Production of particulate polyimide polymers

Abstract

Fusible polyimide powders are prepared by the process of this invention. An amic acid amine is first prepared by treating equimolar amounts of a 2,2-bis[4-(aminophenoxy)phenyl]-hexafluoropropane with an unsaturated carbocyclic monoanhydride. The amic acid amine is then mixed with an aromatic tetracarboxylic dianhydride and the mixture heated to form a polyimide precursor. A chemical dehydrating agent is added in an amount sufficient to substantially, fully imidize the precursor. The resulting fusible polyimide powder is finally separated.

Description

FIELD OF INVENTION

This invention relates to production of finely divided polyimide polymers suitable for use in powder coating operations and for forming composites and molded products.

BACKGROUND

Aromatic polyimides are normally prepared from an aromatic tetracarboxylic dianhydride and an aromatic primary diamine. When these materials are reacted at relatively low temperatures in a suitable solvent, typically a dipolar aprotic solvent such as N-methylpyrrolidone or N,N-dimethylacetamide, an aromatic polyamic acid is formed, usually as a viscous solution sometimes referred to as a varnish. When heated to a temperature above about 140.degree. C. imidization occurs such that a polyimide polymer is formed.

It is known from Japan Kokai 57-200452 and Japan Kokai 57-200453 that finely divided aromatic polyimides from a variety of aromatic tetracarboxylic dianhydride and aromatic diamines can be formed by rapidly heating solutions of polyamic acid in a polar organic solvent such as N-methylpyrrolidone, N,N-dimethylformamide, etc. to 160.degree.-300.degree. C. In this way, polyimide powders suitable for use in compression molding are formed from solutions of polymers such as 3,3',4,4'-biphenyltetracarboxylic dianhydride 4,4'-diaminodiphenyl ether polymer, pyromellitic dianhydride 4,4'-diaminodiphenyl ether polymer, and 3,3',4,4'-biphenyltetracarboxylic dianhydride 4,4'-diamininophenylmethane polymer.

In U.S. Pat. No. 3,708,459 the preparation of polyimide molding powders having surface areas between 100 and 900 square meters per gram is disclosed. These powders are obtained from a polymerizate of a polyamic acid prepolymer and an imide prepolymer. The prepolymers result from the reaction of at least one polyfunctional amine, one or more polyfunctional anhydride and nadic anhydride unsubstituted or substituted with lower alkyl. The prepolymer is thermally polymerized to obtain the desired product.

Polyimide polymers based on use of 2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane as the sole or predominant aromatic primary diamine are of considerable interest because of their desirable high temperature properties. However, the production of such polymers in powder form presents a number of problems. Certain prior methods for converting the polyamic acid to the corresponding polyimide yield the polyimide in the form of solids which require extensive grinding. However, substantial portions of the product may not even be amenable to grinding. Other prior methods are fraught with difficulties caused by the tendency of the wet polyimide polymer to agglomerate into a stringy or tacky mass which can foul reactor and agitator surfaces. Further, such tacky masses cannot be removed from the reactor in any commercially practical manner. Moreover, the solvent tends to remain occluded in such swollen, tacky mass.

A desirable contribution to the art would be a process in which such difficulties may be eliminated, or at least greatly reduced.

SUMMARY OF THE INVENTION

This invention provides a process for the preparation, in particulate form, of aromatic polyimides derived from a 2,2-bis[4-(aminophenoxy)phenyl]hexafluoropropane as the sole or predominant aromatic primary diamine component comprises several sequential steps. As Step 1, diamine 2,2-bis[4-(aminophenoxy)phenyl]hexafluoropropane is reacted with an equimolar amount of a carbocyclic monoanhydride. The resulting amic acid amine is then treated with one or more of an aromatic tetracarboxylic acid dianhydride (Step 2). The mole ratio of aromatic diamine to the combination of dianhydride and carbocyclic monoanhydride is about 1:1. As a final step the product of Step 2 is chemically dehydrated to the polyimide.

DESCRIPTION OF THE PREFERRED EMBODIMENT

As one embodiment of the present invention, fusible powders useful for molding are prepared by treating the reaction product of an aromatic tetracarboxylic dianhydride and a monomaleamic acid or mononadamic acid with at least one chemical dehydrating agent sufficient to substantially fully imidize such reaction product. The fully imidized material is obtained as a fusible powder particularly useful in powder moldings.

The fusible powders are prepared in a number of sequential steps, the first being (step 1) the preparation of a monomaleamic acid or mononadamic acid. In this embodiment, the monomaleamic acid or mononadamic acid is formed by treating a solution of substantially equimolar amounts of fluorine-containing aromatic diamine with an unsaturated carbocyclic monoanhydride.

The fluorine-containing aromatic diamine is a 2,2-bis[4-(aminophenoxy)phenyl]hexafluoropropane such as illustrated by 2,2-bis[4-(3-aminophenoxy)phenyl]hexafluoropropane (3-BDAF) and by 2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane (4-BDAF).

The unsaturated carbocyclic monomeric anhydrides of use in this embodiment of the present invention are nadic anhydride (5-norbornene-2,3-dicarboxylic anhydride) and its halogen or C.sub.1 to C.sub.6 linear or branched alkyl derivatives, i.e., ##STR1## where R=C.sub.1 to C.sub.6 linear or branched alkyl, halo or maleic anhydride and its C.sub.1 to C.sub.6 linear or branched alkyl derivatives ##STR2## where R' is the same or different than R described above. Mixtures of the above compounds may also be used. The R and R' groups in these monomeric anhydrides include both mono substituted compounds as well as disubstituted ones. Thus, compounds such as methyl maleic anhydride (citraconic anhydride) or dimethyl maleic anhydride as well as 5-methyl-5-norbornene-2,3-dicarboxylic anhydride or 5,6-dimethyl-5-norbornene-2,3-dicarboxylic anhydride are useful herein.

As illustrated below for two of the monoanhydrides of Step 1 the present invention, the monomaleamic acid or mononadamic acid may be represented by the following structures: ##STR3## where 4-BDAF is the fluorine-containing aromatic diamine and either nadic anhydride or maleic anhydride is the monomeric anhydride.

While the most preferred anhydrides for use in this embodiment of the present invention are nadic anhydride and maleic anhydride, the preferred anhydrides are either the most preferred unsubstituted ones or the most preferred anhydrides substituted with mono or dimethyl, mono or diethyl and mixtures of these anhydrides.

The term "C.sub.1 to C.sub.6 linear or branched alkyl" is intended to include linear groups such as methyl, ethyl, n-propyl, n-butyl, n-pentyl and n-hexyl as well as branched ones such as isopropyl, sec-butyl, neopentyl, 2-methylpentyl and the like.

The term "halo" as used in this invention is intended to mean chloro, fluoro or bromo substituents.

Examples of solvents which are suitable for use in the formation of the precursor include the dipolar aprotic ones such as N,N-dimethylformamide, N,N-dimethylacetamide, N,N-diethylformamide, N,N-diethylacetamide, N-methylpyrrolidone, dimethylsulfoxide, sulfolane, and the like, including mixtures of two or more such solvents.

Other solvents, such as ketones, ethers, and the like may also be included in the solutions formed above, provided that such co-solvents do not prevent the polyimide from precipitating from solution in proper physical form during the course of the ensuing chemical dehydration reaction in step (3).

As the second step, step (2) in the production of the polyimide fusible powders of the present invention, the monomaleamic acid or mononadamic acid prepared as disclosed above is admixed and reacted with an aromatic tetracarboxylic dianhydride.

A feature of this invention is that no reaction intermediate e.g. the reaction product of Step 1, need be recovered or isolated, and the entire reaction can be, and preferably is, conducted in the same reactor, in effect as a one-stage unit operation.

The aromatic tetracarboxylic dianhydrides which may be employed in the process may be represented by the formula

O(OC).sub.2 --A--(CO).sub.2 O

wherein A is an aromatic group. Illustrative compounds of this type include:

pyromellitic dianhydride;

3,3',4,4'-benzophenonetetracarboxylic dianhydride; (BTDA)

2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride;

4,4'-oxydiphthalic anhydride;

2,3,6,7-naphthalenetetracarboxylic dianhydride;

1,2,5,6-naphthalenetetracarboxylic dianhydride;

2,2-bis(3,4-dicarboxyphenyl)propane dianhydride;

1,1-bis(3,4-dicarboxyphenyl)propane dianhydride;

1,1-bis(3,4-dicarboxyphenyl)ethane dianhydride;

bis(3,4-dicarboxyphenyl)methane dianhydride;

3,4,9,10-perylenetetracarboxylic acid dianhydride; and

3',3',4,4'-biphenyltetracarboxylic dianhydride

Mixtures of two or more such illustrative dianhydrides are also useful reactants. Pyromellitic acid dianhydride, 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride, and 3,3',4,4'-benzophenonetetracarboxylic acid dianhydride are particularly preferred reactants.

Typically, the dianhydride is added to the monomaleamic acid or mononadamic acid at an equimolar ratio to that of the unreacted amino group of the monomaleamic acid or mononadamic acid.

It is preferred that the molar ratio of carbocyclic monoanhydride:2,2-bis[4-(aminophenoxy)phenyl]hexafluoropropane:aromatic tetracarboxylic dianhydride be from about 1:1:0.5 to about 1:1.5:1.

Small amounts of the corresponding tetracarboxylic acid monoanhydride may be present in the reaction system, either as the customary impurity in commercial grades of the dianhydride or as a deliberately added component. The amount present usually will not exceed about 20 mol % of the dianhydride. However, the monoanhydride may be used as a total or partial replacement of the dianhydride, if desired.

In lieu of or in addition to the tetracarboxylic dianhydride, use may be made of its acid halides or esters as reactants in the process.

In a preferred embodiment, a homogeneous solution in step (2) above is formed by stirring or otherwise agitating the mixture at a temperature of up to about 100.degree. C. It should be noted that the order of addition of the components to the reactor is critical. Preferably this operation is conducted at ambient temperatures.

As illustrated below for two of the monoanhydrides of the present invention, the polyimide precursors (i.e. polyamic acids) resulting from the completion of Steps 1 and 2 may be represented by the following structures: ##STR4##

The reaction mass resulting from the procedure of step (2) is next subjected to chemical dehydration and imidization (cyclodehydration) step (3). Surprisingly, by chemically imidizing the compounds prepared by this process, complete (substantially about 100%) imidization occurs and no thermal treatment is necessary to form the imidized particulate product.

The chemical imidization of polyamic acid (PAA) is normally carried out by adding dehydrating agent/catalyst mixture to a solution of the PAA to yield isoimide, imide or mixture thereof. The commonly used dehydrating agent is acetic anhydride or other monobasic acid anhydride such as trifluoroacetic anhydride or phthalic anhydride. These anhydride act as water scavengers by reacting with water eliminated from the imidization process. The chemical removal of water, as opposed to deriving it off by heating, appears to lower the activation energy of the imidization process. This allows imidization to take place at a lower temperature. The commonly used catalysts for chemical dehydration are tertiary, aliphatic, aromatic or heterocyclic amines such as pyridine and triethylamine or sodium acetate. A tertiary amine gives ring closure faster by a factor of 2-10 times than that of the free amic acid under the same conditions. However, the nature of the dehydrating agent/catalyst system will affect the results of the cyclization of PAA. In general, a stronger dehydrating agent (e.g. trifluoroacetic anhydride) and a weaker base (e.g. pyridine) favors isoimide formation. To avoid isoimide formation, a stronger base and a weaker dehydrating agent are usually used. Dehydration of a given PAA with acetic anhydride catalyzed by triethylamine or sodium acetate results in the exclusive polyimide formation. See R. J. Angelo, et al, "Cyclization Studies of Amic Acid to Imides and Isomide: Monomeric and Polymeric Reactions." Second International Conference on PI, October 1985 incorporated herein by reference. The most preferred dehydrating agent/catalyst system for imidization used in this invention is acetic anhydride/triethylamine.

Because the molecular weight of the end-capped polyimide is relatively low, 1000-3000, the imidized material is generally soluble in the solvent used in the reaction. The end-capped polyimide solution is therefore typically poured gradually into a non-solvent such as methanol under vigorous agitation to cause the polyimide to separate in particulate form and to prevent the formation of excessive quantities of agglomerated particles or masses within the system. The particulate polyimide product is filtered and washed with a suitable non-solvent having a boiling point below about 160.degree. C. (preferably below about 100.degree. C.) and then dried in a vacuum oven at a temperature in the range of 25.degree. to 200.degree. C., preferably in the range of 50.degree. to 175.degree. C. to remove residual quantities of solvent(s).

The diamine portion of the polymers is based on a 2,2-bis-[4-(aminophenoxy)phenyl]hexafluoropropane, such as 2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane or 2,2-bis[4-(3-aminophenoxy)phenyl]hexafluoropropane, and mixtures thereof, as the sole or as the predominant (more than 50 mol %) diamine reactant used in producing the polyimide polymer. When forming co-polyimides wherein one or a mixture of 2,2-bis-[4-(aminophenoxy)phenyl]hexafluoropropanes constitute the predominant aromatic primary diamine component, the balance (less than 50 mol %) of the aromatic diamine(s) used will be one or more aromatic or heterocyclic primary diamines such as

p-phenylenediamine,

m-phenylenediamine,

4,4'-diaminobiphenyl,

3,3'-diaminobiphenyl,

4,4'-methylenedianiline,

4,4'-diaminodiphenylsulfide,

3,3'-diaminodiphenylsulfide,

4,4'-diaminodiphenylsulfone,

3,3'-diaminodiphenylsulfone,

4,4'-diaminodiphenylketone,

3,3'-diaminodiphenylketone,

4,4'-oxydianiline,

3,3'-oxydianiline,

1,4-diaminonaphthalene,

2,6-diaminopyridine,

3,5-diaminopyridine,

2,6-diaminotoluene,

2,4-diaminotoluene,

1,1-bis(3-aminophenyl)ethane,

2,2-bis(4-aminophenyl)propane,

2,2-bis(3-aminophenyl)hexafluoropropane,

2,2-bis(4-aminophenyl)hexafluoropropane,

2,2-bis[4-(3-aminophenoxy)phenyl]hexafluoropropane,

2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane and the like.

The particulate polyimide polymer may be separated or recovered from the liquid reaction medium by any suitable procedure, such as filtration, decantation, vacuum distillation or centrifugation.

In the preferred washing step of this invention any inert solvent boiling below about 160.degree. C. (most preferably below about 100.degree. C.) may be employed, including low boiling paraffins, cycloparaffins, chlorinated solvents, ethers, esters, ketones, etc., including mixtures of such solvents. It is important that the solvent used for washing be miscible with the aprotic solvent and thus can remove the aprotic solvent from the polymer particles.

After drying the product under vacuum or under a flow of inert gas (preferably nitrogen, although argon, etc. may be used), preferably using staged drying temperatures in the manner described above, the particulate polyimide powder may be subjected to a final drying in a vacuum tray dryer at a temperature in the range of 50.degree. to about 175.degree. C. and at a pressure in the range of 0 to 20 mm Hg. Ordinarily grinding of the product is not required, but may be resorted to in any situation where it is deemed desirable.

Having described the basic concepts of this invention, reference will now be made to the following specific examples which are illustrative but not limitive of its practice.

GENERAL PREPARATIVE PROCEDURE

To a vigorously stirred solution of one mole of a diamine or a mixture of two or more different diamines in a dipolar aprotic solvent under nitrogen atmosphere, an equimolar amount of a monomeric anhydride (maleic anhydride or nadic anhydride) dissolved in the solvent is added dropwise at ambient temperature to yield the monomaleamic acid or mononadic amic acid of diamine. A 0.5-1.0 mole of a dianhydride or a mixture of two or more different dianhydrides dissolved in solvent is added dropwise to the stirred solution and stirring was continued for another about 0.5 to 4 hours in a stream of nitrogen. Then 0.0 to 0.5 mole of a diamine or a mixture of two or more different diamines dissolved in solvent is added to the stirred solution. It should be understood however mole ratios of aromatic diamine to the combination of dianhydride plus monoanhydride should be about 1:1. To the polyimide precursor (i.e. polyamic acid) solution, a sufficient amount of chemical dehydrating agent/catalyst system is added. The mixture is stirred at 20.degree.-80.degree. C. for a sufficient time (4-24 hours) to yield a substantially fully imidized material. If the resulting imidized material is insoluble in the solvent, a particulate product precipitates. If the imidized material is soluble in the solvent, the solution is then poured gradually into a non-solvent under vigorous agitation to obtain particulate imidized maleimido- or nadimido-end-capped prepolymer. The powder product is separated by vacuum filtration and dried.

In step (3), IR and/or NMR analyses are used to detect the completion of imidization. The temperature used in step (3) will normally fall within the range of about 20.degree.-100.degree. C. and preferably within the range of about 25.degree. to about 80.degree. C.

EXAMPLE 1

52.18 gm (0.101 mole) of 4-BDAF and 160 gm of NMP (N-methyl-2-pyrrolidone) were weighed into a 500 mL flask inside a dry box. The mixture was then stirred until all of the 4-BDAF dissolved. 9.87 gm (0.101 mole) of maleic anhydride (MA) and 60 gm of NMP were weighed into an Erlenmeyer flask, and stirred until the MA dissolved. The solution was then transferred to an addition funnel. 16.21 gm (0.050 mole) of BTDA and 120 gm of the same solvent (NMP) were weighed into an Erlenmeyer flask, and the mixture was stirred and heated (<80.degree. C.) to get the BTDA to dissolve. The flask containing the 4-BDAF solution was moved into a hood and connected to an agitator drive and nitrogen purge. The addition funnel containing the MA solution was attached to the reaction flask. With stirring and with no heating or cooling, the MA solution was added dropwise over an approximately 30 minute period. During the addition, the temperature rose to 30.degree. to 35.degree. C. The BTDA solution was transferred to the addition funnel and after allowing the 4-BDAF/MA reaction mixture to stir about 30 minutes, the BTDA solution was added slowly over an approximately 30 to 60 minute period. There was another exotherm, causing temperature to rise to about 35.degree. C. After stirring another 2 hours at ambient temperature, chemical imidization was begun by adding a mixture of 20.2 gm (0.20 mole) of triethylamine (TEA) and 40.8 gm (0.40 mole) of acetic anhydride (AA). The solution was allowed to stir over night and then added into methanol with high shear mixing in a Waring blender to precipitate the end-capped imide oligomer powder. The volume ratio of methanol/reaction solution was approximately 4:1. The imidization product was a very fine particle dispersion in NMP/methanol mixed solvent.

EXAMPLE 2

Example 1 was repeated except that NMP was replaced with DMAc (N,N-dimethylacetamide). The imidization product from this run was filterable. The powder product was filtered through a medium frit glass filter and then redispersed and washed two times in methanol to remove residual DMAc. The powder product was filtered and dried at a vacuum oven at 50.degree. C. About 67.2 gm of powder was obtained. By NMR analysis, the powder contained 2.33 wt % DMAc and was only 20% imidized. The inherent viscosity (0.5 gm/100 mL) was 0.18 dl/gm. By DSC analysis, an endotherm peak (melting) was observed at about 193.degree. C. The isothermal TGA analysis at 450.degree. C. for 60 minutes in air showed 5.3% weight loss. A small sample (about 1 gm) of the powder was spread on the bottom of a beaker. The beaker was put in an oven at 175.degree. C. for 3 hours to remove the residual solvent and then the temperature was gradually raised to 300.degree. C. During the heating period, curing through the reactive end capping group occurs. The beaker was removed from the oven and allowed to cool to ambient temperature. The original powder had become an amber thin film. The film had good adhesion to the (glass) beaker. After soaking in water at ambient temperature for over night, the film was peeled off. The thin film (about 1 mil) was essentially void-free and had good flexibility and toughness.

EXAMPLE 3

3 gm of the powder obtained from Example 2 was redissolved in 30 mL of DMAc. To the solution a mixture of 0.66 g of TEA and 0.66 gm of AA was added for further chemical imidization. The mixture was stirred over night at ambient temperature. The NMR analysis of a small sample of the reaction solution showed that 100% imidization (i.e. no amid-acid) was obtained. The solution was added into isopropanol to precipitate fully imidized endcapped imide. The fine powder product was filtered and washed with isopropanol and then dried at 50.degree. C. for 7-8 hours in a vacuum oven. The granular solid (agglomeration of powders) product was ground into fine powder which was further dried at 170.degree. C. for 2 hours in a vacuum oven to obtain essentially solvent-free fine powder.

The fusibility of the imide powder was tested by one or more of three different techniques. The first was carried out in a melting point apparatus in which a small amount of powder was placed in a capillary tube, and the temperature at which the solid flowed or fused was observed. The second technique was to place a small sample of powder into an aluminum weighing dish, and then heat the dish from the bottom using a heat gun. The temperature was controlled by varying the distance between the gun and the pan and measuring the temperature at the pan. The third test was to place a powder sample in an aluminum weighing dish in an oven at various temperature or programmed temperatures and observing fusion.

A small amount of the dried, fully imidized endcapped imide powder was placed in a capillary tube for melting point measurement. The powder melted at around 220.degree. C. Another small sample (about 1 gm) of the same endcapped imide powder was placed on an aluminum pan which was placed in an oven at 175.degree. C. The temperature of the oven was gradually raised to about 290.degree. C. At this temperature the tested sample still had enough flowability. After about 15 minutes at this temperature, the aluminum pan was removed from the oven and allowed to cool to ambient temperature. The original powder sample was turned into an essentially void-free, solid thin film.

Claims

1. A process for the production of aromatic polyimide fusible powders derived from a 2,2-bis[4-(aminophenoxy)phenyl]hexafluoropropane, said process comprising

(a) treating a solution of substantially equimolar amounts of

i) a 2,2-bis[4-(aminophenoxy)phenyl]hexafluoropropane,

ii) a monomeric anhydride selected from the group maleic anhydride, maleic anhydride substituted with at least one halogen or C.sub.1 to C.sub.6 linear or branched alkyl group, nadic anhydride or nadic anhydride substituted with at least one halogen or C.sub.1 to C.sub.6 linear or branched alkyl group and mixtures thereof;

(b) admixing the product produced in step (a) with an aromatic tetracarboxylic dianhydride and heating the resulting mixture

(c) adding at least one chemical dehydrating agent sufficient to substantially fully imidize the product of step (b)

(d) separating said polyimide powder.

2. The process of claim 1 wherein the aromatic tetracarboxylic dianhydride is 3,3',4,4'-benzophenonetetracarboxylic dianhydride or pyromellitic dianhydride, or both.

3. The process of claim 1 wherein said monomeric anhydride is nadic anhydride or maleic anhydride.

4. The process of claim 1 wherein said chemical dehydrating agent is acetic anhydride.

5. A process for the production of aromatic polyimide fusible powders comprising

(a) treating a solution of equimolar amounts of 2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane or 2,2-bis[4-(3-aminophenoxy)phenyl]hexafluoropropane with nadic anhydride or maleic anhydride

(b) admixing with the compound prepared in step (a) with solution an aromatic tetracarboxylic dianhydride selected from the group pyromellitic dianhydride, 3,3,',4,4'-benzophenonetetracarboxylic dianhydride and 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride and heating the mixture to a temperature that does not exceed 100.degree. C.,

(c) adding acetic anhydride and triethylamine to substantially completely imidize the product of step (b),

(d) separating a fusible polyimide powder.

6. The compound of the formula ##STR5##

7. The compound of the formula ##STR6## where A is the integer 1 or 2 prepared by the process of claim 1.

8. The compound of the formula ##STR7##

9. A compound of the formula ##STR8##

10. A compound of the formula ##STR9##

11. A compound of the formula ##STR10## where A is the integer 1 or 2 prepared by the process of claim 1.

12. A compound of the formula ##STR11## where A is the integer 1 or 2 prepared by the process of claim 1.