IL38069A - Solution process for the preparation of polyimides from diamines and bisimides - Google Patents

Description

SOLUTION PROCESS FOR THE PREPARATION OP POLIIMIDES FROM DIAMINES AND BISIMIDES D'TSO'an D*»3"»aK'ia D'fa'K' is iuDn1? no'oa ¾17rm 1 UNITED STATES PATENT APPLICATION Of: John D. Hand l Wendell G. Whitehouse 4 For : SOLUTION PROCESS FOR THE 6 PREPARATI ON OF POLYIMIDES FROM 6 DIAMINES AND BISIMIDES 7 S Abstract of the Disclosure,- Polyitnide prepolymers with 9 improved mold flow characteristics and enhanced physical properΪ0 ties when thermoset, are provided by reacting a difunctional amin< 11 and a difunctional iraide of an unsaturated dicarboxylic acid at 12 relatively high concentrations in a solvent at a temperature not 13 in excess of 75°C., 14 6 This inve tion relates to a process for the preparation of 17 pol iraide prepolymers in solution. The prepolymers rapidly cure 18 with heat to provide macroraolecular polyiinides with valuable 19 mechanical and electrical properties and high thermal stability. 20 21 Background of the Invention.- It is known, for example, 22 from the disclosure in French Patent No. 1 , 555 s564 , that the 23 reaction of difunctional amines with at least an equimolar amount 24 of a difunctional imide of an unsaturated dicarboxylic acid pro25 vides so-called polyiraide prepolymers. The prepolymers, on fur2G ther heating, produce a family of macromolecular polyimides with 27 outstandin resistance to heat and solvents and with hi mechanical strength and excellent electrical properties.

Such prepolymers are disclosed to be formed by reacting a difunctional amine of the general formula; H2N — R — NH2 wherein R is a divalent group having up to about 30 carbon atoms with a difunctional imide of the general formula wherein R is a divalent group containing up to about 30 carbon atoms and having at least one carbon-to-carbon double bond. R and R1 can be the same or different.

It is believed that the predominant reaction in the formation of such prepolymers is the addition of the elements of an amine group across the double bond in R1. However, a second type of reaction is also known to occur, that is, vinyl addition polymerization betveen the unsaturated carbon-to-carbon linkages in the groups designated R1 in the above formula.

In any event, it is disclosed that the reaction between the difunctional amine and the difunctional imide may be carried out either in bulk or in an inert polar carrier mediura0 In the bulk process, apparently because reactivity is low unless the mixture is el ed it is necessar to heat the reaction m x e o an elevated temperature, of the order of 100 to 200°C, to induce any reaction at all between the two reagents . In most cases at least about 120°C. is used and it is reported to be preferred to heat at about 160°C. if the reaction is to be completed in a reasonable time. If an inert polar carrier medium is to be used" it is suggested to employ dimethylformamide (b0p0 153°C o ) , N-me hylpyrrolidone. (b.p. 202°C.) and dimethylacetamide (b.p. 163 to 165 °C). If dimethylformamide is used as a carrier it is specifically recommended to heat the solution at 150eC.s i0e„, near the reflux temperature. Thus the reaction, even in a polar carrier medium, is reported to proceed at a temperature from the middle to the upper range of temperature necessary for the bulk process.

In the present state of the art, not only do the reportedly successful processes have a lack of adaptability to production on a large scale, but, in addition, the prepolymer products prepared thereby have disadvantages limiting their full usefulness.

A bulk melting process requires high equipment investment costs and there are safety hazards in handling and grinding the dry reagents, especially the difunctional amine. Furthermore, there are process control problems due to non-homogeniety of the initial mixture of solid reactants and difficulty in achieving adequate mixing upon melting. Moreover, it is difficult to maintain a constant ratio of reactants in a melt process because the reagents, especially the diamine, tend to oxidize, decompose and sublime The prepolymer products prepared by the bulk melt process or in a solvent at the same elevated temperature range, vary widely in final product resin properties. For example, data fro differential scanning calorimetry (DSC) indicate a wide variation in molecular weight dis ribution. Such prepolymers also exhibit less than optimum flow during molding and the ultimate physical properties, such as flexural strength and modulus, are also lower than optimum and vary from batch to batch, probably because of variations in the basic structure of the prepolymer. It is believed tha the variations observed are due to the presence of side reaction products and, possiblyj the presence of varying amounts of vinyl addition and cross -linking in the prepolymers.

The present invention has as a primary object the provisio of an improved process to provide polyimides .

Still another object of this invention is to provide polyimid prepolymers by a process that is economical and safe, A further object of this invention is to provide polyimide prepolymers which can be thermally converted to macromolecular polyimides with better flow, molding and physical properties than those made *by prior art bulk processes.

Description of the Invention.- The above objects and advantages and others that will be apparent to those skilled in the art amine and a difunctional imide of an unsaturated dicarboxylic acid in solution in a solvent at a temperature not in excess of about 75 °C. until the reaction is complete.

It is regarded as critical to achieving the desired results that the reactants be heated in solution at a concentration of not substantially less than about 30% by weight and that the temperature of such heating be no higher than about 75°C. If heating is carried out at a temperature even slightly above this, e.g., at 80 to 100°C, or especially in the range suggested in the prior art, 100-200°C, the reaction product will have poor flow and less than optimum properties. Moreover, at concentrations of below about 307o by weight of the reactants in solution, yields are drastically lower and many advantages are lost. An optimum concentration of reactants in solution for most purposes will be from about 50 to about 6570 by weight.

A solvent will be selected which provides a high percent solids content, and, in general, it is important to select a solvent which is inert to the components of the reaction mixture.

Although it is not critical, it is desirable to employ a solvent which is partially or completely water miscible. It is useful to employ an inert organic polar solvent because the reactants and products are relatively polar in nature. In general, the solvents most useful in the present process will contain carbons hydrogen and oxygen and will have up to about 8 carbon atoms, and a boiling oint of from about 50°C. to about 200°C. A referred famil of 1 solvents contain the bivalent ketone group -CO-. Although the 2 compounds may contain an aromatic group, i0e0s a phenyl group, S they preferably contain only aliphatic groups s straight chain, 4 branched or cyclic, of from about one to about six carbon atoms, δ the maximum total number of carbon atoms in the preferred compound 6 being about eight, including that in the ketone group. Especially 7 preferred inert solvents are (lower)aliphatic ketones of the 8 paraffinic or cycloparaffinic series, having the same or different 9 radicals of from 2 to 8 carbon atoms attached to the ketone group, 0 e.g., acetone, ethylethyl ketone, diethyl ketone, methyl n-propyl 1 ketone, ethyl n-propyl ketone, methyl n-butyl ketone, methyl i-2 butyl ketone, methyl^t-butyl ketone, ethyl n-pentyl ketone, 3 cyclopentanone , cyclohexanone, and the like0 4 Acetone is the most useful inert solvent for the present 6 reaction and is preferred because it has been found to possess a 7 surprising ability to cooperate with the diamine reactant and 8 enhance the solubility of the difunctional imido reactant. It has S been found that the higher the solids content of the reacting mix0 ture, the better the prepolyraer will be in terms of uniformity 1 and ultimate physical properties. In" addition, the refluxing tem2 perature of a solution of the reactants in acetone, about 62°C. , 3 is well below the maximum limit, 75°C., for the process advantages 4 provided by this invention and therefore acetone lends itself to 5 commercial operation on a large scale because the temperature can 5 be easily controlled at the desired level. Acetone is also pre7 ferred because it is inexpensive, safe, non-toxic, water-miscible , easily recovered (because of its low boiling point) and easily removed from the polymer (as in drying) .

Although it is not essential, it is preferred to carry out the reaction between the diamine and the bisimide in the presence of a small but effective amount of acidic catalyst. The nature of the catalyst is not critical; it can be an inorganic or organic acid, such as a mineral acid, e.g., sulfuric acid or hydrochloric acid or an organic carboxylic acid, such as acetic acid or p_-tol~ uenesulfonic acid, and the like. An acid seems to promote the addition of the amine group to the activated double bonds in the unsaturated i ido groups. It is especially preferred to use acetic acid and if the difunctional unsaturated malemide is prepared in the conventional way, i.e., by condensing an anhydride with a diamine in the presence of acetic anhydride and adding wate to precipitate the product, the moist bisimido product contains enough acetic acid (from hyirdlysis of the anhydride) to provide a very desirable reactant for the present process.

The difunctional amines used in the process of the present invention will be, for example, of the formula ; H2N — R — H2 wherein R is a divalent group having up to about 30 carbon atoms and, optionally, oxygen, phosphorus, silicon, nitrogen and similar inert groups. Illustratively, and preferably, R can be straight or branched chain alkylene of up to 12 carbon atoms; cycloalkylene c r e dianiline, i.e., a compound of the formula * because the resins derived from this -'compound have a preferred combination of proper es .

The difunctional imides of unsaturated dicarboxylic acids used in the process of the present invention will be, in general of the formula wherein R is as above defined and illustrated and R1 is a divalent group containing up to about 30 carbon atoms, and, optionally, oxygen, phosphorus, silicon, nitrogen and similar inert groups, and having at least one carbon-to-carbon double bond. The imides can be, 'for example, derived from maleic and substituted, e.g., alkyl of up to 6 carbons, aryl or halogen substituted maleic acid; or derived from citraconic or itaconic acid, from tetrahydro phthalic acid and similar Diels Alder condensation products of a diene with one of these anhydrides of an unsaturated acid, as well as all such products substituted, for example, with alkyl of up to 6 carbon atoms, aryl, e.g., phenyl or halogen, e.g., fluorine s chlorine, bromine and iodine. Illustratively and preferably, R ,1J a b of from t least nd the o split e type al imide h which is pre- CH- and in the dione or pe of o the function-* t the of usin 1 more than about 50 moles of di-imide per mole of diamine and 2 preferably, the amount of di-imide will be about 2.0 to 3.0 and 8 especially about 2.5 moles per mole of diamine 0 4 δ In accordance with the process of this inventions the pre6 paration of prepolymers of polyimides can easily be carried out 7 by dissolving the difunctional amine and the difunctional imide S in the inert solvent. 9 0 There is no particular lower limit on the reaction temperature 1 although in most cases heating to at least 30~35feC« is desirable 2 and most satisfactory results are obtained at a minimum tempera3 ture of about 50°C. The maximum temperature of about 75°C« should 4 not be exceeded. Excellent results will be obtained if the mixture 5 is heated between about 55 and 70°C, and the reaction proceeds 6 particularly smoothly if acetone is used as the inert solvent and 7 the mixture is refluxed. The temperature in such a system will be S about 62°C. 9 0 The reaction time varies depending on such factors as the 1 type of reactants, the nature of the solvent, whether or not a 2 catalyst is employed, etc., but it is sufficient to carry out the reaction until analysis indicates that the amine groups have 4 been substantially completely reacted. In general s satisfactory 5 results are obtained if the reaction is performed for a period of 6 10 minutes to 6 hours. 7 1 Optionally, before or after the prepolymer is recovered, a 2 catalyst capable of generating free radicals can be added and the 3 mixture heated. Such a catalyst will promote vinyl polymerization 4 e.g., in the unsaturated substituent designated R . Even though iS such polymerization to a macromolecular polyimide will proceed in o the absence of a free radical catalyst, merely by heating the 7 prepolymer , e.g., at a temperature from about 100 to about 350°C,3 8 the reaction is facilitated with a catalyst. Examples of catalyst a are well known and among them can be mentioned peroxides , such as benzoyl peroxide, di-cumyl peroxide, acetyl peroxide and the like 11 or azo derivatives, such as azo-bis-isobutyronitrile0 Azo-bis-12 isobutyrcnitrile is preferred because it has a desirable activatio 13 temperature and excellent compatibility with the reaction mixture. 14 The prepolymer can be used in solution, for example, to 16 impregnate webs of paper, glass cloth and the like, which can be 17 layered and consolidated under heat and pressure to form laminates IS in which the macromolecular polyimide resin is the binder. On the 19 other hand, the prepolymer can be recovered from solution by any £0 convenient means. For example, the solvent can be evaporated to 2 leave the prepolymer as a residue. Alternatively a non-solvent 22 for the prepolymer can be added to the solution, or vice, versa , and the prepolymer precipitated. 24 In any case, the recovered prepolymer may be hardened by 28 heating, with or without a free radical-generating catalyst to 27 roduce the mac romolecular ol imide as described above.

One preferred way to recover the product from an acetone reaction, in particulate form, is : to cool the reaction mixture to about 20-25°C. and to reverse precipitate into about 10 volumes of cold water, e.g., at 10-15°C. The particulate material is then filtered, cold water washed, refiltered and dried, e.g., in a vacuum'. — -v Description of the Preferred Embodiments The advantages obtained by preparing polyimide prepolymers according to this invention are illustrated in the following example. The example is not to be construed as limiting the invention thereto.

EXAMPLE Maleic anhydride, 41 g., is dissolved in 200 ml. acetone and 1-5 ml. of water and heated to reflux. Methylene dianiline, 39.5 g., is dissolved in 100 ml. of acetone and fed into the maleic solution during 40 minutes. To this mixture is added 0.40 g. of nickel acetate, 7.0 g. of triethylamine and 60.0 o of acetic anhydride and the mixture is heated at reflux (63°C.) for two hours. Dissolution requires about 20 minutes. Ten volumes of water is added to precipitate the product and it is recovered by filtration but not washed, refiltered or dried.

The moist difunctional imide, containing acetic acid and water, is redissolved in acetone at reflux (62°C_) and methylene dianiline is added until the molar ratio of imide to amine is 2.5:1. The solids content of the solution is 557» by weight. The solids dissolution time in refluxing acetone is a bell-shaped function of the per cent solids, indicating an apparent synergistic: effect of the amine on the solubility of the difunctional imide. After 30 minutes at reflux temperature, formation of the polyimide prepoly er is substantially complete.

To the refluxing mixture of prepolymer there is added a j i catalytic amount of azo-bis- isobutyronitrile . Refluxing is continued for an additional 15 minutes to allow completion of the vinyl addition reaction. i The product is isolated by cooling the mixture to about 25 °C . i and reverse precipitating into 10 volumes of water. The particulate product is filtered, washed with cold water, refiltered and | dried under vacuum.

The product has a melting point (by DSC) of 95°C ; it has J 0. 20 double bonds per 100 grams.; an amine content of l„ 307o and a | volatile content of 2 .57>. When compounded into a formulation com¬ prising 257» by weight of graphite - 757o by weight of resin and j molded under pressure, the flexural strength is 11 ,500 psi and flexural modulus is 700 , 000. i For comparison purposes , corresponding data on polyimide pre- poiymers made from the same reactants by heating in bulk at 160°C . for 20 minutes have a variable melting range (by DSC) of between 95 and 125 °C ; about 0.29 double bonds per 100 grams; an amine content of from 1 .18 to 1.397» and a volatile content of 0 . 17ο .

Maximum properties of molded compositions comprising 257» of graphite - 757, of such resin are flexural strength, 10 , 000 psi, and flexural modulus, 800 , 000 p&i, Obviously, other modifications and variations of the present invention are possible in light of the above teachings.

For example, instead of acetone, there can be used methyl ethyl ketone and r.-.ethyl isobutyl ketone, keeping the reactio temperature below 75°C. in either case.

Instead of methylene dianiline, i.e. , 5 3 -diamino diphenyl methane, there can be used 4,4 '-diamine dicyclohexylmetha e ; 1,4-diamino cyclohexane; 2 , 6-diaminopyridine ; rneta-phenylene-diamine ; para-phenylenediamine ; 2 ,2-bis (4-aminophenyl)propane ; benzidine; ,48 -diaminodiphenyl ether; 4 , ' -diaminodiphenyl sulfide; 4 ,4 ' -diaminodiphenyl sulfone; bis (4=aminophenyi)diphenyl-silane; bis (4-aminophenyl)methylphosphine oxide; bis (3-aminopheny methylphosphine oxide; bis (4-aminophenyl)phenyIphosphine oxide; bis (4-aminophenyl)phenylamine ; 1 ,5-diaminonaphthalene ; meta-xylene diamine; para-xylene diamine; and hexamethylenediamine .

Instead of Ν,Ν'-bis aleimido (4 ,4 ' -diphenylmethane) there can be used Ν,Ν* -bis maleimidoethylene ; Ν,Ν'-bis maleimido hexamethy-lene; Ν,Ν'-bis maleimido etaphenylene ; N,N' -bis maleimido (4,4'-diphenyl ether); Ν,Ν'-bis maleimido (4 s4 '-diphenyl sulfone); ,N 1 -bis maleimido (4 ,4 ' -dicyc lohexyl metane) ; N,N'°bis maleimido( ,4 ' - Because of their excellent physical, mechanical, chemical, electrical and thermal properties, the polymers produced by the s process of this invention have' many and varied uses. For example, 4 they can be used in molding powder formulations s either alone or 5 mixed with other polymers and may contain various fillers, such 6 as wood flour, diatomaceous earth, carbon black5 silica and the 7 like, or reinforcing agents, such as graphite fiber, glass fila¬ 8 ments and glass f bers, to make molded parts, such as spur, 9 helical, worm or bevel gears, ratchets, bearings, cams, impact 10 parts, gaskets, valve seats for high pressure oil and gas systems 11 or other chemical luids, parts requiring resistance to chemicals 12 and the. like. They can be used to prepare molded, cast, ealen ore 13 and extruded articles, films, coatings, threads s filaments, tapes 14 and the like, and are useful in electrical applications, such as cable terminals, terminal blocks, backing for electrical circuits, 16 as components of dynamoelectric machines that operate at high 17 temperatures, and the like. Films of these materials are useful 18 as metal or fiber liners, containers, covers, closures, electrical 19 insulating tapes , magnetic tapes, photographic films , pipe and BO wire tapes, and the like. As a coating material, they can be 21 applied as a solution or suspension to any convenient foundation iJ wnere a surface possessing their excellent properties is desired. 23 They can be used as an encapsulation material, for electrical 24 insulation, for example, as a wire enamel, potting compound, and 215 the like. Fibers produced from the -polymers can be woven into 26 fabrics, useful in many applications, for example, as filter ί ··7 cloths where high chemical and heat-resistance is desired. Their 1 excellent electrical properties make laminates of these materials o useful for electrical equipment, such as slot wedges in the 8 armature of an electric motor, panel boards for printed circuits , 4 electrical appliance panels j, radio and television panels, small 5 punched electrical pieces, transformer terminal boards s transfor6 mer coil spacers, and the like. The compositions may also include 7 various fillers and modifying agents, and the like, such as dyes, S pigments, stabilizers, plastic izers , and the like. 3 0 It is to be understood that changes may be made in the 1 particular embodiments of the invention in light of the above 2 teachings, but that these will be within the full scope of the 3 invention as defined by the appended claims. 4 6 7 8 9 0 1 2 3 4 6 7

Claims (17)

GE.-39 D-1017) Claims

1. In a process for the preparation of a polyiroide pre- polymer which comprises heating a difunctional amine and a di- f nctional imide of an unsaturated dicarboxylic acid until formation of said prepolymer is substantially complete, the improv ment which comprises dissolving said amine and said imide in an inert solvent to form a solution containing not substantially less than about 307c by weight of the reactants, and heating sal solution at a temperature not in excess of about 75°Ce until formation of said prepolymer is substantially complete.

2. , A process as defined in Claim 1 wherein said inert solvent is a (lower)aliphatic ketone .

3. » .A process as defined in Claim 1 wherein said inert solvent is acetone, said solution contains from about 50 to 65% by weight of said reactants and. said heatin is carried out at the reflux temperature of said solutio „

4. A process as defined in Claim 1 wherein the reaction solution includes a small effective amount of an acidic catalys

5. A process as defined in Claim 4 wherein said catalyst is acetic acid.

6. A process as defined in Claim 1 wherein (a) said difunctional amine is of the formula: H2N - R - NH2 wherein R is a divalent group having up to about 30 carbon toms ; and (b) said difunctional imide of an unsaturated dicar boxylic acid is of the formula: I wherein and R1 are as hereinabove defined,,

7. A process as defined in Clairo 6 whereins in said compounds, R is alleylene of up to 12 carbon atoms; cycloaIk iene of from 5 to 6 carbon atoms; a heterocyclic group of from 4 to 5 carbon atoms having at least one oxygen, nitrogen or sulfur atom in the group;, a MHO- or di-carbocyclic aromatic group; or at least two mono- or di-carbocyclic aromatic or cycloalkylene group joined by a direct carbon-to-earbon bond or joined through a divalent group selected from oxygen, sulfur , alkylene of from 1 to 3 carbon atoms or - N = N - N 4· o - P(0)R3 - - CO - 0 - SiR2R3 - - CONH - ; R½ - CO - R5 ~ CO - NRA 0 - CO - R5 - CO - 0 ~ wherein R , R and are alkyl of frora 1 to 6 carbon atoms, cycloalkyl of from 5 to 6 carbon atoms, or mono- or di-carbocyc li aryl; and R^ is divalent alkyl of up to about 12 carbon atoms, cycloalkyl of from 5 to 6 carbon atoms, or mono- or di-carbocycli arylene; and wherein R° , R and R° are, independently, hydrogen or alkyl of from 1 to 6 carbon atoms .

8. A process as defined in Claim 6 wherein R is and R1 is — HC = CK —

9. Λ process as defined in Claim 1 including the step of recovering said prepolymer.

10. A process as defined in Claim 9 including the step of heating said prepoly er until conversion of said prepolymer to a macromolecular polyimide is substantially complete.

11. A proce.ss as defined in Claim 10 wherein said prepolyraer is heated at a temperature of from about 100°Co to about 350°C.

12. A process as defined in Claim 1 including .the step of heating said prepolymer in the presence of a catalyst capable of providing free radicals.

13. A process as'defined in Claim 12 wherein said catalyst is a peroxide or an azo derivative.

14. A process as defined in Claim 13 wherein said catalyst is azo-bis-iso-butyronitrile .

15. A process as defined in Claim 9 wherein said solvent is wate · miscible and said prepolymer is recovered from solution by adding said solution to water until precipitation of said prepolymer is substantially complete and separating the precipitated prepolymer.

16. A process as defined in Claim 15 including the step of heating said prepolymer until conversion of said prepolymer to a macromolecular polyiraide is substantially complete.

17. A process as defined in Claim 16 wherein said prepolymer is heated at a temperature of from about 100°C. to about 350°C. Tel-Aviv, November.1, 1971