WO1992001014A1

WO1992001014A1 - Polyimides and compositions for their preparation

Info

Publication number: WO1992001014A1
Application number: PCT/GB1991/001022
Authority: WO
Inventors: Nigel David Hoyle; Nevin John Stewart
Original assignee: The British Petroleum Company Plc
Priority date: 1990-07-04
Filing date: 1991-06-25
Publication date: 1992-01-23
Also published as: GB9014846D0

Abstract

Novel polyimide materials may be prepared by polymerising a mixture of a defined norbornene derivative (a), a defined benzophenonetetracarboxylic acid derivative (b), and a diamine (c) of formula (III) in which each Rf independently represent a C(1-4) perfluoroalkyl group, a is an integer from 1 to 4, b is 0 or an integer from 1 to 4, c is 0 or an integer from 1 to 4, and d is an integer from 1 to 4; components (b) and (c) being present in substantially stoichiometric amounts, and the molar ratio of component (b) to component (a) being in the range of from 0.25 to 5.

Description

POLYIMIDES AND COMPOSITIONS FOR THEIR PREPARATION

This invention relates to novel polyimides and compositions for their preparation.

Polyimides for advanced structural engineering applications, particularly the production of fibre-reinforced components, are well known. The most widely used material, because of its excellent thermal stability, is PMR-15, developed by NASA and described in US 3 745 149.

PMR-15 is derived from a mixture of three materials:

5-norbornene-2,3-dicarboxylic acid monomethyl ester, 4,4'-methylene dianiline, and 3,3',4,4'-benzophenonetetracarboxylic acid dimethyl ester. These compounds react together to produce a prepolymer which contains a low molecular weight copolymer of the diamine and the tetracarboxylic acid derivative, end-capped by two molecules of the norbornene. On further curing, the pre-polymer cross-links through the norbornene end groups to produce the finished polyimide. The number of moles of norbornene:diamine:tetracarboxylic acid derivative in the finished product is close to 2:3:2.

US 4 203 922 and US 4 111 906 disclose a polyimide having an average molecular weight of at least 5000, consisting of recurring units of a diamine and the dianhydride of a tetracarboxylic acid, in which the diamine is 2, 2-bis[4-(4-aminophenoxy)phenyl]-hexafluoropropane. These polyimides differ fundamentally from the PMR-15 type in that they contain no norbornene units, which in PMR-15 are present in an amount roughly equal to the amount of diamine. They are not significantly cross-linked. EP-A-142149 discloses polyimides of a similar type to those of US 4 203 922 above, that is, long-chain molecules with recurring units of a diamine and the dianhydride of a tetracarboxylic acid. No norbornene units are present. The resulting polymers are stated to be useful as coating film material for semiconductors. One of very many diamines used to prepare the polymers is 2,2-bis[4-(2-trifluoromethyl-4-aminophenoxy)phenyl]hexafluoropropane. A characteristic of the invention is that the dianhydride component has a defined composition including four phenyl groups and a perfluoroalkylene linkage.

WO-84/04102 discloses polyimides very similar to those of

EE-A-142149 above. Again, the polymers are long-chain molecules with recurring diamine and dianhydride units, and their stated utility is as various types of insulating films, particularly in electronics.

We have now found a novel system for producing a cured polyimide having superior high-temperature performance and superior toughness to PMR-15. Most importantly, the system also avoids the use of the highly toxic material 4,4'-methylene dianiline which is used in

PMR-15.

Accordingly, the present invention provides a mixture of:

(a) a compound of the general formula:

in which R¹ is a C(1-4) alkyl group;

(b) a compound of the general formula:

in which each of R² and R³ independently represents a C(1-4) alkyl group; and

(c) a compound of the general formula:

in which each R_f independently represent a C(1-4) perfluoroalkyl group, a is an integer from 1 to 4, b is 0 or an integer from 1 to 4, c is 0 or an integer from 1 to 4, and d is an integer from 1 to 4; components (b) and (c) being present in substantially stoicheiometric amounts, and the molar ratio of component (b) to component (a) being in the range of from 0.25 to 5.

Preferably R¹ represents a methyl group.

Preferably each of R² and R³ represents a methyl group.

Preferably each R_f represents a CF₃ group. Preferably each of a and d is 1 or 2 and each of b and c is 0, 1 or 2. Most preferably a and d are 1 and c and d are 0, and in this case, preferably the R_f groups are meta to the araine groups.

Preferably the compound of the general formula III is 2,2-bis-[4-(2-trifluoromethyl-4-aminophenoxy)phenyl]hexafluoropropane.

The mixture according to the invention contains components (b) and (c) in approximately stoicheiometric amounts - i.e. for each n moles of component (b), (n+1) moles of component (c) are present. The molar ratio of component (b) to component (a) is in the range of from 0.25 to 5, especially 0.75 to 1.5.

The mixture according to the invention may be polymerised by the application of heat to produce a prepolymer. The prepolymer can then be cross-linked to produce the cured polyimide by the application of further heat. This reaction scheme is shown in Figure 1 of the drawings. In Figure 1, n represents the number of repeat units in the polymer. The value of n for individual molecules can vary very widely indeed, but the average n is preferably from 0.5 to 10, especially 1.5 to 3. The scheme of Figure 1 shows one possible structure IV of the prepolymer; it is likely, of course, that the prepolymer also contains other molecules due to the complex nature of the

polymerisation. For example, some of the end groups in the

pre-polymer may begin to cross-link before all the diamine and tetracarboxylic acid derivative have reacted . Following curing , the polyimide has the structure (V) given in Figure 1.

The prepolymer IV and the polymer V are both novel, and both form further aspects of the present invention.

A mixture according to the invention is preferably in the form of a solution in an organic solvent. Any suitable inert solvent, for example an alcohol, ether, ester, hydrocarbon or chlorinated

hydrocarbon, may be used. Preferably an alcohol, especially methanol, or a mixture of alcohols, is used. The solution preferably contains up to 70% by weight of organic solvent, especially from 1 to 15% by weight.

A mixture according to the invention may be converted into a prepolymer by heating, preferably to a temperature in the range of from 100 to 500, especially from 175 to 220. It is preferred to heat under reduced pressure, especially from 100 to 200, in order to aid removal of volatiles from the mixture.

Subsequent curing of the prepolymer is carried out by heating, preferably to a temperature in the range of from 50 to 500, especially 200 to 300. This heating may be carried out under reduced,

atmospheric or elevated pressure; the use of elevated pressure is preferred when producing a fibre-reinforced structure.

If the mixture according to the invention is to be used in the production of a fibre-reinforced composite, it is applied to the desired fibres before curing starts. A typical process would involve applying the mixture, in the form of an organic solution, to the desired fibres and heating the resulting pre-preg to produce a prepolymer and remove voltiles. This may typically be performed in a vacuum oven, or in a mould or vacuum bag. In the latter two cases, the material may then be heated under pressure to produce the finished composite. In the case where the initial heating was performed in a vacuum oven, the material may be transferred to a vacuum bag or mould before heating under pressure to produce the finished composite.

The mixtures according to the invention may be used with any desired fibres, for example carbon, aramid, glass or ceramic fibres. The resulting composites are strong, tough, and stable at high temperatures.

According to a further aspect of the invention, there is provided a fibre-reinforced component comprising reinforcing fibres, especially carbon, aramid, glass and/or ceramic fibres, in a matrix of a polyimide according to the invention.

Compounds of the formulae I, II and III are either commercially available, or can be prepared by methods analogous to known methods. In contrast to the conventional diamine used in PMR-15, 4,4'-methylene dianiline, the preferred diamine used in the process of the present invention, 2,2-bis-[4-(2-trifluoromethyl-4-aminophenoxy)phenyl] hexafluoropropane, has a low order of oral toxicity, is non-irritant to skin and eye, and gives a negative Ames test.

The following Examples illustrate the invention.

Example 1

(i) Preparation of 2,2-bis[4-(2-trifluoromethyl-4-nitrophenoxy)phenyl] hexafluoropropane

To a 1 litre 3-necked round bottomed flask equipped with a mechanical stirrer and glass stirrer rod, Dean-Stark trap, condenser, nitrogen gas inlet adaptor and thermometer, there was placed toluene (400 ml) and N,N-dimethylacetamide (300 ml), and

4,4'-(hexafluoroisopropylidene)diphenol (84.06 g, 0.25 mol) added with stirring. When the diphenol had dissolved (5 minutes),

2-chloro-5-nitrobenzotrifluoride (112.78 g, 0.5 mol) and potassium carbonate (42.13 g, 0.3125 mol, 1.25 equivalents) were added. The apparatus was maintained under a nitrogen blanket and heated with stirring over 45 minutes to the boil, temperature 122ºC. The mixture was boiled under reflux for 3 hours, final temperature 128ºC, and 5.7 ml of an aqueous layer collected in the trap. Toluene (200 ml) was then distilled from the reaction mixture, final temperature 133°C. Further solvent (240 ml, predominantly toluene) was removed by vacuum distillation 50-60°C head temperature/0.2 mbar.

The reaction mixture together with N,N-dimethylacetamide (100 ml) reactor-washings were poured in to distilled water (800 ml) with stirring. A yellow granular solid precipitate formed which was collected by filtration and dissolved in a minimum of toluene

(1600 ml). A lower yellow aqueous layer formed (150 ml) which was separated and discarded. The organic layer was washed with distilled water (400 ml) and the yellow aqueous layer separated and discarded. Filtration of the organic layer through a celite pad gave a bright clear orange solution. A weighed sample was taken, stripped to dryness and the total solids content of the solution calculated

(175.5 g, 98% crude yield). Toluene was removed by distillation until there was 263 ml (calculated by weight) of toluene remaining. The solution was cooled to 65-70ºC. Methanol (329 ml) was added

portionwise whilst maintaining temperature at 65-70ºC. Boiling occurred in the later stages of addition (temperature 66ºC). The solution was allowed to cool with stirring for 18 hours.

Crystallisation was observed to commence at 49ºC. The product, a fine pale yellow solid (125.09 g, 71% recovery, m.pt. 164-166ºC, 95.2%

(area) by HPLC was collected and dried at 80"C under high vacuum, 0.1 mbar, to constant weight. The bulk of this material (121.5 g) was dissolved in toluene (200 ml) at 65-70ºC and methanol (240 ml) added as above. On cooling crystallisation was observed to begin at 57ºC and after 18 hours a fine very pale yellow solid (100.79 g, 83% recovery, m.pt. 166.5-167.5ºC, 99.0% (area) by HPLC) identified as 2,2-bis[4-(2-trifluoromethy1-4-nitrophenoxy)phenyl] hexafluoropropane was collected and dried at 80ºC under high vacuum, 0.1 mbar, to constant weight.

The calculated overall yield of the desired compound was 58%.

(ii) Preparation of

2,2-bis[4-(2-trifluoromethyl-4-aminophenoxy)phenyl]

hexafluoropropane

2,2-bis[4-(2-trifluoromethyl-4-nitrophenoxy)phenyl]

hexafluoropropane (9.49 g, 14.5 mmol) was dissolved in ethyl acetate (232 ml) and charged with 5% palladium on activated carbon (0.51 g) to a rocking hydrogenator (capacity 500 ml). The vessel was sealed and repeatedly evacuated and purged with hydrogen. Hydrogen pressure was increased to 320 psi and rocking and heating started. After 25 minutes the temperature reached 90ºC. The reaction was held at 90ºC for 2¼ hours and then allowed to cool to room temperature over 2 hours. Hydrogen was vented and the reaction mixture degassed.

Catalyst was recovered by filtration and the light orange solution concentrated at 33ºC under reduced pressure on a rotary evaporator to give an orange-red viscous oil. This oil was further dried by stirring at 100ºC (oil bath temperature) for 1.5 hours under high vacuum 0.1 mbar. On cooling an orange-red brittle glass identified as 2,2-bis[4-(2-trifluoromethyl-4-aminophenoxy)phenyl]hexafluoropropane was obtained (8.64 g, 99%, calculated solvent free weight).

Example 2

A mixture of 3 ,3',4,4'-benzophenonetetracarboxylic acid dimethyl ester (1092 g), 5-norbornene-2,3-dicarboxylic acid monomethyl ester (530 g), and 2,2-bis[4-(2-trifluoromethyl-4-aminophenoxy)phenyl]-hexafluroporopane (2727 g) in methanol (584 g) and 2-propanol (199 g) was prepared. This mixture was applied to a carbon fibre (Toho

HTA-76K, Trade Mark) train using a commercial 12" California Graphite prepregging machine to produce a linear prepreg of approximately 60% by weight fibre and 40% by weight of resin. This prepreg was protected on each side by silicone release paper and stored at -18ºC until required.

Prepreg as prepared above was cut into rectangular pieces and stacked. The prepreg stack was placed inside the vacuum bag in an autoclave and subjected to temperature and pressure as depicted in Figure 2. The resulting laminate was then heated to 300ºC at 3ºC/min the temperature increased to 350ºC at 1°C/min and the temperature maintained at 350ºC for 24 hours.

The fracture energy (G_IC ) of the laminate (32 ply,

unidirectional fibre) was measured at 959 J/m². The weight loss after 3000 hours at 300ºC was 18%. Example 3 (Comparative)

This Example illustrates the performance of PMR-15 resin.

A mixture of 3,3',4,4'-benzophenonetetracarboxylic acid dimethyl ester (446 g), 5-norbornene-2,3-dicarboxylic acid monomethyl ester (216 g), and 4,4'-methylenedianiline (338 g) in methanol (1000 g) was prepared. This mixture was applied to a carbon fibre train using a commercial prepregging machine to produce a linear prepreg of approximately 60% by weight fibre and 40% by weight of resin. This prepreg was protected on each side by silicone release paper and stored at -18ºC until required.

Prepreg as prepared above was cut into rectangular pieces and stacked. The prepreg stack was placed inside the vacuum bag in an autoclave and subjected to temperature and pressure as depicted in Figure 3. The resulting laminate was then heated to 290°C at 2ºC/min, the temperature increased to 315ºC at 1ºC/min and the temperature maintained at 315ºC, for 6 hours.

The fracture energy (G_IC) of the laminate (32 ply,

unidirectional fibre) was measured as 633 J/m². The weight loss after 3000 hours at 300ºC was 38%.

Example 4

A sample of polyimide laminate was prepared essentially following the method of Example 2, using 239 g of 2,2'-bis(4-(2-trifluoromethyl-4-aminophenoxy)phenyl)hexafluoropropane and other materials in proportion. The resulting laminate showed flex strength and

interlaminar shear strength (Crag standard test) of 1.79 GPa and 99.27 MPa respectively. The fracture energy (G_IIc) was measured at 755.5 Jm^-2.

Example 5 (Comparative)

A sample of polyimide laminate was made according to Example 4, except that the diamine was replaced with an equimolar amount of 2,2'-bis(4-(4-aminophenoxy)phenyl)hexafluoropropane. During the application to carbon fibres, the amount of methanol in the mixture had to be increased by approximately 40% in order to maintain the necessary viscosity. Autoclave curing took place simultaneously with the laminate sample of Example 4. The resulting material showed flex strength and interlaminar shear strength (Crag standard test) of 1.67 GPa and 93.92 MPa respectively. The fracture energy (G_IIc) was measured at 668.25Jm^-2.

Claims

Claims :

1. A mixture of:

(a) a compound of the general formula:

in which R¹ is a C(1-4) alkyl group;

(b) a compound of the general formula:

in which each of R² and R³ independently represents a C(1-4) alkyl group; and

(c) a compound of the general formula:

2. A mixture as claimed in claim 1, in which R¹ represents a methyl group.

3. A mixture as claimed in either claim 1 or claim 2, in which each of R² and R³ represents a methyl group.

4. A mixture as claimed in any one of claims 1 to 3, in which each

R_f represents a CF₃ group, each of a and d is 1 or 2 and each of b and c is 0, 1 or 2.

5. A mixture as claimed in claim 4, in which the compound of the general formula III is 2,2-bis-[4-(2-trifluoromethyl-4-aminophenoxy) phenyl]hexafluoropropane.

6. A mixture as claimed in any one of claims 1 to 5, in which the molar ratio of component (b) to component (a) is in the range of from 0.75 to 1.5.

7. A prepolymer preparable by heating a mixture as claimed in any one of claims 1 to 6.

8. A prepolymer as claimed in claim 7, having the general formula IV shown in Figure 1, in which R_f, a. b, c and d have the meanings given in claim 1, and n represents the number of repeat units in the prepolymer.

9. A prepolymer as claimed in claim 8, in which each R_f represents a CF₃ group, each of a and d is 1 or 2 and each of b and c is 0, 1 or 2.

10. A prepolymer as claimed in claim 9, in which a and d are 1, b and c are 0, and the R_f groups are meta to the amino groups.

11. A prepolymer as claimed in claims any one of claims 1 to 8, in which the average n in the bulk material is from 0.5 to 10.

12. A polyimide including units of the general formula V in Figure 1, in which R_f, a, b, c and d have the meanings given in claim 1, and n represents the number of repeat units within the unit V.

13. A polyimide as claimed in claim 12, in which each R_f represents a CF₃ group, each of a and d is 1 or 2 and each of b and c is 0, 1 or 2.

14. A polyimide as claimed in claim 13, in which a and d are 1, b and c are 0, and the R_f groups are meta to the amino groups.

15. A polyimide as claimed in any one of claims 12 to 14, in which the average n in the bulk material is from 0.5 to 10.