Thermoplastic polyimides

Abstract

The present invention relates to thermoplastic polyimides and to the synthesis thereof. The invention relates in particular to a method for manufacturing semi-aromatic thermoplastic polyimides by means of the solid-state polymerization of a solid ammonium carboxylate salt formed from an aliphatic diamine and an aromatic tetracarboxylic acid, thereby enabling powders having controlled particle sizes to be produced.

Claims

1. A process for producing solid particles of semicrystalline and semiaromatic (co)polyimide, having a median diameter D50 of between 0.01 and 2 mm, said polyimide being thermoplastic and having a melting temperature of between 50 and 350 C., the process comprising: (a) charging a reactor with a salt formed by reaction of at least one diamine, in which each of the amine functional groups is bound to a respective aliphatic carbon atom, with at least one aromatic tetracarboxylic acid; (b) polymerizing the salt from step (a) in the solid state to give the (co)polyimide, at an absolute pressure of between 0.005 and 1 MPa and at a temperature T which obeys the following relation:
Tf of the salt from step (a)>T>Tg of the (co)polyimide to be obtained; and (c) recovering the solid (co)polyimide particles.

2. The process as claimed in claim 1, wherein the (co)polyimide has a glass transition temperature Tg of less than or equal to 200 C.

3. The process as claimed in claim 1, wherein the at least one aromatic tetracarboxylic acid is selected from the group consisting of pyromellitic acid, 3,3,4,4-biphenyltetracarboxylic acid, 2,3,3,4biphenyltetracarboxylic acid, 2,2,3,3-biphenyltetracarboxylic acid, 3,3,4,4benzophenonetetracarboxylic acid, 2,2,3,3-benzophenonetetracarboxylic acid, 1,2,5,6-naphthalenetetracarboxylic acid, 2,3,6,7-naphthalenetetracarboxylic acid, 2,3,5,6-pyridinetetracarboxylic acid, 3,4,9,10-perylenetetracarboxylic acid, 3,3,4,4tetraphenylsilanetetracarboxylic acid, and 2,2-bis(3,4-bicarboxyphenyl) hexafluoropropanetetracarboxylic acid.

4. The process as claimed in claim 1, wherein the at least one diamine is selected from molecules of formula NH.sub.2RNH.sub.2 with a divalent alkylaromatic or cycloaliphatic, or linear or branched, saturated and/or unsaturated aliphatic hydrocarbon radical R, optionally comprising one or more heteroatoms.

5. The process as claimed in claim 4, wherein the radical R comprises from 2 to 50 carbon atoms, and optionally one or more heteroatoms.

6. The process as claimed in claim 1, wherein the at least one diamine is selected from the group consisting of 1,2-diaminoethane, 1,3-diaminopropane, 1,4-diaminobutane, 1,5-diaminopentane, 2-methyl-1,5-diaminopentane, hexamethylenediamine, 3-methylhexamethylenediamine, 2,5-dimethylhexamethylenediamine, 2,2,4- and 2,4,4-trimethylhexamethylenediamine, 1,7-diaminoheptane, 1,8-diaminooctane, 2,2,7,7-tetramethyloctamethylenediamine, 1,9-diaminononane, 5-methyl-1,9-diaminononane, 1,10-diaminodecane, 1,11-diaminoundecane, 1,12-diaminododecane, 1,13-diaminotridecane, and 1,14-diaminotetradecane.

7. The process as claimed in claim 1, wherein the at least one diamine is selected from the group consisting of isophoronediamine, 1,3-diaminocyclohexane, 1,4-diaminocyclohexane, and diaminodicyclohexylmethane.

8. The process as claimed in claim 1, wherein the at least one diamine is selected from molecules of formula NH.sub.2(CH.sub.2).sub.n-Ph-(CH.sub.2).sub.nNH.sub.2 where n and n are nonzero integers which are independent of one and Ph is a phenyl group.

9. The process as claimed in claim 1, wherein the salt is admixed with an excess of one of the monomers so as to create a stoichiometric imbalance.

10. The process as claimed in claim 1, wherein the absolute pressure during step (b) is between 0.005 MPa and 0.2 MPa.

11. The process as claimed in claim 1, wherein the temperature during step (b) is between 50 C. and 250 C.

12. The process as claimed in claim 1, wherein the number-average molar mass Mn of the (co)polyimide is between 500 g/mol and 50,000 g/mol.

13. Solid particles of (co)polyimide (I), obtained by the process as claimed in claim 1.

14. Solid particles of semicrystalline and semiaromatic (co)polyimide, having a median diameter D50 of between 0.01 and 2 mm, said polyimide being thermoplastic and having a melting temperature of between 50 and 350 C., said (co)polyimide being obtained from the polymerization of a salt formed by reacting at least one diamine in which each of the amine functional groups is bound to a respective aliphatic carbon atom with at least one aromatic tetracarboxylic acid.

15. A process for producing a composition, the process comprising melt or nonmelt mixing of the solid particles of (co)polyimides (I) as claimed in claim 14 with reinforcing or bulking fillers and/or with impact modifiers and/or with additives.

16. A plastics article, wherein the article comprises the solid particles of (co)polyimide (I) as claimed in claim 14.

17. The solid particles of claim 14, wherein the solid particles exhibit less than or equal to a 10% change in relative viscosity after 10 minutes in the melt state, measured with a viscosimeter as a 10 g/L sample in 96% sulfuric acid at a temperature of 25 C.

18. The solid particles of claim 14, wherein the solid particles exhibit less than or equal to a 10% change in number-average molar mass after 10 minutes in the melt state, determined by .sup.1H NMR in D.sub.2SO.sub.4 at 300 K.

19. The process as claimed in claim 1, wherein the salt formed by reaction of at least one diamine with at least one aromatic tetracarboxylic acid has the general structure shown below: ##STR00002## wherein Ar represents the aromatic group from the aromatic tetracarboxylic acid used in the salt forming reaction and R is dependent on the at least one diamine used in the salt forming reaction.

20. The process as claimed in claim 1, wherein the salt is formed in a reaction medium comprising a water and alcohol mixture.

21. The process as claimed in claim 1, wherein the salt is admixed with at least one chain transfer agent.

22. The process as claimed in claim 1, further comprising: reacting at least one diamine, in which each of the amine functional groups is bound to a respective aliphatic carbon atom, with at least one aromatic tetracarboxylic acid to produce the salt; and collecting the produced salt.

Description

EXPERIMENTAL SECTION

Measurement Standards

(1) The melting or fusion temperatures (Tf) and the temperature of crystallization on cooling (Tc) of the (co)polyimides are determined by differential scanning calorimetry (DSC), using a Perkin Elmer Pyris 1 instrument, at a rate of 10 C./min. The Tf and Tc values for the (co)polyimides are determined at the top of the melting and crystallization peaks. The glass transition temperature (Tg) is determined on the same instrument at a rate of 40 C./min (when possible, it is determined at 10 C./min and specified in the examples). The measurements are made after melting of the (co)polyimide formed at T>(Tf of the (co)polyimide+20 C.).

(2) For the determination of the melting temperature of the salt, the end temperature of the endotherm measured by heating the salt at 10 C./min is considered.

(3) Thermo-Gravimetric Analysis (TGA) is carried out on a sample of approximately 10 mg in a Perkin Elmer TGA7 instrument. The precise conditions of use (temperature, time, heating rate) are defined in the examples.

(4) Thermogravimetric analysis makes it possible to determine the thermal stability of the polyimides, but it is used here also to calculate the yield of the reaction for conversion of the salt into polyimide in the following manner: Heating at 10 C./min of a sample of salt polymerized by the process of the invention, from 30 C. to 300 C. Determination of the loss of mass observed, denoted y %. Determination of the degree of reaction n by the calculation n=(1+x)/(1+y)1, where x is the ratio of the molar mass of the PI salt in g/mol to the molar mass of one repeating PI unit. For example, for the PI 12PMA prepared from pyromellitic acid (PMA) and 1,12-diaminododecane, x=15.85%; for the PI 13PMA prepared from PMA and 1,13-diaminotridecane, x=15.38%.

(5) The Fourier transform infrared (FTIR) analysis is performed on a Bruker Vector 22 instrument (in reflection, ATR Diamant) on the powder of formed polyimide.

(6) .sup.1H NMR analysis is carried out on a Bruker AV500 spectrometer in a 50/50 by volume D.sub.2O/CD.sub.3OD mixture at ambient temperature for the salts, and in m-cresol at 110 C. in D.sub.2SO.sub.4 at 300 kelvins for the polyimides.

(7) Particle size analysis is carried out dry on a SYMPATEC HELOS H1302 instrument with a dispersion pressure of 2 bar of nitrogen.

(8) Colorimetric analysis CIE Va*b* is carried out on a Minolta CR-310 colorimeter.

Example 1: Preparation of a Polyimide PI 12PMA from a 12PMA Salt Synthesized in Pure Ethanol

(9) A 5 L reactor is charged with 40 g (0.15 mol) of 94.9% pyromellitic acid (Sigma-Aldrich) and 2 liters of pure ethanol. The reaction medium is stirred and heated to 70 C. while flushing gently with nitrogen. In a 1 L round-bottom flask, 30.5 g (0.15 mol) of 98% 1,12-diaminododecane (TCI Europe N.V.) are dissolved in 500 mL of pure ethanol at ambient temperature. This solution is then placed in a dropping funnel connected to the 5 L reactor and added dropwise over 1 hour to the ethanolic solution of pyromellitic acid. Contact between the diamine and the pyromellitic acid brings about the formation of a salt, which precipitates immediately under vigorous stirring. The reaction medium is maintained with vigorous stirring for 3 hours 30 minutes at 70 C. and under nitrogen.

(10) The salt powder is recovered by filtration on a Bchner funnel, and is washed with ethanol, then ground and dried under vacuum overnight at 50 C. The mass yield is 95%. The powder is fine and white. The melting temperature of the salt is 260 C.

(11) Polymerization

(12) The 12PMA salt powder is placed in a fluted flask attached to a rotary evaporator, and placed under gentle flushing with nitrogen. The pressure is equal to atmospheric pressure. The flask is immersed in an oil bath at 200 C. and rotated for 8 hours. The PI 12PMA powder obtained is white and totally dry. The particles exhibit a median diameter D50 of 73 m. TGA analysis is performed on the final product, by heating from 40 C. to 300 C. at 10 C./min. There is no detectable loss of mass apparent, which indicates that the 12PMA salt powder has undergone quantitative conversion into PI 12PMA powder.

(13) FTIR analysis of the PI 12PMA powder shows the characteristic absorption bands of imide functions at 1699 and 1767 cm.sup.1, and the absence of absorption bands characteristic of amine functions is noted. .sup.1H NMR analysis confirms the presence of imide functions formed, and the 1:1 stoichiometry of the units obtained from the diamines and tetracarboxylic acids, in other words the absence of monomer losses during the production of the PI 12PMA powder.

(14) The PI 12PMA powder has a melting temperature of 303 C. (enthalpy of fusion Hf=35 J/g), a crystallization temperature of 274 C., and a Tg=101 C.

(15) Extrusion

(16) The PI 12PMA powder is extruded in rod form by placement of 10 g of the PI 12PMA thus prepared in a DSM MIDI 2000 twin-screw microextruder (micro-compounder) (volume 15 cm.sup.3) which has been preheated to 340 C. and has a screw speed of 100 revolutions per minute.

(17) The microextruder possesses a recirculation channel which allows the residence time in the twin screw to be adjusted. We selected a residence time of 1 minute: the torque exerted on the rotary screw motor is stable, a sign of little change in the melt medium. After a residence time of 1 minute, the melted polyimide is extruded in the form of a rod and cooled in a water tank, then pelletized. The thermal properties of the polyimide are Tf=305 C., and Tc=287 C.

Comparative Example 1: Polymerization of the PI 12PMA Salt at a Melting Temperature Greater than Tf Salt

(18) The PI 12PMA salt of example 1 is polymerized beyond the melting temperature of the salt: it is heated under nitrogen from ambient temperature to 300 C. at a heating rate of 10 C./min and is maintained at 300 C. for 1 minute, then cooled immediately at ambient temperature.

(19) Thermal analysis of the resulting polyimide shows that the melting temperature is 290 C. and the crystallization temperature is 266 C. It appears that this method of polymerization, relative to the polymerization of the invention as described in example 1, produces a decrease in the melting (13 C.) and crystallization (8 C.) temperatures of the polyimide: the two polyimides, although of similar chemical structure, exhibit a different macromolecular architecture.

Example 2: Preparation of a PI 12PMA Polyimide from a 12PMA Salt Synthesized in a Water/Ethanol Mixture

(20) A 350 mL three-neck round-bottom flask is charged with 2.684 g (0.009 mol) of 84.1% pyromellitic acid (the impurities being water) and 150 mL of a 50/50 by volume water/ethanol mixture. The reaction medium is stirred and heated to 80 C. under gentle nitrogen flushing: the pyromellitic acid is dissolved in this way. In a 100 mL round-bottom flask, 1.83 g (0.009 mol) of 98% 1,12-diaminododecane are dissolved in 50 mL of a 50/50 by volume water/ethanol mixture at ambient temperature. This solution is then placed in a dropping funnel connected to the 350 mL reactor and is added dropwise over 1 hour to the ethanolic solution of pyromellitic acid. This time, no precipitate is formed on contact between the diamine and the pyromellitic acid: the salt formed dissolves immediately. The reaction medium is held with stirring at 80 C. for 3 hours under nitrogen.

(21) Then 100 mL of the water/ethanol mixture are evaporated by heating at atmospheric pressure to 130 C., after which the reaction medium is brought to ambient temperature. The reactor is subsequently immersed in water at 5 C. to cause the salt to crystallize. The salt is recovered by filtration on a Bchner funnel, ground and washed with ethanol, then dried under vacuum at 50 C. overnight. The mass yield is 94%. The product takes the form of a fine white powder.

(22) Polymerization

(23) The 12PMA salt powder is polymerized by heating at 200 C. to give a PI 12PMA powder. DSC analysis of the polyimide thus formed shows that the melting temperature Tf is 308 C., and the crystallization temperature Tc is 273 C. It appears that the melting temperature is slightly greater than that of the PI 12PMA made from a salt in pure ethanol.

(24) It can be seen that when the 12PMA salt is made in solution in a water/ethanol mixture, in other words in a medium in which it is soluble, this influences the eventual thermal properties of the polyimide made in this way.

Example 3: Preparation of a Polyimide PI 13PMA from a Salt 13PMA Synthesized in Pure Ethanol

(25) 1,13-Diaminotridecane is synthesized from a nitrilation of a 99% 1,13-tridecanedicarboxylic acid (Zibo Guangtong Chem), followed by a hydrogenation. The purity of the C13 diamine is 93%.

(26) 40 g (0.15 mol) of 94.9% pyromellitic acid and 2 liters of pure ethanol are placed in a 5 L reactor. The reaction medium is stirred and heated to 70 C. while flushing gently with nitrogen. 34.5 g (0.15 mol) of 93% 1,13-diaminotridecane are dissolved in 500 mL of pure ethanol at ambient temperature in a 1 L round-bottomed flask. This solution is then placed in a dropping funnel connected to the 5 L reactor and added dropwise over 1 hour to the ethanolic solution of pyromellitic acid. The contact between the diamine and the pyromellitic acid gives rise to the formation of a salt which precipitates out immediately with stirring. The reaction medium is held with vigorous stirring for 3 hours 30 minutes at 70 C. under nitrogen.

(27) The salt powder is recovered by filtration through a Bchner funnel and washed with ethanol, ground and then dried under vacuum at 50 C. overnight. The mass yield is 85%, owing to losses during emptying of the reactor. The powder is fine and white. The melting temperature of the salt is 230 C.

(28) Polymerization

(29) The 13PMA salt powder is placed in a fluted flask attached to a rotary evaporator and placed under a gentle flush of nitrogen. The pressure is equal to atmospheric pressure. The flask is immersed in an oil bath at 200 C. and rotated for 8 hours. The PI 13PMA powder obtained is white and totally dry. The particles exhibit a median diameter D50 of 179 m. TGA analysis is performed on the final product, by heating from 40 C. to 300 C. at 10 C./min. No detectable loss of mass appears, indicating that the 13PMA salt powder has been quantitatively converted into PI 13PMA powder.

(30) FTIR analysis of the PI 13PMA powder shows the characteristic absorption bands of imide functions at 1700 and 1767 cm.sup.1, and the absence of absorption bands characteristic of amine functions is noted.

(31) The PI 13PMA powder has a melting temperature of 271 C. (Hf=36 J/g), a crystallization temperature of 238 C., and a Tg=93 C.

(32) Extrusion

(33) The PI 13PMA powder is extruded in rod form by placement of 10 g of the PI 13PMA thus prepared in a DSM MIDI 2000 twin-screw microextruder (micro-compounder) (volume 15 cm.sup.3) which has been preheated to 300 C. and has a screw speed of 100 revolutions per minute.

(34) The melting temperature of the PI 13PMA rod extruded from powder is 270 C., i.e., identical to the melting point of the PI 13PMA powder before extrusion. It will be noted that extrusion is easier to carry out with the PI 13PMA than with the PI 12PMA, which must be employed at a higher temperature.

(35) Injection

(36) The PI 13PMA powder is injected by means of an injection micropress combined with the micro-compounder, by melting of the PI 13PMA at 300 C. and injection in a mold regulated at 200 C., to form bars with dimensions of 80122 mm.sup.3. The bars are rigid but have a certain amount of flexibility.

Example 4: Preparation of Copolyimides PI 10PMA/12PMA of 100/0, 75/25, 50/50, 25/75 and 0/100 Mol/Mol by Synthesis of Co-Salts

(37) Using a procedure like that of example 1, this time an ethanolic solution of pyromellitic acid is added dropwise to a stoichiometric amount of a mixture of 1,10-diaminodecane (C10 diamine) and 1,12-diaminododecane (C12 diamine) in solution in pure ethanol. The molar ratio of the two diamines, C10/C12, that is selected is 100/0 (example 4A), 75/25 (example 4B), 50/50 (example 4C), 25/75 (example 4D), and 0/100 (example 4E). The salts formed precipitate immediately and are recovered by evaporation of the solvent, then dried overnight under vacuum at 50 C. A PI powder formed is produced by heat treatment at 200 C. of the salt powder, then analyzed by DSC. The primary observation is that all of the copolyimides are semicrystalline.

(38) It is also observed, in table 1 below, that the copolyimides exhibit a single melting temperature, which means that they are copolymers capable of cocrystallization. This melting temperature may be between the Tf values of the two homopolyimides or even lower. It also appears that the heat of fusion is lower than the heat of fusion of the homopolymers but that it remains high irrespective of the molar composition of the diamines.

(39) TABLE-US-00001 TABLE 1 Tf Salt TfPI HfPI TcPI TgPI* PI 10PMA/12PMA C. C. J/g C. C. 4A 245 334 47 306 115 4B 242 294 19 274 109 4C 237 269 26 255 104 4D 238 285 30 261 100 4E 260 303 35 274 96 *The Tg is determined at 10 C./min

Example 5: Preparation of Low-Viscosity PI 12PMA Polyimide by Stoichiometric Imbalance from a 12PMA Salt Synthesized in Pure Ethanol

(40) In order to obtain polyimides prepared from polyimide salt having a lower viscosity, an excess of one of the two monomers was introduced.

(41) Example 5A: a 2 L reactor is charged with 17.538 g (0.0674 mol) of 97.6% pyromellitic acid and 600 g of pure ethanol. The reaction medium is stirred with gentle flushing with nitrogen. In a 500 mL round-bottom flask, 13.373 g (0.066 mol) of 98.9% 1,12-diaminododecane (TCI Europe N.V.) are dissolved in 260 g of pure ethanol at 60 C. This solution is then placed in a dropping funnel connected to the 2 L reactor, and is added dropwise over 30 minutes to the ethanolic solution of pyromellitic acid. Contact between the diamine, the monoamine, and the pyromellitic acid brings about the formation of a salt, which precipitates immediately under vigorous stirring. The reaction medium is heated to 70 C. and kept with vigorous stirring for 2 hours 30 minutes under nitrogen. The salt powder is recovered by evaporation of the ethanol, then ground and dried under vacuum at 45 C. overnight. The mass yield is 99.1%. The powder is fine and white. The stoichiometric ratio R=[pyromellitic acid]/[diamine] is 1.02.

(42) The salt powder is placed in a fluted round-bottom flask attached to a rotary evaporator, and is placed under gentle flushing with nitrogen. The pressure is equal to atmospheric pressure. The flask is immersed in an oil bath at 200 C. and rotated for 5 hours. The PI 12PMA powder obtained is white and completely dry.

(43) The resulting polyimide powder is analyzed by measurement of the relative solution viscosity in 96% sulfuric acid of a 10 g/L polyimide solution in an Ubbelohde tube with a diameter of 1.03 mm in combination with a SHOTT viscosimeter having the reference AVS350 and at a temperature of 25 C. The relative viscosity is 2.06.

(44) Examples 5B, 5C and 5D are carried out using the same method as for example 5A, but by changing the stoichiometric ratio Requal to 1.04, 1.08, and 1.12, respectively. In all of these examples, there is more pyromellitic acid than diamine. For comparison, an example 5E is carried out with a ratio R of 0.93, in other words a case where there is less pyromellitic acid than diamine.

(45) Table 2 reports the viscosities of the resulting polyimides. It is apparent that when there is more diamine, the resulting polyimide is insoluble, a sign of branching or of substantial crosslinking. When there is more pyromellitic acid, the resulting polyimide is highly soluble and the viscosity in solution decreases in line with the size of the ratio R. This route for stoichiometric imbalance of salts allows the viscosity of the polyimides obtained by the salt route to be modified.

(46) TABLE-US-00002 TABLE 2 Viscosity in Examples R solution 5A 1.02 2.06 5B 1.04 1.56 5C 1.08 1.49 5D 1.12 1.45 5E 0.93 Insoluble

(47) The number-average molar mass of samples 5A and 5D is determined by .sup.1H NMR in D.sub.2SO.sub.4 as solvent at 300 kelvins. The samples exhibit no amine ends, as expected, because of the stoichiometric imbalance selected in favor of the acid functions. The number-average molar mass can therefore be calculated using the following equation: Mn=DPnM where M=(Mdiamine+MPMA-184)/2=191 g/mol and DPn=(Ndiamine+NPMA)/(Nchain end)/2, i.e., Mn=13 800 g/mol for example 5A and Mn=5600 g/mol for example 5D.

Example 6: Preparation of Low-Viscosity PI 12PMA Polyimide by Use of Chain Transfer Agent from a 12PMA Salt Synthesized in Pure Ethanol

(48) In order to prepare polyimides prepared from polyimide salt having a lower viscosity, a chain transfer agent, 1-aminododecane, was introduced.

(49) Example 6A: a 2 L reactor is charged with 17.07 g (0.0655 mol) of 97.6% pyromellitic acid and 550 g of pure ethanol. The reaction medium is stirred with gentle flushing with nitrogen. In a 500 mL round-bottom flask, 13.15 g (0.0649 mol) of 98.9% 1,12-diaminododecane (TCI Europe N.V.) and 0.2530 g (0.00137 mol) of 98% 1-aminododecane are dissolved in 200 g of pure ethanol at 60 C. This solution is then placed in a dropping funnel connected to the 2 L reactor, and is added dropwise over 1 hour to the ethanolic solution of pyromellitic acid. Contact between the diamine, the monoamine, and the pyromellitic acid brings about the formation of a salt, which precipitates immediately under vigorous stirring. The reaction medium is heated to 70 C. and kept with vigorous stirring for 2 hours 30 minutes under nitrogen. The salt powder is recovered by filtration on a Bchner funnel and washed with ethanol, then ground and dried under vacuum at 45 C. overnight. The mass yield is 98.8%. The powder is fine and white. NMR analysis of the salt indicates the presence of 1.8 mol % of 1-aminododecane, relative to the mixture of 1-aminododecane and 1,12-diaminododecane.

(50) The salt powder is placed in a fluted round-bottom flask attached to a rotary evaporator, and is placed under gentle flushing with nitrogen. The pressure is equal to atmospheric pressure. The flask is immersed in an oil bath at 200 C. and rotated for 5 hours. The PI 12PMA powder obtained is white and completely dry.

(51) The resulting polyimide powder is analyzed by measurement of the relative solution viscosity in 96% sulfuric acid of a 10 g/L polyimide solution in an Ubbelohde tube with a diameter of 1.03 mm in combination with a SHOTT viscosimeter having the reference AVS350 and at a temperature of 25 C. The relative viscosity is 4.9.

(52) Example 6B: the same protocol is used, but using 1.6725 g (0.00642 mol) of 97.6% pyromellitic acid in solution in 60 g of ethanol, and a mixture of 1.2475 g (0.00616 mol) of 98.9% 1,12-diaminododecane and 0.1023 g (0.00054 mol) of 98% 1-aminododecane. The polyimide obtained has a relative viscosity of 2.61.

(53) The color of the PI 12PMA powder from example 6B is analyzed. The powder has the colorimetric characteristics CIE V=98.17, a*=0.20, b*=2.66, thereby indicating that the powder is very white. A powder is considered here to be white when b*10 and slightly yellow when b*>10. The polymerization process of the invention therefore prevents instances of coloring. Without wishing to be tied to any one theory, this may be due either to degradations, or to the presence of residual solvents, as is the case in the polymerizations of polyimides by a melt route, above the melting temperature of the polyimide, or by a solvent route.

(54) The polyimide 6B is injected by means of an injection micropress combined with the micro-compounder, by melting of the PI 12PMA at 320 C. and injection in a mold regulated at 180 C., to form bars with dimensions of 80122 mm.sup.3. The bars are rigid but have a certain amount of flexibility. A dynamic mechanical analysis in three-point bending (imposed strain of 0.01%, frequency 1 Hz) is carried out on a TA Instrument RSA3 apparatus. At 23 C., the E modulus is 2.2 GPa.

(55) Example 6C: the same protocol is used, but using 28.1459 g (0.10809 mol) of 97.6% pyromellitic acid in solution in 535 g of ethanol, and a mixture of 21.3347 g (0.10531 mol) of 98.9% 1,12-diaminododecane and 1.0556 g (0.00558 mol) of 98% 1-aminododecane. This time, the salt precipitate is recovered by evaporation of the ethanol under reduced pressure, and then ground. The polyimide obtained has a relative viscosity of 1.76.

(56) 1 g of the polyimide from example 6C is placed in a test tube and inertized with nitrogen. The tube is placed in a block heated to 320 C. (above the melting temperature of the polyimide) for 40 minutes. After 40 minutes, the relative viscosity is determined at 1.66, which shows that the viscosity of the polyimide has changed by only 6% during its passage to the melt state over 40 minutes. The first conclusion is that the viscosity has not increased, meaning that the synthesis process allows sufficient and controlled molar masses to be obtained, and the second conclusion is that the PI 12PMA blocked with 1-aminododecane hence exhibits a high thermal stability.

Example 7: Preparation of PI 12PMA/Carbon Fabric Composite

(57) A batch of 200 g of PI 12PMA polyimide powder blocked with 12-aminododecane, with a relative viscosity of 1.7, is used to make a thermoplastic PI 12PMA/carbon fabric composite. The polyimide used was synthesized by the processes described above in example 6C.

(58) Prior to use, the powder is dried at 90 C. under vacuum overnight.

(59) The reinforcement used in this example is in the form of preforms made of carbon fabrics, cut to the dimensions required for the manufacture of plaques, namely 100150 mm. The reinforcing fabric used is a balanced fabric made of carbon fiber (0-90) originating from Hexcel, having a grammage of 200 g/m.sup.2 (3K).

(60) The composite components are produced by means of a force-controlled two-plate 100 tonne hydraulic press equipped with an induction heating mold (RocTool technology) and with cooling means (water circulation). The metal mold has a cavity with dimensions 150 mm150 mm.

(61) To produce a composite containing 55% by volume of carbon fibers with the fabrics having a grammage of 200 g/m.sup.2 (3K), a preform is prepared by stacking carbon layers, each layer being given a relatively uniform dusting of the polyimide powder. In the example under consideration, 10 layers of carbon (200 g/m.sup.2) were used. The preform, consisting of the stack of dusted layers, is then introduced into the mold.

(62) Following introduction of the preform and closing of the mold, under very low pressure, the temperature of the press plateaus is then raised to 320 C. in 80 seconds. A plateau is performed under very low pressure for 60 seconds at 320 C. At the end of the plateau (60 seconds) a pressure is applied for 20 seconds: 25 bar jack. Cooling is carried out under pressure over 6 minutes and 30 seconds: the plaques are demolded at approximately 50 C. The total cycle time is less than 10 minutes.

(63) The plaques obtained have a thickness of 2.12 mm. The composite articles of the invention exhibit a very good surface appearance.

(64) It is therefore possible to obtain composite articles by using the polyimides of the invention, in particular with extremely short manufacturing cycles being operated.