MXPA00004996A - Polyamide production process - Google Patents

Abstract

A process is disclosed for producing polyamides. The extracted water produced from polyamide extraction is concentrated up to an extract content of maximum 85 wt.%and is mixed with fresh lactam up to a 0.5-13 wt.%water content, at least one adiabatic stress relief being carried out during polymerization.

Description

PREPARATION OF POLYAMIDE DESCRIPTION OF THE INVENTION The present invention relates to a process for the preparation of polyamides from a mixture of at least one lactam and water under conditions of polyamide formation wherein at least one adiabatic expansion is carried out during the polymerization. The polyamide, largely prepared by polymerization of caprolactam, has an equilibrium content, depending on the temperature, from 8 to 15% of caprolactam monomer and oligomers of caprolactam. These cause undesirable effects in further processing of the polymer product and are therefore usually removed by extraction with water, water containing caprolactam or alcohol or treatment with inert gases or vacuum treatment after pelletization. Extraction with water typically produces a wash water extract comprising from 2 to 15% by weight of caprolactam monomer and caprolactam oligomer, which is processed for economic and environmental protection reasons to recover the monomer and caprolactam oligomer for recycling in the polymerization. According to DD-A-213 936, DE-A-43 21 683 US-A-4 049 638, for example, describe processes for the preparation of polycaprolactam that allow the use in the polymerization of caprolactam having a content of water up to 15%. EP-A-745 631 describes the reuse of aqueous extract solutions through the addition of small amounts of a di- or polycarboxylic acid, because the extract will polymerize more slowly than will the caprolactam of another shape. Because the extract also comprises appreciable proportions of cyclic oligomers that remain unchanged in the polymerization, several processes have been proposed for the cracking of these oligomers for conversion thereof into linear oligomers. The oligomers are typically cracked with phosphoric acid or by means of high temperatures. For example, US-A-5 077 381 describes a process for cracking oligomers ranging from 220 to 290 ° C, preferably under superatmospheric pressure. The use of acetic acid and orthophosphoric acid for cracking oligomers is described in DD-A-213 936, for example. Before recycling in the polymerization, a weight-reinforced extract, typically in about 10%, must first be worked, that is, typically concentrated. The work normally takes the form of water removal by distillation. DE-A-25 01 348 describes the concentration of more than 70% by weight in the absence of atmospheric oxygen by the addition of fresh caprolactam to the wash water extract before concentration. EP-A-123 881 describes the addition of caprolactam to the extract before concentration to avoid any precipitation of oligomers. The concentration is typically carried out in two stages. In the first stage, the 10% strength solution is concentrated to about 75-80% of caprolactam and oligomers using conventional evaporation means. In addition the additional evaporation of the solution to > 98%, the solution polymerizes at once. The aqueous solution of caprolactam and oligomers of caprolactam is not stable in storage of concentrations above 70 to 80%, because the oligomers in particular are precipitated. However, when this concentration method is applied to the wash water extract from pigmented polyamides, problems arise. The polyamide comprises dulling the aggregated Ti02 pigments before pelletizing, preferably during the polymerization. These tarnished Ti02 pigments are additionally surface treated or coated with inorganic additives to improve UV light stability and impart certain properties, such as dispersibility and particle size. In the extraction of pieces of polyamide, these inorganic helpers are partially extracted with the caprolactam and the caprolactam oligomers and then separated during the concentration of the washing water extract, especially on the surfaces of the heat exchanger. This significantly shortens the service life of the steam equipment and additionally the possibility of blocking the heat exchanger arises. Similar problems with the coating of heat exchange surfaces arise during the evaporation of the water in the polymerization stage when the concentrate is retro-recycled in the polymerization of caprolactam, in that the concentrate similarly includes also these inorganics. By way of solving these problems, EP 306 862 proposes to use pigments comprising less than 0.1% by weight of 100 ° C of water-soluble. However, the restriction to the choice of pigments is disadvantageous. It is an object of the present invention to provide an improved process for working and further processing the washing water extract of polyalmides, especially pigmented polyamides, to produce, simply and without the aforementioned problems, a concentrated extract which can be recycled in the polymerization. We have found that, surprisingly, this object is achieved, when the wash water extract is concentrated to a removable content of not more than 85% by weight and this solution is mixed with fresh lactam in a ratio of 1: 1 to 1: 8 The resulting mixture then comprises from 0.5 to 13% by weight of water. This can be recycled in the polymerization without the aforementioned problems when the water is removed by adiabatic expansion during the polymerization. The present invention accordingly provides a process for the preparation of polyamides from at least one lactam especially caprolactam, with or without additional monomers and additives and custom filler, which comprises: a) Concentrating the wash water extract of the extraction of polyamide at a content of extractables of not more than 85% by weight. b) Adjust the water content of the resulting concentrate to from 0.5 to 13% by weight by the addition of fresh lactam, c) Submit the resulting mixture to a polymerization under conditions of polyamide formation. d) Perform at least one adiabatic expansion during the polymerization to reduce the water content. The process of the present invention is significantly more robust, simpler and more economical than the prior art. With the extract of wash water from the extraction of pigmented polyamide, the precipitation of inorganics from the extract and the formation of a coating of these inorganics, especially on the heat exchange surfaces, are avoided. Furthermore, it has been found that the composition of the mixture used makes the polymerization faster. The extract waters from the polyamide extraction generally have an organic content, with or without inorganics, ranging from 4 to 15% by weight. To recycle them in the polymerization, these extract waters have to be concentrated first. This takes place in a conventional manner in a simple or multistage evaporation apparatus having a short residence time for example in a Robert evaporator, a falling film evaporator, a thin film evaporator or a recirculation evaporator. The evaporation is carried out on a content of extractables of not more than 85% by weight, because this concentration is still sufficiently low for the non-precipitation of solutes that have been observed. Preferably, the evaporation is carried out on a content of extractables ranging from 60 to 85% by weight, especially from 70 to 85% by weight. Evaporation temperatures are generally in the range of 103 to 115 ° C, preferably in the range of 107 to 112 ° C (atmospheric pressure). Evaporation is generally carried out continuously.

It is particularly preferable for the fresh lactam to be added to the wash water extract before concentration, especially if the polyamide extraction was not carried out with water comprising caprolactam. This has the advantage that the concentrated extract is stabilized against the precipitation of oligomers even during the concentration. The proportion by weight of caprolactam added to the content of extractables is chosen in the range from 0.1 to 1.5, preferably in the range of 0.5 to 1. The concentrated extract obtained after the concentration generally has a temperature in the range of 107 to 112 ° C and is subsequently mixed with a lactam monomer to be polymerized. Sufficient lactam is used so that the resulting mixture has a water content ranging from 0.5 to 13% by weight, preferably ranges from 0.5 to 10% by weight, particularly preferably from 0.8 to 7% by weight, especially from 1 to 4% by weight, and particularly preferably from 1.9 to 3.5% by weight. To establish this water content, the concentrated extract is generally mixed with the lactam in a weight ratio of 1: 1 to 1:12, preferably 1: 1 to 1:10, especially 1: 1 to 1: 8. The high content of lactam (generally in the range of 79 to 95% by weight) improves the solubility of the oligomers present in the mixture, so that no precipitation is observed. The mixture is therefore stable and can be stored for at least several hours until it is further processed without blockages being observed in the equipment. The mixture is then subjected to a S polymerization, which is generally carried out continuously, and essentially in accordance with the process described in DE-A 43 21 683. The polymerization is preferably carried out under conditions where the mixtures form a simple liquid phase, i.e. elevated temperature and superatmospheric pressure. In general, the temperature used will be in the range of 230 to 310 ° C, preferably in the range of 240 to 290 ° C, and the pressure in the range of 5 to 40 bar, preferably in the range of 12 to 20 bar. 15 The water present in the mixture catalyzes the polymerization under polymerization conditions. However, to obtain high molecular weight polyamide, the water content has to be considerably reduced, and this is accomplished by adiabatically expanding the mixture. 2nd polymerization, preferably at a pressure in the range from 0.1 mbar to 1.8 bar, especially in the range from 1 bar to 1.3 bar, particularly preferable at atmospheric pressure. Pertaining to the adiabatic release or, to be more precise, the evaporation of water, can not be separated oligomers or additives on the equipment. Otherwise, they remain dissolved or suspended in the polymer matrix ensuring a problem free polymerization progress. In a first embodiment, the mixture to be polymerized is brought to a temperature in the range of 230 to 310 ° C and at a pressure in the range of 5 to 40 bar, initiating the polymerization. Once the desired temperature and the desired pressure have been reached, at least one adiabatic expansion. { as more particularly described below), it is carried out. The product obtained after said expansion is subjected to atmospheric or reduced pressure post-polymerization according to known processes in at least one reaction zone. Preferably, however, the product obtained after said expansion is further polymerized in a first reaction zone at a temperature in the range of 230 to 310 ° C and at a pressure in the range of 5 to 40 bar, then subjected to further expansion adiabatic and finally post-polymerized in a second reaction zone as described above. An additional polymerization in the first reaction zone is generally carried out in a reaction vessel as will be described more particularly below. In an additional mode, the mixture to be polymerized is brought to a temperature of 230 to 310 ° C and to a pressure of 5 to 40 bar, for example when passing the mixture continuously through a heated heat exchanger in a few minutes. The product is subsequently incipiently polymerized in a first reaction zone by maintaining the pressure and temperature conditions. The polymerization is generally carried out in a "reaction vessel having internal adjustments, for example a tubular reactor having mixing elements." These may be structured mixing elements (for example Sulzer packagings) or unstructured mixing elements such as random packing. (eg Raschig rings, balls or pall ring) This reaction zone is the site of an exothermic polymerization of the reaction mixture, and appropriate temperature and pressure conditions are employed for the above-mentioned reasons to ensure that the reaction system form a simple phase The heat of the developed reaction raises the temperature of the reaction mixture from about 10 to 50 ° C in the course of the zone.The residence time is generally in the range of 0.5 to 3 hours, preferably in the range of 1 to 2 hours such that the conversion is not 85%, preferably more than 87% .The pressurized reaction mixture is subsequently expanded adiabatically in the separation zone. The pressure in this separation zone is generally in the range of 0.1 mbar to 1.5 bar, preferably in the range of 10 to 1300 mbar. The expansion results in a flash evaporation of the water still present in the polymer through the use of reaction heat or enthalpy previously stored in the polymer melt. The reaction mixture is cooled to temperatures in the range of 220 to 300 ° C, preferably in the range of 230 to 270 ° C. In contrast to the conventional evaporation of water on the heat exchange surface, it is impossible, as already mentioned, for precipitation on the heat exchanger surfaces and other surfaces of the apparatus to take place from the polymer matrix in the course of flash evaporation. The obstruction due to organic or inorganic precipitations is avoided. In addition, the heat released in the process is directly used for water evaporation, producing energy savings and additional costs. Moreover, cooling the reaction mixture is desirable, because the lower temperature will change the polycondensation equilibrium to the higher molecular weight product side. The water vapor released in the course of expansion comprises volatile constituents such as caprolactam monomer and caprolactam oligomers. The rectification of the column can be used to remove water vapor from the system and recycle the organics in the process. The polycaprolactam obtained after the separation zone when the caprolactam is used as monomer has a molecular weight in the range of 3000 to 18000 g / mol, preferably in the range of 6000 to 1200 Q g / mol. The melt viscosity is in the range of 1 to 200 pa.s (at 270 ° C). At the exit from the separation zone, the melt of the polymer is immediately transferred to the post-reaction zone, where the molecular weight is increased further, or directly in piece form in a conventional manner. Flash evaporation in two or more stages is especially advantageous when the concentration of water in the concentrated, diluted lactam extract is above 8%. Flash evaporation in two or more stages prevents any drop in the temperature of the reaction mixture below the melt temperature of the polymer during adiabatic evaporation. The mixture of concentrated extract and lactam with a water content of more than 8% is heated as described above and passed within the first reaction zone, where the reaction warms it additionally. The pressure is preferably again chosen such that the reaction mixture is present as a simple liquid phase. The pressurized reaction mixture is then adiabatically expanded within the first separation zone, where the pressure is within the range of 6 to 15 bar, preferably within the range of 8 to 12 bar. Some of the water present in the polymer evaporates by flash through the use of the heat of the reaction and the previously stored enthalpy. The reaction mixture is cooled to approximately 220 to 300 ° C, preferably to within the range of 230 to 270 ° C. The residence time in the first separation zone is generally chosen within the range of 10 to 60 minutes, preferably in the range of 20 to 30 minutes. The pressurized mixture is then passed through the heat exchanger and heated in a few minutes to temperatures in the range of 230 to 310 ° C, preferably in the range of 240 to 290 ° C. The pressure is preferably again established in such a way that the reaction mixture is present in a simple liquid phase; the pressure is generally in the range of 6 to 20 bar, preferably in the range of 12 to 18 bar. The reaction mixture is again expanded adiabatically in a second separation zone. The pressure in the second separation zone is established in the range of 10 to 1300 mbar. This operation of reheating the reaction mixture followed by flash evaporation can be repeated, if necessary. The amount of water evaporated in the various zones of separation and the decrease in the temperature of aid can be controlled in a specific way by means of the particular pressure employed. The lactam used can be, for example, Caprolactam, enanctactam, caprylactam and laurilactam and also mixtures thereof, preferably caprolactam. Possible additional monomer units are, for example, dicarboxylic acids, such as alkanedicarboxylic acids and have from 6 to 12 carbon atoms, Especially from 6 to 10 carbon atoms, such as adipic acid, pimelic acid, suberic acid, azelaic acid, or sebasic acid, and also terephthalic acid and isophthalic acid, diamines, such as C-C alkyldiamines. 2, especially having 4 to 8 carbon atoms, 2o such as hexamethylenediamine, tetramethylenediamine or octamethylene diamine, also m-xylylenediamine, bis (4-paminophenyl) methane, bis (4-aminophenyl) -2, 2-propane or bis (4-aminoclose) methane and also mixtures of dicarboxylic acids and diamines, advantageously in a proportion Equivalent, such as hexamethylene diammonium adipate, examethylene diammonium terephthalate or tetramethylene diammonium adipate preferably hexamethylene diammonium adipate and hexamethylene diammonium terephthalate, in amounts ranging from 0 to 60%, preferably from 10 to 50%, by weight, based on the total monomers. The particular industrial importance has been acquired by the caprolactam and the polymerized pliomides of caprolactam, hexamethylenediamine and also adipic acid, isophthalic acid and / or terephthalic acid. In a preferred embodiment, caprolactam and hexamethylene diammonium adipate ("SALT 66") is used, salt 66 is employed in the form of an aqueous solution. The moral ratio of caprolactam to salt 66 is usually chosen in the range of 99.95: 0.05 to 80:20, preferably in the range of 95: 5 to 85 15. Possible customary fillers and fillers are pigments, such as titanium dioxide, silicon dioxide or talc, chain regulators, such as aromatic and aliphatic carboxylic and carboxylic acids. , such 2o as propionic acid or terephthalic acid, stabilizers, such as copper (I) halides and alkali metal halides, nucleating agents, such as magnesium silicate or boron nitride, catalysts, such as phosphorous acid, and also antioxidants in amounts which they vary from 0 to 5% by weight, Preferably from 0.5 to 1% by weight, based on the total monomers. The additives are generally added before pelletizing and before, during or after, preferably after polymerization. The polymer obtained according to the present invention can then be additionally processed in a conventional manner, for example converted in the form of feet in the conventional manner by extruding it in the form of molten profiles, passing these through a water bath until cooled down. and then pelletize them into pieces. The pieces can then be commercially extracted and subsequently or simultaneously converted into high molecular weight polylactam. The extraction can be carried out with, for example, water or aqueous caprolactam solution. Another possibility and gas phase extraction, see EP-A-284 968. The viscosity of the desired finished product is generally in the range of 120 to 350 ml / g. This can be established in a conventional manner.

BRIEF DESCRIPTION OF THE FIGURES Figure 1 schematically shows a modality of the process of the present invention, characterized by the evaporation of the soap at an implicit stage; Figure 2 schematically shows the modality of Figure 1 which is characterized by evaporation of two-stage stello.

Figure 1 describes schematically by means of the example one embodiment of the process of the present invention. An extract that has been concentrated to a content ranging from 70 to 85% is passed through via 1 in a heated mixing vessel 4. The extract is then mixed in a ratio of 1: 6 with pure caprolactam 2 and optionally regulators of chain and other additives 3 in the heated mixing vessel 4 on recirculating the mixture by means of the pump 5. The prime pump 5 then feeds the mixture into a heat exchanger 6 and into the reaction zone 7 while at the same time the desired pressure from 5 to 40 bar is generated. The heat exchanger 6 then provides heating at temperatures of 220 to 320 ° C. The reaction mixture is polymerized in the first reaction zone 7. Here the pressure is adjusted in such a way that the solution is present in a simple liquid phase at the usual temperatures. Corresponding to exothermic polymerization, the temperature of the reaction mixture rises to about 20 to 50 ° C at the end of the first reaction zone 7, while the reaction mixture is established in a simple liquid phase. The water content goes from 2 to 7%, catalyzes the polymerization in such a way that the conversion of >87% is reached after one h. The polymer produced is then subjected via a valve 8 to flash evaporation to release the predominant portion of water. In the course of this adiabatic expansion, the prepolymer is cooled from about 6 to 7 ° C by 1% of liberated water. If desired after the addition of additional caprolactam and other suitable monomers and miscellaneous additives through a safety gate 9, the polymerization is then generally continued in the postcondensation zone 10 at about atmospheric pressure and a water content of < 0.4%. The water vapor released in 8 is separated from the volatile organics entered in a column 11 and removed in 12. The organics are returned to the process in 13. The polyamide discharged from the postcondensation zone 10 can be further processed in a conventional manner. Figure 2 shows schematically one embodiment of the process of the invention for recycling wash water extract concentrates having an increased water content. Flash evaporation is used in two stages, instead of that of a stage shown in figure 1. As above, the concentrated extract 1 is mixed with the caprolactam 2 and optionally additives 3 in the heated vessel. The water content in the cup 4 is > 8% This may be the result, for example, of less concentration or recycling of increased wash water extract. The mixture in the vessel 4, heated in the heat exchanger 6 and the polymerization in the first reaction zone 7 is then followed by an adiabatic expansion in 8 at a pressure of 6 to 15 bar in the first separation zone 10 '. A pump 5"and a heat exchanger 6 'then restore one or more conditions from 230 to 310 ° C and from 6 to 20 bar, a further adiabatic expansion is carried out in 8' from 10 to 1300 mbar in an area of postcondensation 10 by continuing the polymerization with or without addition of additional monomers and / or additives, in 9. The water vapor is released in a column 11 of volatile organic, which are returned to the process in 13 while the water vapor is removed 12. The polyamide is charged from the postcondensation zone 10, it can be processed in a conventional manner The following examples illustrate the process of the present invention Unless otherwise stated, the amounts and percentages are by weight.

EXAMPLE 1 22.5 kg / h of non-extracted nylon 6 pieces produced with the addition of 0.3% titanium dioxide pretreated as white pigment are extracted with 22.5 kg / h of countercurrent hot water. The resulting 10% strength aqueous extract includes not only about 7.5% caprolactam monomer and about 2.5% oligomers, but also traces of inorganic compounds (silica, manganese, phosphorus and aluminum compounds) in order of 1 at 8 ppm, which originate from the pretreatment of titanium dioxide. The hot extract is then concentrated in a single lid evaporator at 108 ° C at an organic and inorganic content of 78%. As shown in figure 1, hot solution 1 is pumped at a rate of 3.2 kg / h in a mixing glass heated to 4 and mixed there with 20 kg / h of fresh lactam 2. The mixture is achieved by the recirculation pumped out of the solution. The recirculation ratio is 2 to 3 m3 / h. The mixing vessel 4 develops a temperature in the range of 90 to 95 ° C and a water concentration of 3.0%. The recirculation and especially the excess of high lactam are responsible for the stable simple phase solution that is obtained and the formation of organic precipitates that is avoided. The reaction solution is fed via a 5 'pump at a rate of 23.2 kg / h in a heated heat exchanger 6 having a transfer area of 6m2 and an inlet temperature of 270 ° C and heated up to 260 ° C in the course of 2 minutes. The pressure side of the pump 5 'is adjusted to a pressure of 17 bar to ensure the simple phase nature of the reaction system. The heated reaction mixture is then pumped through a heated cylindrical tube 7 of 120 mm in an internal diameter and 2500 mm in length, which is packed with 5 mm Raschig rings with transverse pieces and has a cover temperature of 270 ° C. The residence time in the tube is l, lh. The temperature of the product at the downstream end of the tube is 275 ° C. The reaction mixture, which is under a pressure of about 17 bar, is continuously decompressed at the downstream end of the tube to atmospheric pressure via a control valve 8 and passes to a heated cylindrical preparation vessel 10. The mixture of The reaction is separated into two phases in the process, and the water present there is evaporated through the use of the enthalpy of the polymer melt. The temperature of the polymer melt therefore decreases to 15 ° C to 260 ° C. The vapors released in the course of decompression are passed through a packed rectification column 11 having 6 theoretical plates. The water vapor 12 is removed from the overhead of the system, while the caprolactam monomer and the oligomers are removed from the bottom product 13 and returned to the separator 10. The water vapor from the overhead 12 comprises less than 0.1% of caprslactama. The pretreated titanium dioxide is added as a white pigment concentrate to the nylon 6 in the stirred separation vessel 10 via a latched door 9 and uniformly mixed. Following a residence time of lOh in the separation vessel 10, while also serving as a postreaction zone, the polymer is continuously pumped by a fusion pump from the base of the postreactor via a nozzle into the water bath in the shape of molten profiles that are solidified in the water bath and pelletized. The polymer thus prepared has a viscosity number (measured as a solution reinforced by weight 0.5% in 96% strength sulfuric acid reinforced by weight at 96% at 25 ° C of 115 ml / g and a low molecular weight content of 11). % The titanium oxide content is 0.3%, the polymer is subsequently extracted with countercurrent hot water and then tempered dried until a viscosity number of 130 ml / g is reached.The waters of diluted extract produced in the Extraction stage are concentrated by means of the process described above and returned to the polymerization stage.

EXAMPLE 2 22.5 kg / h of pieces of nylon 6 not extracted produced with the addition of 0.3% of titanium dioxide pretreated as white pigment are extracted with 22.5 kg / h of hot water in countercurrent. The resultant 10% reinforced aqueous extract does not include only about 7.5% caprolactam monomer and about 2.5% oligomers, but also traces of inorganic compounds (silicon, manganese, phosphorus and aluminum compounds) which originates the pretreatment of titanium dioxide. The hot extract is diluted with 1.2 kg / h of fresh lactam to stabilize it against the precipitations of the oligomer and then concentrated in a simple stage evaporator at 109 ° C to an organic and inorganic content of 84%. The hot solution 1 is pumped at a rate of 4.4 kg / h in a heated mixing vessel 4 and mixed there with an additional 18.8 kg / h of fresh lactam 2. The mixture is achieved by recirculation pumped from the solution. The recirculation ratio is 2 to 3 m3 / h. The mixing vessel 4 develops a temperature in the range of 90 to 95 ° C and a water concentration of 3.0%. The recirculation especially the high excess of lactam are responsible for obtaining a stable single phase solution and the formation of precipitates and that the precipitation of organic is avoided. The reaction solution is fed via a 5 'pump at a rate of 23.2 kg / h in a heated heat exchanger 6 having a 6m2 transfer area and an internal temperature of 270 ° C and heated up to 260 ° C. C in the course of 2 minutes. The pressure side of the pump 5 'is adjusted to a pressure of 17 bar to ensure the simple phase nature of the reaction system. The heated reaction mixture is then pumped through a heated cylindrical tube 7 of 120 mm in internal diameter and 2500 mm in length, which is packed with 5 mm Raschig ring with cross pieces and has a cover temperature of 270 ° C . The residence time in tube 7 is 1.1 h. The temperature of the product at the downstream end of the tube is 275 ° C. The reaction mixture, which was under pressure of about 17 bar, is continuously decompressed at the downstream end of the tube to atmospheric pressure via control valve 8 and passes inside the heated cylindrical separation vessel 10. The reaction mixture it is separated into 2 phases in the process, and the water present there is evaporated through the use of the enthalpy of the polymer melt. The melt temperature of the polymer therefore decreases to 14 ° C to 361 ° C. The vapors released in the course of decompression are passed through a packed rectification column 11 having 6 theoretical plates. The water vapor 12 is removed from the over head of the system, while the caprslactam monomer and the oligomers are removed from the bottom product 13 and returned into the separator 10. The water vapor in the overhead 12 comprises less than 0, 1% caprolactam. The pretreated titanium dioxide is added as a white pigment concentrate to the nylon 6 in a stirred separation vessel 10 via an insurance gate 9 and uniformly mixed there. Following a residence time of lOh in the separation vessel 10, which also serves as a postreaction zone, the polymer is continuously pumped by means of a melt pump from the base of the postreactor via a nozzle inside the water bath in the form of molten profiles, which are solidified in the water bath and pelleted. The polymer thus prepared has a viscosity number (measured as a solution fortified by weight of 0.5% in 96% strength sulfuric acid reinforced by weight at 96% at 25 ° C) of 117 ml / g and a low molecular weight content. 10.8% The content of titanium dioxide is 0.3%. The polymer is subsequently extracted with countercurrent hot water and then dried and quenched and until a viscosity number of 127 ml / g is reached. The waters of the diluted extract produced in the extraction stage are concentrated by means of the process described above and returned to the polymerization stage. The process described in Examples 1 and 2 was continuously operated for a period of 12 weeks with closed cycles corresponding to an average number of 50 recycles of the extract concentrated in the polymerization. Through this period, no organic or inorganic was observed to precipitate during the process.

Especially not on heat exchange surfaces.

Claims (12)

R E I V I N D I C A C I O N S

1. A process for the preparation of polyamides from at least one lactam with or without additional monomers and customary additives and fillers, comprising: a) concentrating the extract of washing water from the extraction of polyamide to a content of extractables of no more 85% by weight; b) adjusting the water content of the resulting concentrate from 0.5 to 13% by weight by the addition of fresh lactam; c) subjecting the resulting mixture to polymerization under conditions of polyamide formation; and d) effecting at least one adiabatic expansion during the polymerization to reduce the water content.

2. A process according to claim 1, wherein the wash water extract is concentrated at a content of extractables of 70 to 85% by weight.

3. A process according to claim 1 or 2, wherein the wash water extract comes from the extraction of pigmented polyamide.

4. A process according to any of the preceding claims, wherein a portion of fresh lactam added in step b) is added before said concentration. A process according to any of the preceding claims, wherein the water content is adjusted to be from 0.5 to 10% by weight preferably from 0.8 to 7% by weight, especially from 2 to 4% by weight , in step b). 6. A process according to claim 1, wherein the mixture in step c) is dropped at a temperature in the range of 230 to 310 ° C and at a pressure in the range of 5 to 40 bar, then at least performs an adiabatic expansion, and the product obtained after said expansion is post-polymerized in at least one reaction zone. 7. A process according to claim 6, wherein the product obtained after said expansion is polymerized, in addition in a first reaction zone at a temperature in the range of 230 to 310 ° C and at a pressure in the range of

5. at 40 bar, then subjected to another adiabatic expansion and finally post-polymerized in a second reaction zone. 8. A process according to claim 1, wherein the mixture obtained from step b) is polymerized in a first reaction zone at a temperature in the range of 230 to 310 ° C and at a pressure in the range of 5 to 40 bar, then adiabatically expanded and post-polymerized in an additional reaction zone. 9. A process according to any of claims 6 to 8, wherein said adiabatic expansion is carried out at a pressure in the range of 0.1 mbar to 1.5 bar. 10. A process according to claim 8, wherein the mixture obtained from step b) is polymerized in a first reaction zone at a temperature in the range of 230 to 310 ° C and at a pressure in the range of 5 to 40. bar, then an adiabatic expansion is effected at a pressure in the range of 6 to 15 bar, and the expanded product is again brought to a temperature in the range of 230 to 310 ° C and at a pressure in the range of 5 to 40. bar, then it is subjected to an additional adiabatic expansion at a pressure in the range of 0.1 mbar to 1.5 and finally post-polymerized. 11. A process according to any of claims 6 to 10, wherein the post-polymerization is carried out at a temperature that is 5 to 20 ° C below that of the preceding polymerization, especially 260 to 270 ° C. 12. A process according to any of the preceding claims, wherein the lactam used is caprolactam.