diaminodiphenyl sulfone
4,4'-diaminodiphenyl sulfide
4,4'-diaminodiphenyl sulfone
3,5-diaminopyridine
2,6-diaminopyridine
3,~'-diaminodiphenyl ether
4,4'-diaminodiphenyl ether
m-phenylenediamine
p-phe~ylenediamine
, !
The monomeric precursors are prepared by first reactin
3,3',4,4'-benzophenonetetracarboxylic acid, or preferably, its
dianhydride and an esterfying agent to form an alkyl diester.
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The preferred esterfying agents are methyl, e~hyl, propyl,
and isopropyl alcohols (other alkyl alcohols can also be used
as the esterfying agent-solvent. Changing the alkyl group of
the esterfying agent effects the curing rate of the product
and properties associated with the resinous nature of the
material such as tackiness, drying time, etc.). Ethanol is
in many cases preferred because of its widespread availability,
low cost, lack of toxicity and other attributes.
The esterification reaction is followed by the addi-
tion of the aromatic diamines, which are allowed to dissolve
in the reaction mixture, the temperature being kept below the
reflux temperature of the esterfying agent and low enough to
avoid polymerization. Excess alcohol can be removed from the
resulting product at reduced pressure until it becomes a thick
syrup.
Graphite and/or other fibers and fillérs can be added
to the resulting composition to impart wanted properties to
the final product. A surfactant can also be added with stirring
to control the pore size and/or the cellular structure of the
foam which will ultimately be made. From 0.1 to 10 parts by
weight of surfactant for each 100 parts of resin constituent
can be employed for this purpose.
One suitable surfactant is Union Carbide L-5420 sili-
cone surfactant. That company's L-5410 and 1,-530 surfactants
are also suitable as are various silicone surfactants available
from Dow Chemical and General ~lectric.
Any excess solvent remaining after the dissolution
of the diamines is removed by drying the viscous composition
at a temperature in the ran~e of ca. 76-104C. This leaves an
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amorphous resinoid which can be converted to a copolyimide
foam by heatiny it to a temperature in the range of ca. 230-
¦ 315C for ca. 15-30 minutes. In a typical application of the
¦ present invention the viscous composition existing after the
¦ dissolution of the diamines will be coated onto a metallic or
non-metallic substrate and then dried and foamed as just des-
cribed in a single, one-step operation.
In accord with the present invention the foam resulting
l from the heating of the resinoid precursor is compressed by
¦ applying a pressure of 3-20 psig to it, preferably in a preheated
mold, at a temperature generally equal to the maximum tempera-
ture reached in the foaming step and typically on the order
of 315C. The application of the pressure is continued until
a permanent set of the foam is achieved. The amount of ~ime
this will require will depend upon the dimensions of the work-
piece; but it will, in general, be between three and ten
minutes.
The novel post-treatment ~ust described produces
l a strong, rigid foam which has the important advantage that it
¦ is capable of keeping fire from spreading. This is of obvious
. -importance in the protection of structures and equipment and
in the preservation of human and other animal life.
If longer periods of application of pressure (i.e.,
those tending toward 10 minutes) are employed, the product
foam will have a hard, high density skin enveloping a low
density core. This composite construction is advantageous for
many applications of my invention.
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In applications where a skin is not wanted, the
pressure is applied for a shorter period, viz., one tending
toward three minutes. This results in the product foam being
of uniform density throughout.
Instead of burning, the novel foams of the present
invention form a fire resistant, surface char when subjected
to intense heat. ~urthermore, the surface char forms without
producing smoke or toxic byproducts unlike conventional cellu-
losic or plastic insulating materials. This characteristic
is of obvious importance in applications involving the presence
of human or other animal life in closed or artificially sup-
ported environments.
Also, the novel polyimide foams disclosed herein
remain rigid and structurally intact in the presence of intense
heat. This makes them significantly superior as fire contain-
ment barriers to the metallic, ceramic, cellulosic, plastic,
and glassy materials conventionally used for this purpose as
the latter soften, or melt, and collapse under the same condi-
tions causing catastrophic failures and allowing fire to
propagate.
Thus, the invention contemplates a strong, high
density cellular material which is rigid and resistant to
intense heat sources without distortion, loss of foam integrity,
or formation of smoke or toxic gases, and the material is com-
posed of a permanently set polyimide containing essentially
stoichiometric amounts of aromatic and/or heterocyclic diamine
and tetracarboxylic acid constituents and a filler composed of
particulate solids uniformly dispersed in the polyimide.
The invention also contemplates a process of making
a rigid, intumescent, polyimide foam which comprises the steps
of preparing a resinous precursor by forming a composition
which is essentially a stoichiometric mixture of aromatic diamine
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and tetracarboxylic acid ester constituents, heating the
resinous precursor to a temperature in the range of 210-315C
to produce a polyimide foam artifact of selected configuration,
and then heating the artifact under a pressure in the range of
3-20 psig and at a temperature in the range of 230-315C for
a time sufficient to produce a permanent set in the foam and
thereby increase the rigidity and structural strength of the
- artifact.
That inventive process can also include the step of
dispersing a filler composed of particulate solids in the
composition containing the aromatic diamine and tetracarboxylic
acid ester constituents prior to heating the composition to
prepare the resinous precursor.
In a further embodiment, the invention comprehends a
copolyimide foam made by a process which includes the steps
of preparing a resinous precursor by forming a composition which
is essentially a stoichiometric mixture of aromatic diamine
and tetracarboxylic acid ester constituents and dispersing a
filler composed of particulate solids in the composition, heating
the resinous precursor to a temperature in the range of 210-315C
to produce a polyimide foam artifact of selected configuration,
and then heating the artifact under a pressure in the range of
3-20 psig and at a temperature in the range of 230-315C for a
time sufficient to produce a permanent set in the foam and
thereby increase the rigidity and structural strength of the
artifact.
Certain important objects of the present invention have
been identified above. Other important objects and advantages
and additional novel features of the present invention will be
apparent to those skilled in the relevant arts from the foregoing
general description of the invention, from the appended claims,
and from the following example, which is intended to illustrate
and not restrict the scope of the invention.
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3,3',4,4'-Benzophenonetetracarboxylic acid dianhydride
(322.2 g 1.0 M) was dissolved in 330 ml of reagent grade ethanol
by reflu~ing the mixture for 30-60 minutes. To the solution
was added 124.1 g of 4,4'-diaminodiphenyl sulfone (0.5~ and
54.6 g of 2,6-diaminopyridine (0.5M). That mix~ure was refluxed
for 15-30 minutes. L-5420, a Union Carbide silicone surfactant,
11.8 g, was added to the batch which was then stirred for one
hour.
To a 300 g portion of the resulting syrup composition
was added 90 g of 1/4 inch long graphite fibers and 90 g reagent
grade ethanol. The mixture was stirred to insure complete
wetting of the fibers.
The putty-like composition which resulted was dried
by spreading a 0.6 cm (0.25 in.) thick layer of the compo.sition
on an aluminum foil and heating it at 76-104C (170-220F) in
a circulating air oven for 2-16 hours. Foaming was accomplished
by heating the dried resin at 315C (600F) for 30 minutes.
This was followed by a compression of the foam to a
thickness of 1.25-2.5 cm~(0.5-1.0 in.) in a press preheated at
315C (600F). Seven pounds of pressure (gage) was applied,
and the pressure was maintained for 4 minutes.
The process just described produced a high density
cellular material in which graphite fibers were homogeneously
distributed. The foam was rigid and had exceptional mechanical
properties. It exhibited high resistance to intense heat
sources without distortion, loss of foam integrity, or forma-
tion of smoke or toxic gases.
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The invcntion may be embodiccl in other specific
forms t~ithout departing from the spirit or essential charac-
teristics thereof. The present embodiments are therefore
to be considered in all respects as illustrative and not
restrictive, the scope of the invention being inclicated by
the appended claims rather than by the foregoing description;
and all changes which come within the meaning and range of
equivalency of the claims are therefore intended to be embraced
therein.
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