EP0549494B1 - Process for spinning solutions of polyamide-imides based on metaphenylene diisocyanate and fibres made thereof - Google Patents

Abstract

Process for obtaining PAI fibres by spinning PAI in solution and the fibres thus obtained. They are obtained by dry or wet spinning in dimethylalkyleneurea, followed by removal of the solvent and overdrawing at elevated temperature. The yarns and fibres obtained originate from PAI based on tolylene or meta-phenylene diisocyanate and on an aromatic acid anhydride and/or an aromatic dianhydride, and optionally on one or a number of diacidic compounds. They exhibit an excellent thermomechanical behaviour and allow access to very low filament counts.

Description

The present invention relates to a process for obtaining fibers thermostable by spinning in polyamide-imide solution and the fibers as well obtained.

It is known according to FR 2 079 785 to manufacture shiny wires with base of polyamide-imides containing at least 3% of sequences originating from 3,5-dicarboxy alkali or alkaline earth metal benzene sulfonate by spinning wet of a solution of polymer in N-methyl-pyrrolidone, in a bath aqueous also containing N-methyl-pyrrolidone then drawing, washing, drying.

But such threads have, in addition to an important yellow coloring, a insufficient thermomechanical resistance for certain applications.

To improve the mechanical properties of polyamide-imides it has also prepared, according to FR 2 643 084 fibers based on polyamide-imides preferably derived from diphenyl ether-4,4 'diisocyanate. But such fibers have a low stretchability which does not allow access to low strand titles.

In addition, 4,4 'diphenyl ether diisocyanate is a product with difficulty commercially accessible and expensive.

• des enchaínements amides-imides (A) de formule :
  
• éventuellement des enchaínements amides (B) de formule :
  
• éventuellement des enchaínements amides (C) de formule : -NH-Ar₁-NH-CO-R-CO-
• des enchaínements imides (D) de formule :
  
  dans lesquels:
  • Ar₁ représente un radical aromatique divalent métaphénylène,
  • Ar₂ représente un radical aromatique trivalent,
  • Ar₃ représente un radical aromatique tétravalent,
  • R représente un radical aromatique divalent,
  • M représente un métal alcalin ou alcalino-terreux.
• les enchaínements (A) étant présents à raison de 0 à 100 %, de préférence 20 à 100 %,
• les enchaínements (B) étant présents à raison de 0 à 5 %,
• les enchaínements (C) étant présents à raison de 0 à 80 %,
• les enchaínements (D) étant présents à raison de 0 à 80 %, la somme des enchaínements (A) + (B) + (C) + (D) étant égale à 100 %, dans un milieu coagulant aqueux contenant 30 à 80 % en poids de diméthylalkylène urée (DMAU), de préférence 50 à 65 %,
• étirage des filaments obtenus à un taux d'au moins 2 X,
• élimination du solvant résiduel,
• séchage par tout moyen connu,
• surétirage à un taux de 2 X au moins, de préférence au moins 3 X, à une température d'au moins 250 °C, généralement au moins 300 °C ou même plus, le taux d'étirage total étant d'au moins 5 X, de préférence au moins 6 X.

The present invention relates to a process for obtaining yarns and fibers based on polyamide-imides by spinning a solution in dimethylalkylene urea of a polymer comprising:

• amide-imide sequences (A) of formula:
  
• optionally amide sequences (B) of formula:
  
• optionally amide sequences (C) of formula: -NH-Ar ₁ -NH-CO-R-CO-
• imide sequences (D) of formula:
  
  wherein:
  • Ar ₁ represents a divalent metaphenylene aromatic radical,
  • Ar ₂ represents a trivalent aromatic radical,
  • Ar ₃ represents a tetravalent aromatic radical,
  • R represents a divalent aromatic radical,
  • M represents an alkali or alkaline earth metal.
• the sequences (A) being present at a rate of 0 to 100%, preferably 20 to 100%,
• the sequences (B) being present at a rate of 0 to 5%,
• the sequences (C) being present at a rate of 0 to 80%,
• the sequences (D) being present at a rate of 0 to 80%, the sum of the sequences (A) + (B) + (C) + (D) being equal to 100%, in an aqueous coagulating medium containing 30 to 80% by weight of dimethylalkylene urea (DMAU), preferably 50 to 65%,
• drawing of the filaments obtained at a rate of at least 2 X,
• elimination of the residual solvent,
• drying by any known means,
• over-drawing at a rate of at least 2 X, preferably at least 3 X, at a temperature of at least 250 ° C, generally at least 300 ° C or even more, the total drawing rate being at least 5 X, preferably at least 6 X.

Preferably the polyamide-imide used has a viscosity inherent ≥ 0.8 dl / g.

The dimethylalkylene urea used is preferably dimethylethylene urea or dimethylpropylene urea.

The yarns and fibers according to the present invention can also be prepared by dry spinning of a solution of concentration 15 to 35%, preferably 20 to 30% in the dimethylalkylene urea of a polyamide-imide containing the chains of a copolymer A, B, C and D of the formula described above, with Ar ₁ , Ar ₂ , Ar ₃ , R and M having the same meaning, in an evaporative atmosphere maintained at a temperature close to or above the boiling point of the solvent, the filaments at the outlet of the evaporative enclosure being freed of their residual solvent. For this they can be washed with water, optionally boiling and under pressure, dried in the usual way, preferably at a temperature above 80 ° C. They can also be heat treated at a temperature ≥ 160 ° C under reduced pressure, and / or under an inert atmosphere. After being freed from their residual solvent, they are drawn at a temperature above 250 ° C, preferably above 300 ° C, preferably in the absence of oxygen.

The total drawing rate practiced is at least 5 X, preferably at minus 6 X.

Such polymers can be obtained by reaction (a), in substantially stoichiometric proportions and in the absence of catalyst, in an anhydrous polar solvent of metaphenylene diisocyanate, with at least minus an acid reagent comprising an aromatic anhydride-acid optionally an aromatic dianhydride, optionally a 3,5-dicarboxy alkali or alkaline earth benzene sulfonate, and optionally a diacid aromatic under the operating conditions described in the patent application French 1,600,067 filed on December 30, 1968.

These polymers can also be obtained by reaction (b) of diisocyanate indicated above and an acid reagent comprising a dianhydride aromatic, and an aromatic diacid, optionally of a dicarboxy-3,5-benzene sulfonate alkali or alkaline earth, in the absence aromatic acid anhydride, in stoichiometric proportions and in the absence of catalyst.

• enchaínements (A) : 20 à 100 %
• enchaínements (B) : 0 à 5 %
• enchaínements (C) : 0 à 80 %
• enchaínements (D) : 0 à 80 %

When using reaction (a) the proportions of the different sequences are as follows:

• sequences (A): 20 to 100%
• sequences (B): 0 to 5%
• sequences (C): 0 to 80%
• sequences (D): 0 to 80%

• enchaínements (A) : 0 %
• enchaínements (B) : 0 à 5 %
• enchaínements (C) : 0 à 80 %
• enchaínements (D) : 0 à 80 %.

La somme des enchaínements (A) + (B) + (C) + (D) = 100 %.When using reaction (b) the proportions of the sequences are as follows:

• sequences (A): 0%
• sequences (B): 0 to 5%
• sequences (C): 0 to 80%
• sequences (D): 0 to 80%.

The sum of the sequences (A) + (B) + (C) + (D) = 100%.

Optionally, to the metaphenylenediisocyanate, a minor proportion of another aromatic, aliphatic diisocyanate, or cycloaliphatic in order to improve certain properties of articles consistent; for example it can be useful for improving the properties fibers obtained from substituting up to 30% m-PDI with paraphenylenediisocyanate (p-PDI).

As the acid anhydride, anhydride is preferably used trimellic and as aromatic dianhydride mention may be made of dianhydrides pyromellic acid, diphenyl acid 3,3 '4,4'-tetracarboxylic acid naphthalene-2,3,6,7-tetracarboxylic acid, diphenyl ether acid-3,3 ', 4,4' tetracarboxylic, 3,3 'diphenylsulfone, 4,4' tetracarboxylic and preferably, diphenylketone-3,3'4,4'-tetracarboxylic acid dianhydride. Several of these dianhydrides can be used as a mixture; and among the aromatic diacids, terephthalic and isophthalic acids are often used and, although terephthalic acid is preferred, other diacids may suitable, such as biphenyl-dicarboxylic acids or naphthalene-dicarboxylic acids. The trimellic anhydride used must be pure and in particular should not not contain more than 5 mol% of trimellic acid.

The alkali metal dicarboxy-3,5-benzene sulfonate or alkaline earth is preferably sodium or potassium sulfonate.

• anhydride-acide aromatique : de 0 à 100 % par rapport à l'ensemble des réactifs acides, de préférence 20 à 100 %,
• diacide aromatique : de 0 à 80 %,
• dicarboxy-benzène sulfonate en proportion de 0 à 5 %
• dianhydride aromatique : de 0 à 80 % par rapport à l'ensemble des réactifs acides. Les polymères ainsi obtenus possèdent de préférence une viscosité inhérente d'au moins 0,8 dl/g, de préférence au moins 0,9 dl/g, pour pouvoir être filés et donner des fils présentant de bonnes propriétés mécaniques.

The various acid or anhydride-acid and dianhydride compounds are present in the following molar proportions:

• anhydride-aromatic acid: from 0 to 100% relative to all of the acid reactants, preferably 20 to 100%,
• aromatic diacid: from 0 to 80%,
• dicarboxy-benzene sulfonate in proportion from 0 to 5%
• aromatic dianhydride: from 0 to 80% relative to all of the acid reactants. The polymers thus obtained preferably have an inherent viscosity of at least 0.8 dl / g, preferably at least 0.9 dl / g, in order to be able to be spun and to give strands having good mechanical properties.

Below these viscosity values, the molecular weights are insufficient and the son obtained are difficult to use.

t0 (en secondes) étant le temps d'écoulement du solvant pur

t1 (en secondes) étant le temps d'écoulement de la solution.

Polyamide-imides also have a glass transition temperature of at least 290 ° C, generally higher than 300 ° C, which contributes to gaining wires with good thermomechanical resistance. The inherent viscosity represents the measurement of the flow time of a 0.5% (weight / volume) polymer solution in the 25% DMEU in a 0.8 mm diameter capillary. η = 4.6 (log t1 - log t0)

t0 (in seconds) being the flow time of the pure solvent

t1 (in seconds) being the solution flow time.

We use a dimethylalkylene urea, for example dimethylethylene urea or dimethylpropylene urea, and polyamide-imide spinning solutions have the advantage of being lightly colored. They must also have a viscosity allowing their spinning, generally between 40 and 100, preferably 50 and 80 Pa.s (between 400 and 1000, preferably 500 and 800 poises) measured using a viscometer known in the trade under the brand EPPRECHT RHEOMAT 15 for wet spinning, and 150 to 300 Pa.s (1,500 to 3,000 poises) for dry spinning.

The spinning solution may have a polymer concentration between 10 and 35%, preferably between 15 and 25%. It can contain various additives intended to modify the appearance or the final properties of the yarns obtained such as, dyes, matifiers, stabilizers.

The temperature of the spinning solution can vary within large limits depending on the viscosity of the spinning solution. For example a solution having a low viscosity can easily be extruded at temperature ordinary, while it is preferable to hot extrude, for example at 120 ° C or even more, a solution of high viscosity to avoid using too large sector pressures.

In the case of wet spinning the coagulating bath used in the process according to the invention is an aqueous solution containing from 30 to 80% by weight of dimethylalkylene urea (DMAU) although it is often beneficial to use a bath containing more than 50% by weight of DMAU to obtain filaments of better stretchability, therefore better final properties.

The speed of passage of the filaments in the coagulating bath can vary within wide limits, depending on its solvent concentration and the distance of travel of the filaments in this bath. This passage speed filaments in the coagulating bath can easily be chosen between 10 and 60 m / min, for example, although higher speeds can be reached. There is generally no advantage in spinning at lower speeds for reasons of profitability of the process. In addition, excessively high speeds of passage of the filaments in the coagulating bath reduces the stretchability of the filaments in the air. The speed of passage of the filaments in the coagulating bath will therefore chosen to take into account both profitability and desired qualities on the thread finished.

The temperature of the coagulating bath can be chosen between 15 and 40 ° C. for example, it is generally between 20 and 30 ° C.

The filaments thus obtained are then drawn, preferably in air, at a rate of at least 2 X or more.

After stretching, preferably in air, generally carried out by passage between two series of rollers, the residual solvent is removed from the filaments by known means, generally by washing with water running against the current or on washing rollers, at room temperature.

The wires obtained by dry spinning are pre-stretched in the spinning and then the residual solvent is removed, either by heat treatment at a temperature above 100 ° C, either by washing with water, preferably with water boiling under pressure.

In both spinning processes, the washed filaments are then dried by known means, for example in a dryer or on rollers. The temperature of this drying can vary within wide limits as well as the speed which is all the greater the higher the temperature high. It is generally advantageous to carry out drying with elevation progressive temperature, this temperature can reach and even exceed 200 ° C for example.

The filaments from which the solvent and water have been removed undergo a second stretching to improve their mechanical qualities and to allow reach fine stranded titles, which may be less than 1 dtex / strand.

The overstretching can be carried out by any known means: oven, plate, rollers, at a temperature of at least 250 ° C, preferably at least 300 ° C up to 400 ° C, preferably in the absence of oxygen.

Over-drawing, generally carried out at a rate of at least 2 X, of preferably at least 3 X, up to 4 or 5 X, so that the overall rate stretching is at least 5 X, preferably at least 6 X.

According to the present invention, the PAI yarns derived from metaphenylene diisocyanate has the unexpected characteristic of exhibiting excellent stretchability and therefore allow access to titles that are thinner than those polyamide-imides from other diisocyanates such as 4,4 'diphenylmethane diisocyanate or 4,4 'diphenyl ether diisocyanate described previously in French patents 2,079,785 and 2,643,084. They also have the advantage less coloring and above all better thermomechanical behavior, as will be seen later in the description.

• enchaínements amides-imides (A) de formule :
  
• éventuellement des enchaínements amides (B) de formule :
  
• éventuellement des enchaínements amides (C) de formule : -NH-Ar₁-NH-CO-R-CO-
• des enchaínements imides (D) de formule :
  
  dans lesquels :
  • Ar₁ représente un radical aromatique métaphénylène,
  • Ar₂ représente un radical aromatique trivalent,
  • Ar₃ représente un radical aromatique tétravalent
  • R représente un radical aromatique, divalent,
  • M un métal alcalin ou alcalino-terreux,
• les enchaínements (A) étant présents à raison de 20 à 100 %, de préférence de 50 à100 %,
• les enchaínements (B) étant présents à raison de 0 à 5 %,
• les enchaínements (C) étant présents à raison de 0 à 50 %,
• les enchaínements (D) étant présents à raison de 0 à 80 %, de préférence 20 à 80%,
• la somme des enchaínements A + B + C + D étant égale à 100 %,

possédant une excellente tenue thermomécanique et une faible coloration.The present invention also relates to yarns and fibers based on polyamide-imides consisting of:

• amide-imide sequences (A) of formula:
  
• optionally amide sequences (B) of formula:
  
• optionally amide sequences (C) of formula: -NH-Ar ₁ -NH-CO-R-CO-
• imide sequences (D) of formula:
  
  wherein :
  • Ar ₁ represents a metaphenylene aromatic radical,
  • Ar ₂ represents a trivalent aromatic radical,
  • Ar ₃ represents a tetravalent aromatic radical
  • R represents a divalent aromatic radical,
  • M an alkali or alkaline earth metal,
• the sequences (A) being present in an amount of 20 to 100%, preferably from 50 to 100%,
• the sequences (B) being present at a rate of 0 to 5%,
• the sequences (C) being present at a rate of 0 to 50%,
• the sequences (D) being present at a rate of 0 to 80%, preferably 20 to 80%,
• the sum of the sequences A + B + C + D being equal to 100%,

with excellent thermomechanical resistance and low coloring.

The yarns and fibers according to the invention preferably have a inherent viscosity ≥ 0.8 dl / g, preferably 0.9 dl / g.

The thermomechanical behavior is highlighted by the retention of the level of the modulus of elasticity by linear rise in temperature with a temperature increase from 50 to 400 ° C approximately. The retention of modulus of elasticity is ≥ 40% at 310 ° C. PAI-based wires from m-PDI have a very low initial coloration, allowing them to be dyed in very light shades, unusual for these types of product.

They also have tenacity retention after exposure 1000 hours at 200 ° C of at least 75%, preferably at least 80%, and after 5,000 hours exposure to 200 ° C of at least 65%, preferably at minus 70%.

The yarns according to the invention also have excellent stretchability, making it possible to reach very low strands, less than 1 dtex / strand, which is quite unusual for thermostable wires and their gives a very soft textile feel. They also have excellent mechanical properties, toughness at break, modulus of elasticity, and a low elongation. Thus they combine a textile touch and good characteristics mechanical and thermomechanical. They can be dyed easily with basic dyes.

They can be used alone or in mixtures with natural threads or synthetic in order to improve or modify certain properties. They find their use in a wide range of applications, especially work and protective clothing.

When the wires are free of units (B), they can also enter in the composition of many composites, especially for applications dielectric.

Finally, they have an important economic advantage because the metaphenylene diisocyanate is known for its accessibility and its price of low market, which represents a significant industrial advantage; this is particularly significant compared to yarn from polyamide-imides prepared from 4,4 'diphenyl ether diisocyanate.

In the examples that follow the values of Mn and Mw are determined by gel exclusion chromatography (GPC) in NMP at 80 ° C and 0.1 mole / liter of lithium bromide, the masses being expressed relative to a polystyrene calibration.

Polydispersity index 1 corresponds to the ratio Mw / Mn

- EXAMPLES 1 TO 4 (TDI) (comparative)

A 21% solution in dimethylethylene urea of a sulfonated copolyamide-imide is prepared by reaction, in the absence of catalyst, of: - DMEU 257.1 g 244 ml - toluylene diisocyanate 87.0 g 0.5 mole - trimellic anhydride 76.8 g 0.4 mole (80 mol%) - terephthalic acid (AT) 13.28 g 0.08 mole (16 mol%) - dicarboxy-3,5 benzene - sodium sulfonate 5.36 g 0.02 mole (4 mol%) - DMEU of dilution 236.7 g 250 ml Molar mass Mn 50,020 Polydispersity I 1.78 Inherent viscosity 0.97 dl / g

A 21% concentration solution with a viscosity of 60.3 Pa.s is obtained. (603 poises) measured on an EPPRECHT RHEOMAT 15 viscometer. D + E tank at 25 ° C.

The solution maintained at a temperature of 70 ° C. is extruded at through a die with 62 orifices, 0.06 mm in diameter, into a bath DMEU / water coagulant containing 62% by weight of DMEU and 38% by weight of water, maintained at 27 ° C., the length of the path of the filaments in this bath being approximately 1 meter. At the end of the coagulating bath the filaments are taken up by a first train of rollers and drawn into the air between the first and second train of rollers at a rate of 2 X. They are then washed with water against the current in a washing tub, dried in an oven maintained at around 150 ° C, then over-stretched in an oven maintained at a temperature of around 350 ° C.

• Tenue thermique :
  Rétention de la ténacité après exposition de 1 000 heures à 200 °C : 80 % et après 5 000 heures à 200 ° C = 70 %. Un fil obtenu selon le FR 2 643 084 constitué d'un polyamide-imide issu des mêmes monomères que ci-dessus excepté le toluylène diisocyanate qui a été remplacé par le diphénylméthane-4,4' diisocyanate, possède une rétention de la tenacité après exposition pendant 1 000 heures à 200 °C : 38 %.Tenue thermomécanique : rétention du module d'élasticité en fonction de la température :
• la fibre conserve 50 % de son module à 310 °C,
• une fibre préparée selon le FR 2 643 084 à base de diphénylméthane-4,4' diisocyanate conserve seulement 22,5 % de son module à 310 °C

Several rates of over-drawing have been achieved. The characteristics of the wires are gathered in the following table I: Ex. 1 Ex. 2 Ex. 3 Ex. 4 - over-stretching rate 4.5 3.5 4 4.2 - overall rate 9 7 8 8.4 - stranded title (dtex) 1.03 1.03 1.21 0.87 - fracture toughness g / tex 21.6 24.7 18.7 23.3 - elongation at break% 16.2 15.7 17.8 17.6

• Thermal resistance:
  Tenacity retention after 1000 hours exposure to 200 ° C: 80% and after 5000 hours at 200 ° C = 70%. A wire obtained according to FR 2 643 084 consisting of a polyamide-imide derived from the same monomers as above except the toluylene diisocyanate which has been replaced by diphenylmethane-4,4 'diisocyanate, has a retention of tenacity after exposure for 1000 hours at 200 ° C: 38%. Thermomechanical resistance: retention of the elastic modulus as a function of the temperature:
• the fiber retains 50% of its modulus at 310 ° C,
• a fiber prepared according to FR 2 643 084 based on 4,4 'diphenylmethane diisocyanate retains only 22.5% of its modulus at 310 ° C

- EXAMPLE 5 (mPDI):

Is prepared in the manner indicated in Example 1 a polyamide-imide by replacing the TDI with metaphenylene diisocyanate in identical molar proportion.

The PAI obtained has a molar mass Mn : 36,560 and a polydispersity index: 2.05 and has an inherent viscosity of 0.86 dl / g.

The viscosity of the solution measured on an EPPRECHT viscometer RHEOMAT is: 56.6 Pa.s (566 poises).

The polyamide-imide solution thus obtained, concentration: 21% is spun and processed as shown in Example 1, the air-drawing being performed at a rate of 2.3 X and overstretching also at a rate of 2.3 X, the rate overall stretch being 5.29 X.

• titre au brin (dtex) : 2,3
• Ténacité rupture : 26 g/tex
• Allongement rupture : 18 %
• Rétention de la ténacité après vieillissement thermique pendant 1 000 heures à 250 °C : 75 %
• Tenue thermomécanique : rétention du module d'élasticité à 310 °C : 40 %.

The yarns obtained have the following characteristics:

• strand title (dtex): 2.3
• Breaking toughness: 26 g / tex
• Elongation at break: 18%
• Tenacity retention after thermal aging for 1000 hours at 250 ° C: 75%
• Thermomechanical behavior: retention of the modulus of elasticity at 310 ° C: 40%.

- EXAMPLE 6 (mPDI):

A 21% solution in dimethylpropylene urea of a sulfonated copolyamide-imide is prepared by reaction, in the absence of catalyst, of: - Dimethylpropylene urea (DMPU) 279.8 g (d = 1.064) = 263.0 ml - trimellic anhydride 61.44 g (0.32 mole) - terephthalic acid 13.28 g (0.08 mole) - Phenylene diisocyanate-1,3 (mPDi) 64 g (0.04 Mole) - DMPU of dilution 109.6 g (103 ml)

A solution with a concentration of 21% in DMPU is obtained, with a viscosity of 81.0 Pa.s (810 poises) measured as indicated in Example 1. The PAI thus obtained has a molar mass Mn = 37,840, a polydispersity of 2.34 and an inherent viscosity of 0.89 dl / g.

The solution thus obtained is spun as indicated in Example 1 and the filaments obtained are drawn at a drawing rate of 2 X, washed and dried, then over-stretched in an oven maintained at 335 ° C at a rate of 3 X.

The filaments obtained have the following characteristics: Strand title (dtex) 2 Breaking toughness 30 g / tex Elongation% 15 Thermomechanical behavior - Retention of the elastic modulus: at 310 ° C: conservation of 43% of the initial modulus.

Claims (16)

1. Thermally stable yarns and fibres based on polyamide-imides, characterized in that they have:

   amide-imide chain sequences (A) of formula:

   

   optionally, amide chain sequences (B) of formula:

   

   amide chain sequences (C) of formula: -NH-Ar₁-NH-CO-R-CO-

   imide chain sequences (D) of formula:

   

   in which:

   Ar₁ denotes a meta-phenylene divalent aromatic radical,

   Ar₂ denotes a trivalent aromatic radical,

   Ar₃ denotes a tetravalent aromatic radical,

   R denotes a divalent aromatic radical,

   M denotes an alkali metal or alkaline earth metal,

   the chain sequences (A) being present in a proportion of 20 to 100 %,

   the chain sequences (B) being present in a proportion of 0 to 5 %,

   the chain sequences (C) being present in a proportion of 0 to < 80 %,

   the chain sequences (D) being present in a proportion of 0 to < 80 %

   the sum of the chain sequences (A) + (B) + (C) + (D) being equal to 100 %,
   and in that they have an at least 40% retention of the modulus of elasticity at 310°C.
2. Yarns and fibres according to Claim 1, characterized in that the polyamide-imide has an inherent viscosity ≥ 0.8 dl/g, preferably ≥ 0.9 dl/g.
3. Yarns and fibres according to Claim 1, characterized in that the chain sequences (A) are present in a proportion of 50 to 100 %, the chain sequences (B) in a proportion of 0 to 3 %, the chain sequences (C) in a proportion of 0 to 50 % and the chain sequences (D) in a proportion of 0 to 50 %.
4. Yarns and fibres according to Claim 1, characterized in that Ar₂ is a radical of formula

   
5. Yarns and fibres according to Claim 1, characterized in that R is a radical of formula

   
6. Yarns and fibres according to Claim 1, characterized in that M is an alkali metal.
7. Yarns and fibres according to Claim 1, characterized in that the retention of the modulus of elasticity at 310°C is at least 50 %.
8. Yarns and fibres according to Claim 1, characterized in that they have a linear density ≤ 1 dtex.
9. Process for obtaining yarns and fibres based on polyamide-imide, which are of improved thermomechanical behaviour, according to one of Claims 1 to 8, characterized in that:

   a) - a solution of a polymer in dimethylalkyleneurea (DMAU) is spun, the polymer comprising:

   amide-imide chain sequences (A) of formula:

   

   optionally, amide chain sequences (B) of formula:

   

   amide chain sequences (C) of formula: -NH-Ar₁-NH-CO-R-CO-

   imide chain sequences (D) of formula:

   

   in which:

   Ar₁ denotes a meta-phenylene divalent aromatic radical,

   Ar₂ denotes a trivalent aromatic radical,

   Ar₃ denotes a tetravalent aromatic radical,

   R denotes a divalent aromatic radical,

   M denotes an alkali metal or alkaline-earth metal,

   the chain sequences (A) being present in a proportion of 20 to 100 %,

   the chain sequences (B) being present in a proportion of 0 to 5 %,

   the chain sequences (C) being present in a proportion of 0 to 80 %,

   the chain sequences (D) being present in a proportion of 0 to 80 %,

   the sum of the chain sequences (A) + (B) + (C) + (D) being equal to 100 %, in an aqueous coagulating medium containing 30 to 80 % by weight of dimethylalkyleneurea (DMAU) and from 20 to 70 % by weight of water,

   b) - the filaments obtained are drawn to a ratio of at least 2x,

   c) - the residual solvent is removed from the filaments and they are dried by any known means,

   d) - they are overdrawn at a temperature of at least 250°C, to a ratio of at least 2×, with the result that the total draw ratio is at least 5×.
10. Process according to Claim 9, characterized in that the solvent is removed by washing.
11. Process according to Claim 9, characterized in that the coagulating bath contains 50 to 65 % by weight of solvent.
12. Process according to Claim 9, characterized in that the overdrawing is performed to a ratio of at least 3×, with the result that the total draw is 6×.
13. Process according to Claim 9, characterized in that the overdrawing is performed at a temperature of at least 300°C, in the absence of oxygen.
14. Process for obtaining yarns and fibres based on polyamide-imide, which are of improved thermomechanical behaviour, characterized in that a solution of a polyamide-imide polymer according to Claim 1 in dimethylalkyleneurea (DMAU) is spun, into an evaporation atmosphere maintained at a temperature close to or higher than the boiling point of the solvent, the residual solvent being removed from the filaments, the latter being then drawn at a temperature above 250°C, with the result that the total draw ratio is at least 5×.
15. Process according to Claim 14, characterized in that the residual solvent is removed from the filaments by heat treatment at a temperature ≥ 160°C.
16. Thermally stable articles comprising yarns and fibres according to Claim 1.