IL36176A - Process for preparing glossy heat-stable polyamide-imide fibres and fibres obtained thereby - Google Patents

Description

ata «333 0*181 yn o»i?»ia» »D»K-T*as» 9 »a»o imnV i* nn πτ i» nn niysasa o»i?sion a»3*oi PROCESS FOR PREPARING- G-LOSST HEAT-STABLE PO LYA IDE- I'lIDE FIBRES AND FIBRES OBTAINED THEREBY This invention relates to glossy heat-resistant fibres obtained by wet spinning polyamide-imides.

It is known to manufacture heat-resistant filaments by dry spinning polyamide-imides, but in this process it is rather difficult to remove the solvent. It is also possible to manufacture heat-resistant filaments by wet spinning polyamide-imides, but the filaments obtained tend to be porous and very matt. Furthermore, it is known from French Patent No. 1,54-8,029 to introduce into the chain of the aromatic polyamides units which carry sulphonate groups, so as to impart an affinity for basic dyestuffs to the polyamides We have now found, according to the present invention, a new process for obtaining glossy heat-resistant polyamide-imide fibres. This process comprises: (1) extruding a solution, in N-methylpyrrolidone , of a polymer containing: amide-imide recurring units (A) of formula: amide recurring units (B) of formula: and, optionally, amide recurring units (C) of formula: -NH-Ar-j-Mi-CO-R-CO-in which: Ar-, represents a dival , optionally containing a small pro or cycloaliphatic Arg represents a trivalent aromatic radical; E represents a divalent, aromatic, cycloaliphatic or araliphatic radical; and M represents an alkali metal or alkaline earth metal, with the units (B) representing at least 3 mole % of the total amount of units (A), (B) and (c)5 into an aqueous coagulating bath containing 30 to 75% by weight of N-methylpyrrolidone, (2) stretching the resulting filaments in air at least 1·5 times, (3) and then washing them to remove the N-methyl-pyrrolidone and (4) drying them.

The polymers used in the process of this invention, which will hereinafter be described as the "sulphonated polyamide-imides" can be obtained by reacting, in substantially stoichiometric proportion, at least one aromatic diisocyanate with an acid reagent containing at least one aromatic acid anhydride, an alkali metal or alkaline earth metal 35 -dicarboxybenzenesulphonate in an amount of at least 3 mole %, preferably 3 to 10 mole %, relative to the total molar amount of the acid reagent, and, optionally, at least one aromatic, cycloaliphatic or araliphatic diacid, in an anhydrous polar organic solvent, under the conditions described in Application No. 3363 , eorrooponding to ffronoh Application No. 181, 9^8 filed on 30th December 1968* Suitable diisocyanates which can be used to obtain these polymers include monocyclic diisocyanates such as the toluylene diisocyanates, and dicyclic diisocyanates, preferably symmetrical dicyclic diisocyanates, such as diisocyanatodiphenylmethane, diisocyanatodiphenylpropane and diisocyanatodiphenyl-ether.

Minor proportions (up to 5 mol %) of an aliphatic or cycloaliphatic diisocyanate can optionally be added to the aromatic diisocyanate so as to improve such properties solubility of the finished product, and the flexibility and elasticity of shaped articles made therefrom.

Trimellitic anhydride is preferably used as the acid anhydride i.e. Ar? represents Amongst the diacids, terephthalic acid and lic acid are preferably used (i.e. R represents a the proportions of the diacide in the mixture being generally between 5 and 95 mol , preferably 20 to 80 mol %, relative t the total molar amount of the acid reagent.

The sulphonated copolyamide-imides can also be obtained by reacting, in substantially stoichiometric proportions, a diamine with a reagent comprising at least one compound containing an acid chloride group and an anhydride group, a dichloride of an alkali or alkaline earth metal 3,5-dicarboxybenzenesulphonate , and, optionally, at least one dichloride of a carboxylic diacid which can be aromatic, cycloaliphatic or araliphatic, in an organic polar solvent.

Suitable polar organic solvents xvhich can be used^ include dimethylformamide, dimethylacetamide , hexamethyl-phosphotriamide , tetramethylenesulphone and, preferably, N-methylpyrrolidone 0 Depending on the process and the solvent used to prepare the polymer, it is possible either to use the solution of sulphonated polyamide—imide directly (as obtained), or first to isolate the polymer and then redissolve it in N-methylpyrrolidone for extrusion purposes, so as to remove the by-products which are formed during the polycondensation.

The sulphonated polyamide-imides which can be used in the present invention must have an intrinsic viscosity-greater than 0.5 and, preferably, no exceeding 1.4. This intrinsic viscosity is measured at 25*0 as a 0. strength weight/volume solution in N-methylpyrrolidone. Polymers of intrinsic viscosity between 0,8 and 1,2 are preferably used.

The solutions of sulphonated polyamide-imides which can be used in the process of this invention should generally have a viscosity of 150 to 3t000 poises at 25*C (measured with a DRAGE viscometer, using speed II, and rotor .2) .

It is however preferable to use solutions of viscosity not exceeding 1, 500 poises. The solution to be spun suitably has a polymer concentration of between 10 and 50%, preferably between 15 and , andean contain various adjuvants such as pigments and delustering agents.

The temperature of the solution being extruded can vary within wide limits depending on the viscosity of the solution to be spun. For example, a solution of relatively low viscosity can easily be extruded at ordinary temperature, whilst a solution of relatively high viscosity is preferably extruded hot, for example at 120*0 or even above, so as to avoid using excessively high pressures at the spinneret.

The coagulating bath used in the process of the invention is an aqueous solution containing 50 to 75% by weight of N-methylpyrrolidone, though it is frequently advantageous to use a bath containing more than 50% by weight of N-methylpyrrolidone so as to obtain filaments which can be stretched better and hence have better final properties. The length of the coagulating bath can vary within wide limits depending on its concentration of N-methylpyrrolidone. Thus, 10 cm of bath can suffice to give good coagulation where the bath contains less than 50% of N-methylpyrrolidone, whereas bath lengths of over 40 cm are preferred if the concentration of solvent in the coagulating bath exceeds 65%. Furthermore, it is generally preferable to increase these lengths if a high number of strands is spun from one and the same spinneret.

The speed of travel of the filaments in the coagulating bath can vary within wide limits, depending on the concentration of N-methylpyrrolidone in the bath and on the length of the path of the filamenits in this bath. This speed of travel of the filaments in the coagulating bath can easily be chosen to be between 10 and 60 m/minute, for example, though higher speeds can be achieved. There is generally no advantage in spinning at lower speeds, for reasons of economy. Furthermore, excessively high speeds through the coagulating bath reduce the stretchability of the filaments in air. For a given proportion of units (B) in the initial polymer, the filaments obtained are glossier, and have the better mechanical properties, the higher is the degree of stretching in air of the base filaments. The speed of travel of the filaments in the coagulating bath will thus be chosen so as to take into account both the economics and the desired properties of the finished filament. The temperature of the coagulating bath can, for example be between 15 and 0eC.

The filaments thus obtained are then stretched in air at least 1.5 times, for example between 1.7 and 2 times or even higher; the filaments obtained are the glossier the higher is this stretching ratio. Since the stretchability of the filaments is the better the higher is the content of N-methylpyrrolidone in the coagulating bath, there is an advantage in using rather high concentrations of N-methylpyrrolidone in the coagulating bath, and high stretching ratios, so as to obtain glossier filaments for a given proportion of units (B) in the polymer. However, as the proportion of units (B) in the polymer increases, the glossiness of the filaments also increases without it being necessary to use a very high stretching ratio.

The proportion of units (B) in the polymer must be at least 3 mole % relative to the total amount of units (A), (B) and (C) and, preferably, between 3 and 10 mole . It can also exceed 10 mole % if desired, though there is generally no value in using an excessively high proportion, for economic reasons. The correct proportion of units (B) in the polymer varies in particular with the gauge of the filaments which it is desired to obtain. Thus 4 mole % can suffice for rather fine filaments, for example of less than 10 dtex gauge after stretching in air, whilst about mole % may be necessary for coarser filaments, for example of gauge about 40 dtex.

After stretching in air, the filaments are washed in known manner so as to free them from N-methylpyrrolidone, This washing can, for example, be effected in successive ou?'' through which water circulates in counter-current, or on washing rollers, or by any other means.

The washed filaments are then dried in known manner, for example in a drier or on rollers. The temperature of this drying process can vary within wide limits, as can the speed, which is the greater the higher is the temperature. It is generally advantageous to carry out the drying with a gradual increase in temperature, and this temperature can, for example, reach, or even exceed, 260CC.

If desired, the washed filaments can be subjected to a further stretching after drying, so as to improve their mechanical properties, in particular their tenacity. This subsequent or secondary stretching can be effected in any known manner such as in a oven, plate, roller, roller or plate. It should be carried out at a temperature of at least 300°C, for example up to 4-20°C or even above. The stretching ratio should generally be at least 1.5 times but this can vary within wide limits depending on the properties desired of the finished tow. This secondary stretching can be carried out in one or. more stages, continuously or discontinuously with the preceding operations. Furthermore, it can be combined with the drying operation. For this, it suffices to provide, at the end of the drying operation a zone of higher temperature which allows the secondary stretching to be carried out.

The filaments thus obtained are very glossy; in contrast wet spinning of polyamide-imides without observing all the essential conditions of the process of the present invention, i.e. starting from polymers which do not contain, or contain insufficient, (B) units, and/or spinning with coagulating baths of a different composition and/or using different stretching ratios from those specified yields very porous and very matt filaments.

It is, in factj very surprising to find- that the introduction of units into the polymer chain together with the other characteristics of the present invention, yields glossy filaments, since the introduction of other sulphonated units of similar formulae such as (the introduction of which into aromatic polyamides is 'described in French Patent No. 1,548,029), even if the other conditions of the process of the present invention are complied with, yields matt and porous filaments.

The filaments obtained according to the present invention furthermore have good physical properties and are very resistant to exposure to high temperatures. They also have good affinity for basic dyestuffs.

In the following Examples, which further illustrat the present invention, parts and percentages are expressed by weight unless otherwise indicated. The viscosity of the solutions was measured with a DHAGE viscometer, using speed II and rotor A-?.2. The intrinsic viscosity was measured at °C as a 0.5% weight/volume solution in N-methylpyrrolidone The diisoc anates rsntioned are th '- EXAMPLE 1 A 21.5% strength solution in N-methylpyrrolidone of a sulphonated polyamide-imide was prepared by reacting: diisocyanatodiplienylmethane 25 molar parts trimellitic anhydride 24.25 " " sodium 3, 5-dicarboxybenzenesulphonate 0.75 " " in dried N-methylpyrrolidone.

The viscosity at 25°0 of the solution obtained was n 2,83 poises and the intrinsic viscosity of the polymer was I.25.

The solution xTas extruded at a speed of 10 m/minute through a spinneret with 600 orifices of 0.06 mm diameter, into a coagulating bath containing 60 of N-methylpyrrolidone and 0% of water. After travelling 20 cm in this bath, the filaments issued therefrom in the form of a gel and were stretched in air at ordinary temperature 1» 5 times» They were then washed with water to remove the solvent and dried on two drying rollers which are at 4-0° and at 150°C, respectively.

The "tow obtained was very glossy. It had a gauge of 2,400 dtex, a dry tenacity of 16 g/tex and a dry elongation of 24%.

By way of comparison, a 21% strength solution of a polyamide-imide obtained by reaction of 25 molar parts of diisocyanatodiphenylmethane and 25 molar parts of trimellitic anhydride in N-methylpyrrolidone was spun in the same manner.

The tow obtained was matt and crazed and was dry to the touch.

EXAMPLE 2 A 17.3% strength solution in N-methylpyrrolidone of a sulphonated copolyamide-imide was prepared by reacting: diisocyanatodiphenylmethane 100 molar parts trimellitic anhydride 80 " " terephthalic acid 3 " 11 sodium 355-&icarboxybenzenesulphonate 17 " " in dried N-methylpyrrolidone, and the concentration was then adjusted by addition of this solvent at the end of the reaction.

A solution of viscosity 180 poises at 25°C was obtained; the polymer had an intrinsic viscosity of 1.65· The solution was extruded at a speed of 10 m/minute through a spinneret having 64 orifices of 0.05 nmi diameter, into a coagulating bath containing 30% of N-methylpyrrolidone and 7% of water, the length of the path of the filament in ^ this bath being 20 cm. On issuing from this bath, the filament in the form of a gel, were stretched in air at ordinary temperature 1.5 times. They were- then washed with water to remove the solvent and dried on a roller at 60°G.

The tow obtained was very glossy. It had a gauge of 260 dtex/64 filaments a dry tenacity of 9 g/tex and a dry elongation of 13%· It was then stretched again, at 380°C, 6.9 times.

It then had a dry tenacity of 33 g/tex and a dry elongation of 14%.

E, iP3LE 3 A 20.3% strength solution in N-methylpyrrolidone of a sulphonated copolyamide-imide was prepared by reacting: diisocyanatodiphenylmethane 200 molar parts trimellitic anhydride 153· 5 " " terephthalic acid 38.5 " " potassium 3* 5-d.icarboxybenzenesulphonate 8 " " in dried N-methylpyrrolidone.

The viscosity at 25°C of the solution obtained was 4-58 poises and the intrinsic viscosity of the polymer was 1.00.

The solution x^as extruded at a speed of 10 m/minute through a spinneret having 200 orifices of 0.055 mm diameter, into. he same coagulating bath as used in Example 1. The tow " . in the form of a gel, was stretched 2 times, then washed with water and finally dried on two rollers which were, at 40* and' 150°C respectively.

The tow obtained was very glossy. It had a gauge of 800 dtex/200 filaments a dry tenacity of 1 g/tex and a dry elongation of 20%.

After a further stretching 3.1 times at 00°C the ° to had a gauge of 278 dtex/200 filaments, a dry tenacity of 33 g/tex and a dry elongation of 9%· EXAMPLE 4 A 20.2% strength solution of a sulphonated copoly-amide-imide xvas prepared by reacting: diisocyanatodiphenylmethane 200 molar parts trimellitic anhydride 1 3»5 " " terephthalic acid 38« " " lithium 3} 5-dicarboxybenzenesulphonate 8 " " in dried N-methylpyrrolidone, adjusting the concentration after reaction.

The viscosity at 25°C of the solution was 5^-2 poises and the intrinsic viscosity of the polymer was 1.08.

The solution was extruded as in Example 3 and the filaments were thereafter super-stretched under the same conditions.

The tow obtained was very glossy and had a gauge of 282 dtex/200 filaments a dry tenacity of 36 g/tex and a dry elongation of 9%· EXAMPLE 5 A 19. strength solution of a sulphonated polyamide-i ide was prepared by reacting: diisocyanatodiphenylmethane 7 molar parts trimellitic anhydride 5 »6 " " terephthalic acid 1 . " " sodium 3» 5-cLicar'boxybenzenesulphonate 3 " " in dried N-methylpyrrolidone, and then adjusting the concentration.

The viscosity at 25°C of the solution obtained was 413 poises and the intrinsic viscosity of the polymer was 1.20.

The solution was extruded at a speed of 10 m/minute through a spinneret of 15, 000 orifices of 0.08 mm diameter, into a coagulating bath containing 65 of N-methylpyrrolidone and 35% of water, the path of the tow in this bath being 80 cm. On issuing from this bath, the tow, in the form of a gel, was stretched in air at ordinary temperature 1.75 times. It was then washed with water in counter-current, in a succession of wash troughs, and placed without tension on the perforated, conveyor belt of a ventilated drier, wherein the dry tempera ture varied from 70° to 105°C from one end of the drier to the other.

The tow obtained was very glossy. It had a gauge of 45, 000 dtex/15, 000 filaments a dry tenacity of 12 g/tex and a dry elongation of 12%.. It could be super-stretched 2 times in an oven at 400"C. It then had a gauge of 22,600 tex, a dry tenacity of 29 g/tex and a dry elongation of 13%· J^XAKPLE 6 A 12.9% strength, solution of a sulphonated copolyamide-imide i^as prepared by reacting: diisocyanatodiphenyl-ether 35 molar parts trimellitic anhydride 25.55 " " •terephthalic acid 7 " " sodium 3 , 5-dicarboxybenzenesulphonate . 2.4-5 " " in dried N-methylpyrrolidone.

The viscosity at 25°C of the solution obtained was 720 poises and the intrinsic viscosity of the polymer was 1.81.

The solution, was extruded at a speed of 10 m/minute through a spinneret having 200 orifices of 0.055 mm diameter, into a coagulating bath containing 50% of N-methylpyrrolidone and 50% of water, the path of the tow in this bath being 20 cm. The filament, in the form of a gel, were stretched 1.8 times in air at ordinary temperature, and then washed and dried on two rollers, the respective temperatures of which were 40° and 150°C.

The tow · obtained was very glossy. It had a gauge of 800 dtex/200 filaments a dry tenacity of 19 g/tex and a dry elongation of 12%.

After super-st etching 2.6 times at 4-00JC, it had_ a dry tenacity of 40 g/tex and a dry elongation of %* EXAMPLE 7 A solution of a sulphonated copolyamide-imide was prepared by reacting: diisocyanatodiphenylmethane - 200 molar parts trimellitic anhydride 157 " " terephthalic acid 38 " " in anhydrous N-methylpyrrolidone.

The solution obtained had a concentration of 19.6% and a viscosity at 25°C of 1,500 poises. It was diluted to adjust the concentration to 17%j its viscosity dropping to 300 poises. The intrinsic viscosity of the polymer was 1.4-3.

The solution v/as extruded at a speed of 10 m/minute through a spinneret having 64- orifices of 0.05 m diameter, into a coagulating bath containing 60% of N-methylpyrrolidone and 40% of water, the path of the filament in this bath being 20 cm. The filament, in the form of a gel, were then stretched 2 times in air, washed with water and dried on a roller at 50'C. The tow obtained was glossy and silky, and very soft to the touch. It had a gauge of 280 dtex/64 strands, a dry tenacity of 17 g/tex and a dry elongation of 30%. It was thereafter super-stretched 2.3 times at 340°C. Its gauge was then 120 dtex, its dry tenacity was 39 g/tex and its dry elongation was 12%.

EXAMPLE 8 A 19.5% strength solution of a sulphonated copolyamide-imide was prepared by reacting: diisocyanatodiphenylmethane 75 molar parts trimellitic anhydride 57·6 " " terephthalic acid 14.4 " " sodium 3» 5-cLicarboxybenzenesulphonate 3 " " in dried N-methylpyrrolidone, and then adjusting the concentration.

The viscosity at 25°C of the solution obtained was 400 poises and the intrinsic viscosity of the polymer was 1.21.

The solution was extruded at a speed of 35 m/minute through a spinneret of 64 orifices of 0.06 mm diameter, into a coagulating bath containing 52% of N-methylpyrrolidone and 48% of water, kept at 20°C, the path of the filament in this bath being 40 cm. On issuing from this bath, the filament, in the form of a gel, were stretched in air 1.94 times before being passed over a washing roller of peripheral speed 68« m/minute, on which they weiewound several times. On the washing roller, the filament were washed in counter-current with demineralised water at a flow rate of 170 cm^/minute, and then passed over a drying roller around which they were wound several times. The drying roller had several different temperature levels: 40"C where the tow entered and 420°C where it left. The dry and hot tow issuing from this drying roller was then stretched 2.1? times before being wound up at a speed of 149 m/minute.

The tow obtained was very glossy. It had a gauge of 191 dtex, a dry tenacity of JO. g/tex and a dry elongation of 8%.

EXAMPLE 9 A 19.5% strength solution of a sulphonated copolyamide-imide was prepared as in Example 8.

The viscosity at 25°C of the solution obtained was 440 poises and the intrinsic viscosity of the polymer was 1.28.

The solution was extruded at a speed of 45 m/minute through a spinneret having 64 orifices of 0.05 mm diameter, into a coagulating bath containing 48% of water and 52% of N-methylpyrrolidone at a temperature of 20°C, On issuing from the coagulating bath, the filaments were stretched in air 1.8 times before being passed over a combination of two washing rollers rotating at a speed of 81 m/minute, on which rollers the tow was wound several times. The filament was thereafter dried on the same drying roller as in Example 8, the temperature of which was however kept at 250°C where the tow left then passed over a plate at 300°C and finally wound up at a speed of 14-2 m/minute, which stretched it 1.75 times.

The tow obtained was very glossy. It had a gauge of 212 dtex, a dry tenacity of 28.1 g/tex and a dry elongation of 9-4-%. 1 EXAMPLE 10 A 19.8% strength solution of a sulphonated copoly-amide-imide was prepared as in Example 8.

The viscosity at 25°C of the solution obtained was 3OO poises and the intrinsic viscosity of the polymer was 1.10.

The solution was extruded at a speed of m/minute through a spinneret of 64- orifices of 0.06 mm diameter, into a coagulating bath containing 4-8% of water and % of N-methyl pyrrolidone at a temperature of 20°C. On issuing from this bath, the tow, in theform of a gel, was stretched in air at ordinary temperature 1.8 times, before being washed on washing rollers over which it passed, being wound several times. It thereafter passed over a drying roller similar to that of Example 8, but the temperature of which was 360°C where the filament left, and then passed over a feed roller at a speed of 14-5 m/minute, xhich stretched it 1.8 times between the drying roller and this feed roller. The tow thereafter passed over a plate kept at 4O0°C where it was stretched 1.4 times before being wound up at a speed of 210 m/minute.

The tow obtained was very glossy and had a gauge of 155 dtex, a dry tenacity of 31·8 g/tex and a dry elongation of 10.7%· EXAMPLE 11 A 19.8% solution of a polyamide-imide was prepared by reaction of: diisocyanatodiphenylmethane 50 molar parts trimellitic anhydride 360 " " terephthalic acid 72 " " sodium 3, 5-dicarboxybenzene- sulphonate 18 " " in anhydrous N-methylpyrrolidone , then adjusting the .concentration. The viscosity of the solution obtained was 325 poises at 25°C and the polymer had an inherent viscosity of 1.07· The solution \^as extruded at a rate of 8 metres per minute through four spinnerets having 10,000 apertures of Ο.Ο5521m diameter into a coagulating bath containing 62% of N-methylpyrrolidone and 38% of water, at a temperature of 27°C, the path of the filaments in the bath being 2 metres. At the end of the bath the filaments were in the form of a gel and were drawn 1.8 times. Then they were washed in counter-current in water in successive wash tanks and then wound, without tension, on perforated rollers of a ventilated dryer of which the drying temperature varied from 100° to 1 0°C from the beginning to the end of the dryer, and of which the wet temperature was 85°C The tow obtained was very shiny; it had a titre of 176000 dtex/ 0000 'filaments a unitary filamentsfcenacity of g/tex and an elongation of 50%. The tow could be super-drawn in a furnace at 400°C two times. The tenacity of a unitary filaments was then 40 g/tex and its elongation was 20%.

Claims (2)

1. •WE CLAIM 1.. Process for preparing glossy heat-stable polyaiiiide-imide fibres, which comprises extruding a solution, in N-methylpyrrolidone, of a copolymer comprising amide-imide units (A) of formula: amide units (B) of formula: and, optionally, amide units (C) of formula: -KH-Ar^NH-CO-E-CO-in which: r^ represents a divalent aromatic radical, optionally iTip to 5 mole ¾S, containing a small proportionVof aliphatic or cyclo-aliphatic radical; J?2 represents a trivalent aromatic radical; R represents a divalent aromatic, cycloaliphatic or araliphatic radical; and M represents an alkali metal or alkaline earth metal, the copolymer containing at least 3 mole %, based on the units (A), (B) and (C), of units (B), into an aqueous coagulating bath containing 30 to 75% by weight of N-methylpyrrolidone, stretching the resulting filaments in air at least 1»5 times, and then washing them to remove the N-methylpyrrolidone and drying them.
2. 2. Process according to claim 1, in which the copolymer has an intrinsic viscosity greater than 0.5, measured as a 0.5 w/v solution in N-methylpyrrolidone at 25°C 3· Process according .to claim 2 in which the copolymer has an intrinsic viscosity between 0.8 and 1.2, measured as a 0.5% w/ solution in N-methylpyrrolidone at 25°C. 4-, Process according to any one of claims 1 to 3 in which the units (B) are present in an amount between 3 and 10 mole % of the total amount of units (A), (B) and (C)e i 5· Process according to any one of claims 1 to in which the coagulating bath contains over 5?» by weight of N-methylpyrrolidone. 6o Process according to any one of claims 1 to in which the dried filaments are stretched at a temperature of at least 300°C. 7· Process according to any one of the preceding claims in which the solution has a viscosity not exceeding 1,500 poises. 8. Process according to any one of the preceding claims in which the copolymer is obtained by reacting, in substantially stoichiometric amounts, at least one aromatic diisocyanate with an acid reagent comprising at least one aromatic acid anhydride, an alkali metal or alkaline earth metal salt of 3,5-dicarboxybenzenesulphonic acid in an amount of at least 3 mole % and, optionally, at least one aromatic, cycloaliphatic or araliphatic diacid, in an anhydrous polar organic solvent. 9· Process according to claim 8 in which the organic solvent is N-methylpyrrolidone . 10„ Process according to claim 8 or 9 in which the dii'socyanate is a toluylene diisocyanate , diisocyanato-diphenylmethane , diisocyanatodiphenylpropane or diiso-cyanatodiphenylethe . lie Process according to any one of claims 8 to 10 in which the aromatic acid anhydride is trimellitic anhydride. 12. Process according to any one of claims 8 to 11 in which the diacid is terephthalic or isophthalic acid. 13. Process according to any one of claims 8 to 12 in which the diacid is used in an amount from 20 to 80 mole % based on the total molar amount of the acid reagent. 14. Process according to claim 1 substantially as hereinbefore described. 15· Process according to claim 1 substantially as described in any one of the Examples. 16. Glossy heat-stable polyamide-imide fibres whenever prepared by a process as claimed in any one of the preceding claims. :.; 17. Glossy heat-resistant fibres of a copolymer comprising: amide-imide units (A) of formula: amide units (B) of formula: and, optionally, amide units (C) of formula: -NH-Ar^-NH-CO-R-CO-in which: Ar^ represents a divalent aromatic radical, optionally \up to 5 mole j containing a small proportionΓοΓ aliphatic or cycloaliphatic radical; represents a trivalent aromatic radical; R represents a divalent aromatic, cycloaliphatic or araliphatic radical; and M represents an alkali metal or alkaline earth metal, the copolymer containing at least 3 mole %, based on the units (A), (B) and (C), of units (B). 18. Fibres according to claim 17 in which the copolymer contains between 3 and 10 mole % of units (B). 19. Fibres according to claim XI- or 18 in which Ar-^ represents a divalent toluylene, diphenylmethane, diphenylpropane or diphenylether radical. 20. Fibres according to any one of claims 17 to in v/hich Aiv, represents a , radical. 21, Fibres according to any one of claims 17 to in v/hich R represents a . radic 22. Fibres according to claim 17 substantially escribed in any one of the Exaraple