NASA CASE NO. LAFI_SD20_--

PRINT FIG.

NO_TJCE

The invention disclosed in this document resulted from research in aeronautical

and space activities performed under programs of the National Aeronautics and Space

Administration. The invention is owned by NASA and is, therefore, available for

licensing in accordance with the patent licensing regulations applicable to U.S.

Government-owned inventions (37 CFR 404.1 et SeCl.).

To encourage commercial utilization of NASA-owned inventions, it is NASA

policy to grant licenses to commercial concerns. Although NASA encourages

nonexclusive licensing to promote competition and achieve the widest possible

utilization, NASA will provide the necessary incentive to the licensee to achieve early

practical application of the invention.

Address inquiries and all applications for license(s) for this invention to the

Technology Applications Team, NASA Langley Research Center, Code 200, Hampton,

Virginia 23681-0001.

Serial No.: 08/299,385

Filed: 09/01/94 LaRC

(NASA-Case-LAR-15020-1) A DIRECT

PROCESS FOR PREPARING

SEMI-CRYSTALLINE POLYIMIDES Patent

Application (NASA. Langley

Research Center) 16 p

_._/z7

N95-20407

Unclas -

0039285
AWARDS ABSTRACT 

A DIRECT PROCESS FOR PREPARING SEMI-CRYSTALLINE POLYIMIDES 

NASA Case Number: LAR 15020-1 

Linear aromatic polyimides are typically prepared by reacting a dianhydride with a 

diamine in the presence of a polar aprotic solvent. The resulting product is a polyamic 

acid which is subsequently imidized to form a polyimide. lmidization can take place 

either through chemical dehydration or through the application of heat. The resulting 

polyimides have exceptional thermal and oxidative stabilities and are useful for such 

applications as adhesives, matrix resins for composites and molding powders. 

By the present invention, a direct process was developed for preparing semi-crystalline 

polyimides. This process involves reacting a dianhydride and a diamine in the 

presence of a polar aprotic solvent to form a mixture. Next, glacial acetic acid is added 

to the mixture at a ratio ranging from 90:10 to 75:25 (polar aprotic solvent : glacial 

acetic acid) to form a solution. The solution is heated from about 11 O0 to about 140°C 

to form a polyimide precipitate. The polyimide precipitate is recovered as a semicrystalline polyimide powder. 

The novelty of this invention lies in the addition of glacial acetic acid to the mixture. 

Glacial acetic acid acts as both a non-solvent and a dehydrating agent in the reaction, 

thus allowing the polyimide to be formed directly from the dianhydride and the diamine 

without first forming the polyamic acid. The nonobviousness is found in that when the 

glacial acetic acid is added at the aforementioned ratios, the polyimides formed by the 

reaction are semi-crystalline. When the ratio lies in favor of the glacial acetic acid, the 

polyimides are amorphous. 

Inventors: Alice C. Chang 

308 Marl Ravine Road 

Yorktown, VA 23692 

SS: 

Employer: Lockheed Eng. & Sci. Co. 

Initial Evaluator: Darrell R. Tenney 

Terry L. St. Clair 

17 Roberts Landing Drive 

Poquoson, VA 23662 

SS: 

NASA LaRC 

Serial No.: 08/299,385 

Filing Date: 9/1/94 
LAR 15020-1 -1- PATENT APPLICATION

A DIRECT PROCESS FOR PREPARING

I

SEMI-CRYSTALLINE POLYIMIDES

5

Cross-Reference to Related Applications

This application is related to co-pending patent application serial

number , filed , entitled "A Solvent Resistant Copolyimide",

and co-pending patent application serial number , filed ,

entitled "Copolyimides Prepared from ODPA, BTDA and 3,4'-ODA".

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Origin of the Invention

The invention described herein was jointly made by an employee of

the United States Government and in the performance of work under a NASA

Contract and is subject to the provisions of Section 305 of the National

Aeronautics and Space Act of 1958, as amended Public Law 85-568 (72 Stat.

435; 42 U.S.C. 2457).

20

Background of the Invention

1. Field of the Invention

The present invention relates to a process for preparing polyimides.

particular, it relates to a direct process for preparing semi-crystalline

polyimides.

In

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2. DescriDtion of the Related Art

Typically, linear aromatic polyimides are prepared by reacting a

dianhydride with a diamine in the presence of a polar aprotic solvent. The

resulting product is a polyamic acid which is subsequently imidized to form a

polyimide. Imidization can take place either through chemical dehydration or

through the application of heat. The resulting polyimides have exceptional

thermal and oxidative stabilities and are useful for such applications as

adhesives, matrix resins for composites and molding powders.
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LAR 15020-1 -2- PATENT APPLICATION

Despite these properties, many of these polyimides are diIficult to

process into useful form because of their limited melt-flow properties. Most of

the existing polyimides require processing temperatures approaching 400°C

and pressures near 10,000 psi in order to sinter to form a well consolidated

part. Thus, in order for linear aromatic polyimides to be useful as adhesives,

matrix resins and molding powders, it is desired that the polymers exhibit flow

at lower temperatures and pressures.

St. Clair (U.S. 5,061,783) sought to achieve this through providing a

two-part process for converting the polyamide-acid precursors of polyimides

to polyimides which exhibit maximum levels of crystallinity, so that these

crystalline forms can then be melted at relatively moderate temperatures to

afford adequate flow for molding and consolidation operations. This process

involves first dissolving a polyamide-acid in an amide solvent, or a mixture of

an ether and an amide solvent, at low percent solids, followed by treating this

solution with an aprotic organic base, such as triethylamine or pyridine. The

polyamide-acid employed is the reaction product of 3,3',4,4'-benzophenonetetracarboxylic dianhydride and an aromatic diamine, especially 3,3'-diaminobenzophenone prepared in diglyme, or 4,4'-oxydianiline prepared in dimethyl

acetamide, or 3,3'-diaminodiphenylsulfone prepared in diglyme. Finally, this

I

solution is treated with an organic dehydrating agent. St. Clair found that the

solubility of the resulting polyimide in the reaction mixture affects the resulting

degree of crystallinity. Also, the time that the dehydrated polymer is in

contact with the final solution affects the degree of crystallinity.

An object of the present invention is to provide a direct process for

preparing semi-crystalline polyimides directly from a mixture of a dianhydride

and a diamine.

Another object of the invention is to provide a direct process for

preparing semi-crystalline polyimides which may be processed at

temperatures just above their melting points.
LAR 15020-1 -3- PATENT APPLICATION

Another object of the invention is to form finely divided polyimide

particles directly from the reaction mixture.

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Summary_ of the Invention

By the present invention, the foregoing and additional objects are

attained by providing a direct process for preparing semi-crystalline

polyimides. This process comprises providing a polar aprotic solvent; adding

a dianhydride and a diamine to the solvent to form a mixture; stirring the

mixture at ambient temperature; adding glacial acetic acid to the mixture to

provide a ratio of polar aprotic solvent to glacial acetic acid which ranges from

about 90 to 10 to about 75 to 25 by volume to form a solution. The solution is

then heated to a range from about 110 ° to about 140°C to form a polyimide

precipitate. The polyimide precipitate is recovered as a semi-crystalline

polyimide powder.

In addition to the steps given above, the stoichiometry of the reaction

may be offset in order to control the final molecular weight of the polyimide.

When there is a molar deficiency in the dianhydride, a monofunctional

aromatic anhydride, such as phthalic anhydride, may be added to the mixture

or to the solution in order to compensate. Likewise, if there is a molar deficit

I

in the diamine, a monofunctional aromatic amine may be added to the

mixture or to the solution to compensate.

25

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Description of the Preferred Embodiments

The process of the present invention requires the use of a polar aprotic

solvent. Examples of this solvent include but are not limited to: N-methylpyrrolidinone(NMP), N,N-dimethyl acetamide (DMAc) and gammabutyrolactone (GBL). The preferred solvent is NMP.

A dianhydride and a diamine were added to the solvent to form a

mixture. The process will work with any dianhydride but in particular those

selected from the group consisting of: isophthaloyldiphthalic anhydride
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LAR 15020-1 -4- PATENT APPLICATION

(IPDA), 3,3'4,4'-benzophenonetetracarboxylic dianhydride (BTDA) and 4,4'-

oxydiphthalic anhydride (4,4'-ODPA). The most preferred dianhydride is 4,4'-

ODPA. Similarly, any diamine may be employed in the process of the present

invention but the preferred diamines include those selected from the group

consisting of: m-phenylene diamine (m-PDA), 3,4'-oxydianiline (3,4'-ODA)

and 3,3'-diaminodiphenylsulfone (3,3'-DDS). 3,4'-ODA is the most preferred

diamine.

The stoichiometry for the mixture may be balanced or offset depending

on the final desired molecular weight of the polyimide. By offsetting the

stoichiometry, a molar deficit of up to about 5 mole percent may exist for

either the dianhydride or the diamine. A molar deficit in the dianhydride was

compensated for by adding up to about 10 mole percent of a monofunctional

aromatic anhydride (endcapper) to the mixture. In a most preferred

embodiment, up to about 10 mole percent of phthalic anhydride (PA

endcapper) was added to the mixture to compensate for a molar deficit in the

dianhydride. In a like manner, a molar deficit in the diamine is compensated

for by adding up to about 10 mole percent of a monofunctional aromatic

amine (endcapper) to the mixture. The monofunctional aromatic anhydride or

amine may be added to either the mixture (prior to the addition of glacial

I

acetic acid) or the solution (after the addition of glacial acetic acid). When the

endcapper was added to the mixture, it was found that the mixture must first

be stirred for one hour at ambient temperature prior to adding the endcapper.

After the endcapper was added, the mixture had to be stirred for two more

hours at ambient temperature prior to the addition of glacial acetic acid.

Glacial acetic acid (GAA) was added to the mixture to form a solution.

The ratio of polar aprotic solvent to GAA ranges from about 90 to 10 to about

75 to 25 by volume. When the polyimides were prepared at a ratio of 50 to

50 or with the ratio in favor of the GAA (25 to 75) they were not semicrystalline, but instead, where amorphous. In a preferred embodiment, the

ratio of polar aprotic solvent to GAA is 90 to 10 by volume. In a most
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LAR 15020-1 -5- PATENT APPLICATION

preferred embodiment, NMP is selected as the polar aprotic solvent and GAA

is added to provide a ratio of 90 to 10 (NMP to GAA) by volume. It was found

that the GAA acts as both a dehydrating agent and as a non-solvent for the

formation of the semi-crystalline polyimide.

When the stoichiometry was offset and the endcapper was added to

the solution (after the addition of the GAA), favorable results were obtained

when the solution was stirred for one hour at ambient temperature. After the

monofunctional aromatic anhydride or amine was added to the solution, the

solution was stirred for two more hours at ambient temperature prior to the

application of heat.

In order for the polyimide to be formed, the solution was heated to a

temperature ranging from about 110°C to about 140°C. The addition of heat

caused the formation of a polyimide precipitate. The preferred temperature.

for this reaction is about 120°C. It was observed that at this temperature, the

polyimide began to precipitate from the solution after only one hour.

Once the reaction was complete, the polyimide precipitate was

recovered as a semi-crystalline polyimide powder. Any method known to

those skilled in the art may be employed in recovering the polyimide powder.

An example of one method which was used to recover the polyimide involved

i

collecting the precipitate by filtration, stirring it in hot ethanol for several hours

and drying it under vacuum at a temperature of 180°C overnight. Another

method involved merely filtering the precipitate from the solution and allowing

it to dry at ambient temperature under vacuum.

The following examples are merely illustrative of the invention and are

intended to enable those skilled in the art to practice the invention in all of the

embodiments flowing therefrom, and do not in any way limit the scope of the

invention as defined by the claims. The results for examples 1 - 12 are

summarized in Table 1.
LAR 15020-1 PATENT APPLICATION

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Example 1

Isophthaloyldiphthalic anhydride (IPDA) (4.9626g, 0.0116 mole, 3 mole

percent stoichiometric offset) was added to a stirred solution of m-phenylene

diamine (m-PDA) (1.2978g, 0.012 mole) in a 50/50 (by volume) solvent

mixture comprising NMP (17 ml) and GAA (17 ml) under a nitrogen

atmosphere. The solids content was 15% by weight. The solution was

stirred for one hour at ambient temperature. Phthalic anhydride (PA)

(0.1066g, 0.0007 mole, 6 mole percent) was added to the solution and stirring

was continued at ambient temperature for two more hours. The solution was

then heated to 120°C and maintained at that temperature for three hours. It

was observed that the polyimide began to precipitate from the solution after

one hour at 120°C. The precipitated polyimide was collected by filtration,

stirred in hot ethanol for two hours, and dried under vacuum at 180°C

overnight. An infrared spectrum showed that the imidization was complete.

The polymer had a glass transition temperature (Tg) of 253.2°C and the x-ray

diffraction pattern showed that the polymer was amorphous.

Example 2

The procedure of example 1 was followed to prepare a polyimide from

3,3',4,4'-benzophenonetetracarboxylic dianhydride (BTDA) (3.1257g, 0.0097

mole, 3 mole percent offset) and 3,3'-diaminodiphenylsulfone (DDS)

(2.4830g, 0.01 mole) in a 50/50 (by volume) solvent mixture of NMP (15 ml)

and GAA (15 ml). Phthalic anhydride (0.0889g, 0.0006 mole, 6 mole percent)

was added to the solution as an endcapper. The resulting polyimide had a Tg

of 262.3°C. The infrared spectrum showed that the imidization was complete

and the x-ray diffraction pattern showed that the polyimide was amorphous.

3O
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LAR 15020-1 -7- PATENT APPLICATION

Example 3

The procedure of example 1 was followed to prepare a polyimide from

4,4'-oxydiphthalic anhydride (ODPA) (3.0092g, 0.0097 mole, 3 mole percent

offset) and 3,4'-oxydianiline (3,4'-ODA) (2.0024g, 0.01 mole) in a 50/50 (by

volume) solvent mixture of NMP (15 ml) and GAA (15 ml). Phthalic anhydride

(0.0889g, 0.0006 mole, 6 mole percent) was added to the solution as an

endcapper. The resulting polyimide had a Tg of 209.6°C. The infrared

spectrum showed that the imidization was complete and the x-ray diffraction

pattern showed that the polyimide was amorphous.

Example 4

The procedure of example 1 was followed to prepare a polyimide from

ODPA (3.0092g, 0.0097 mole, 3 mole percent offset) and 3,4'-ODA (2.0024g,

0.01 mole) in a 75/25 (by volume) solvent mixture of NMP (21 ml) and GAA

(6.8 ml). Phthalic anhydride (0.0889g, 0.0006 mole, 6 mole percent) was

added to the solution as an endcapper. The resulting polyimide had an

enthalpy of 38.9 J/g, a melting temperature (Tm) of 290.2°C and a Tg of

231.4°C. The x-ray diffraction pattern showed that the polyimide was semicrystalline.

Example 5

A polyimide was prepared by adding IPDA (12.7767g, 0.0233 mole, 3

mole percent offset) to a mixture of m-PDA (2.5955g, 0.024 mole) in NMP (53

ml) in the presence of a nitrogen atmosphere. The mixture was stirred at

ambient temperature for one hour and phthalic anhydride (0.2559g, 0.00173

mole, 7.2 mole percent) was added. The mixture was stirred for an additional

two hours at ambient temperature. GAA (17 ml) was added to the mixture to

yield a NMP to GAA ratio of 75 to 25 (by volume) and forming a solution. The

solution was heated to 120°C and the temperature was maintained for 10

hours. The polyimide precipitate was filtered from the solution. The resulting
LAR 15020-1 -8- PATENT APPLICATION

polyimide had a Tg of 248.5°C, a Tm of 299.9°C and an enthalpy of 28.9 Jig.

The x-ray diffraction pattern indicated that the polyimide was semi-crystalline

and the infrared spectrum showed that the imidization was complete.

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Example 6

The procedure of example 5 was followed to prepare a polyimide from

ODPA (24.0733g, 0.0776 mole, 3 mole percent offset) and 3,4'-ODA

(16.0194g, 0.08 mole) in NMP (201 ml). Phthatic anhydride (0.71 lOg, 0.0048

mole, 6 mole percent) was added to the mixture after it had stirred for one

hour at ambient temperature. Stirring at ambient temperature was continued

for two more hours after the addition of phthalic anhydride. GAA (22 ml) was

added to the mixture to yield a NMP to GAA ratio of 90 to 10 (by volume) and

the solution was heated to 120°C and maintained at that temperature for 3

hours. The polyimide was recovered by filtration. The polyimide powder had

a Tg of 234.1°C, a Tm of 298.9°C, and a melt viscosity of 6,431 poise at

350°C. The x-ray diffraction pattern indicated that the polyimide was semicrystalline.

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Example 7

I

The procedure of example 6 was followed to prepare a polyimide

where the stoichiometric offset was 2 mole percent for the ODPA and the

phthalic anhydride endcapper was added at 4 mole percent. The ratio of

NMP to GAA was maintained at 90 to 10 (by volume). The resulting

polyimide had a Tg of 235.7°C and a Tm of 300.5°C. A portion of the

polyimide powder was dried at ambient temperature for 10 hours. The x-ray

diffraction pattern showed that the polyimide was semi-crystalline.

3O

Example 8

A polyimide was prepared by adding ODPA (57.081g, 0.184 mole) to a

mixture of 3,4'-ODA (40.0484g, 0.2 mole) and NMP (400 ml). The mixture
5

LAR 15020-1 -9- PATENT APPLICATION

was stirred overnight under nitrogen at ambient temperature. GAA (130 ml)

was added to the mixture to provide a NMP to GAA ratio of 75 to 25 (by

volume) and the solution was heated to 120°C for three hours. It was

observed that the polyimide began to precipitate from the solution after the

solution had been heated for two hours. The polyimide powder was

recovered from the solution. The resulting polyimide powder had a Tg of

217.6°C, a Tm of 278.4°C and the x-ray diffraction pattern showed that the

polyimide was semi-crystalline.

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Example 9

The procedure of example 8 was followed to prepare an anhydride

terminated polyimide from IPDA (85.2688g, 0.2 mole), m-PDA (20.5474g,

0.19 mole), NMP (436 ml) and GAA (167 ml) (NMP to GAA ratio was 75 to 25

by volume). The reaction time at 120°Cwas 24 hours. The resulting

polyimide had a Tg of 242.9°C and the x-ray diffraction pattern showed that it

was semi-crystalline.

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Example 10

The procedure of example 4 was followed to prepare theksame

polyimide but DMAc was substituted for NMP as the solvent. The solvent

ratio was 75 to 25 by volume (DMAc to GAA). All other variables and

amounts remained the same. The polyimide powder had a Tg of 226.8°C, a

Tm of 294.0°C and an enthalpy of 39.4°J/g. The polyimide was semicrystalline as determined by x-ray diffraction.

Example 11

The procedure of example 7 was followed to prepare the same

polyimide but the solvent NMP was replaced with DMAc. The solvent ratio

was 90 to 10 by volume (DMAc to GAA). All other variables and amounts

remained the same. The polyimide powder had a Tg of 230.8°C, a Tm of
LAR 15020-1

297.8°C and an enthalpy of 33.3 J/g.

determined by x-ray diffraction.

-10- PATENT APPLICATION

The polyimide was semi-crystalline as

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Example 12

The procedure of example 6 was followed to prepare a polyimide

having a 3 mole percent stoichiometric offset for the ODPA and 6 mole

percent of phthalic anhydride endcapper. The NMP was replaced with

gamma-butyrolactone (GBL) to provide a ratio of 90 to 10 by volume (GBL to

GAA). The resulting polyimide powder had a Tg of 229.6°C, a Tm of 295.2°C

and an enthalpy of 33.2 J/g. The infrared spectrum showed that the

imidization was complete and the x-ray diffraction pattern showed that the

polyimide was semi-crystalline.

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Example 13

Neat resin moldings were prepared from the polyimide powder of

example 6. Ten grams of the polyimide were placed in a matched metal die

mold. The mold was heated to 300°C and maintained for 30 minutes at 0 psi.

Next, the mold was pressurized to 1000 psi and the temperature was raised

to 330°C where it was held for 1.5 hours. The mold was then cooled to room

temperature. The neat resin had a fracture toughness (G,o) of 4956 J/m 2 at

room temperature.

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Example 14

Neat resin moldings were prepared from the polyimide powder of

example 7. Ten grams of the polyimide were placed in a matched metal die

mold. The mold was heated to 300°C and maintained for 30 minutes at 0 psi.

Next, the mold was pressurized to 1000 psi and the temperature was raised

to 330°C where it was held for 1.5 hours. The mold was then cooled to room

temperature. The neat resin had a fracture toughness (G,o) of 6860 J/m 2 at

room temperature.
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LAR 15020-1 PATENT APPLICATION

A DIRECT PROCESS FOR PREPARING

SEMI-CRYSTALLINE POLYIMIDES

5

10

Abstract of the Disclosure

The invention is a direct process for preparing semi-crystalline

polyimides. This process comprises the steps of: providing a polar aprotic

solvent, adding a dianhydride and a diamine to the solvent to form a mixture,

stirring the mixture at ambient temperature, and adding glacial acetic acid to

the mixture to provide a ratio of porar aprotic sotvent to gtaciat acetic acid

which ranges from about 90 to 10 to about 75 to 25 by volume to form a

solution. The solution was heated to a range from about 110°C to about

140°C to form a polyimide precipitate. The polyimide precipitate was

recovered as a semi-crystalline polyimide powder.

I