CA1131399A

CA1131399A - Polyimides

Info

Publication number: CA1131399A
Application number: CA319,961A
Authority: CA
Inventors: John Gagliani
Original assignee: International Harverster Corp
Current assignee: Navistar Inc
Priority date: 1978-02-13
Filing date: 1979-01-19
Publication date: 1982-09-07
Also published as: GB2014166A; GB2014166B; FR2416914B1; FR2416914A1; DE2903347C2; DE2903347A1

Abstract

ABSTRACT OF THE DISCLOSURE
Polyimide foams derived from mixtures of aromatic diamines and partial esters of a benzophenonetetracarboxylic acid. The precursors are converted into polyimide foams by the application of heat; and the foams are then compressed, yielding dense, rigid, and structurally strong, intumescent materials.

Description

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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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11;~1399 .
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.

1:~31399 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 _3_ Il 11;~1399 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.

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

-5-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.

-6-~31399 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.

B i 11313~9 . .

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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Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. 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 and tetracarboxylic acid ester constituents; heating said resinous precursor to a temperature in the range of 210-315°C to produce a polyimide foam artifact of selected configuration; and then heating said artifact under a pressure in the range of 3-20 psig and at a temperature in the range of 230-315°C for a time sufficient to produce a permanent set in said foam and thereby increase the rigidity and structural strength of the artifact.

2. The process defined in Claim 1 which includes the step of dispersing a filler composed of particulate solids in the composition containing the aromatic diamine and tetra-carboxylic acid ester constituents prior to heating said composition to prepare the resinous precursor.

3. The process defined in Claim 2 wherein the filler comprises graphite fibers.

4. The process defined in Claim 1, Claim 2 or Claim 3 wherein, in heating the foam artifact under pressure, it is maintained at a temperature approximating the highest temperature reached in converting the precursor to the foam.

5. The process defined in Claim 1, Claim 2 or Claim 3 in which the foam is heated under pressure in a press and in which said press is preheated prior to processing the foam artifact therein.

6. The process defined in Claim 1, Claim 2 or Claim 3 in which the polyimide foam artifact is heated under pressure for a period ranging from 3 to 10 minutes.

7. The process defined in Claim 1, Claim 2 or Claim 3 in which any tetracarboxylic acid ester present in the precursor is a C1-C3 ester of 3,3',4,4'-benzophenonetetracarboxylic acid and the diamines are selected from the group consisting of 3,3'-diaminodiphenyl sulfone, 4,4'-diaminodiphenyl sulfide, 3,3'-diaminodiphenyl ether, 4,4'-diaminodiphenyl ether, meta-phenylenediamine, para-phenylenediamine, 2,6-diaminopyridine, and 3,5-diaminopyridine.

8. The process defined in Claim 1, Claim 2 or Claim 3 together with the step of adding to the precursor before it is foamed from 0.1 to one part, based on the weight of the ester and diamine constituents, of a surfactant which is effective to control the pore size and/or cellular structure of the foam artifact into which said precursor is to be converted.

9. The process defined in Claim 1, Claim 2 or Claim 3 in which the tetracarboxylic acid ester constituent comprises at least one C1-C3 diester of 3,3',4,4'-benzophenonetetra-carboxylic acid.

10. The process defined in Claim 1, Claim 2 or Claim 3 together with the step of applying the precursor to a substrate before said precursor is foamed.

11. A copolyimide foam made by a process which includes the steps of: preparing a resinous precursor by forming a compo-sition which is essentially a stoichiometric mixture of aromatic diamine and tetracarboxylic acid ester constituents and dispersing a filler composed of particulate solids in said composition; heating said resinous precursor to a temperature in the range of 210-315°C
to produce a polyimide foam artifact of selected configuration; and then heating said artifact under a pressure in the range of 3-20 psig and at a temperature in the range of 230-315°C for a time sufficient to produce a permanent set in said foam and thereby increase the rigidity and structural strength of the artifact.

12. A copolyimide foam as defined in claim 11 which has an essentially homogeneous physical structure.

13. A copolyimide foam as defined in claim 11 which has a hard, high density skin enveloping a core of materially lower density.

14. A copolyimide foam as defined in claim 11 and made by a process as therein described wherein, in heating the foam arti-fact under pressure, it is maintained at a temperature approximating the highest temperature reached in converting the precursor to the foam.

15. A copolyimide foam as defined in claim 11 and made by a process as therein described in which the foam artifact is heated under pressure in a press and in which said press is pre-heated prior to processing the foam therein.

16. A copolyimide foam as defined in claim 11 and made by a process as therein described in which the foam artifact is heated under pressure for a period ranging from 3 to 10 minutes.

17. A copolyimide as defined in claim 11 in which any tetracarboxylic acid ester present in the precursor thereof is a C1-C3 ester of 3,3',4,4'-benzophenonetetracarboxylic acid and the diamines are selected from the group consisting of 3,3'-diamino-diphenyl sulfone, 4,4'-diaminodiphenyl sulfide, 3,3'-diamonodiphenyl ether, 4,4'-diamonodiphenyl ether, meta-phenylenediamine, para-phenylenediamine, 2,6-diaminopyridine, and 3,5-diaminopyridine.

18. A copolyimide foam as defined in claim 11 and made by a process as therein described in which there is added to the precursor before it is foamed from 0.1 to one part, based on the weight of the ester and diamine constituents, of a surfactant which is effective to control the pore size and/or cellular structure of the foam artifact into which said precursor is to be converted.

19. A copolyimide foam as defined in claim 11 in which the tetracarboxylic acid ester constituent comprises at least one C1-C3 diester of 3,3',4,4'-benzophenonetetracarboxylic acid.

20. A copolyimide foam as defined in claim 11 wherein the filler comprises graphite fibers.

21. 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, said material being composed 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 said polyimide.

22. A material as defined in Claim 21 which has an essentially homogeneous physical structure.

23. A material as defined in Claim 21 which has a hard, high density skin enveloping a core of materially lower density.

24. A material as defined in Claim 21 in which any tetracarboxylic acid ester present in the polyimide is a C1 - C3 ester of 3,3', 4,4' -benzophenonetetracarboxylic acid and the diamines are selected from the group consisting of 3,3' -diaminodiphenyl sulfone, 4,4' -diaminodiphenyl sulfide, 3,3' -diaminodiphenyl ether, 4,4' -diaminodiphenyl ether, meta-phenylenediamine, para-phenylenediamine, 2,6-diaminopyridine, and 3,5-diaminopyridine.

25. A material as defined in Claim 21 in which the polyimide includes from 0.1 to one part, based on the weight of the ester and diamine constituents, of the residue of a surfact-ant which is effective to control the pore size and/or cellular structure of the foam artifact into which said precursor is to be converted.

26. A material as defined in Claim 21 in which the tetracarboxylic acid ester constituent comprises at least one C1 - C3 diester of 3,3',4,4' -benzophenonetetracarboxylic acid.

27. A copolyimide foam as defined in Claim 21 wherein the filler comprises graphite fibers.