EP3143069A1

EP3143069A1 - Production of polyamides by hydrolytic polymerization and subsequent treatment in a kneader

Info

Publication number: EP3143069A1
Application number: EP15723899.9A
Authority: EP
Inventors: Ning Zhu; Rüdiger HÄFFNER; Achim Stammer; Cesar Ortiz; Silke Biedasek; Faissal-Ali El-Toufaili
Original assignee: BASF SE
Current assignee: BASF SE
Priority date: 2014-05-16
Filing date: 2015-05-13
Publication date: 2017-03-22
Also published as: KR20170008805A; JP2017521508A; WO2015173310A1; US20170081472A1; CN106536595A

Abstract

What is proposed is a process for producing polyamides having the following steps: a) providing a monomer composition comprising at least one lactam or at least one aminocarbonitrile and/or oligomers of these monomers, b) reacting the monomer composition provided in step a) in a hydrolytic polymerization at elevated temperature in the presence of water to obtain a reaction product comprising polyamide, water, unconverted monomers and oligomers, and c) postpolymerizing the reaction product obtained in step b) in at least one kneader (3), the kneader (3) used in step c) having at least two screws, with at least one reaction zone and one discharge zone arranged in the direction of the longitudinal axes of the screws, and with kneading elements arranged in succession on each of the screws in the at least one reaction zone, wherein a free space zone having a free space volume in the range from 10% to 70%, based on the total volume of the at least one reaction zone, is provided in the kneader (3) above the at least two screws.

Description

Production of polyamides by hydrolytic polymerization and subsequent treatment in a kneader

BACKGROUND OF THE INVENTION

The present invention relates to a process for producing polyamides comprising hydrolytic polymerization and subsequent treatment in a kneader. STATE OF THE ART

Polyamides are among the world's most widely produced polymers and serve in addition to the main applications of fibers, materials and films a variety of other uses. Among polyamides, polyamide 6 is the most widely produced polymer at about 57%. The classical method for the production of polyamide 6 (polycaprolactam) is the hydrolytic polymerization of ε-caprolactam, which is still of great industrial importance. Conventional hydrolytic manufacturing processes are z. In Ullmann's Encyclopaedia of Industrial Chemistry, Online Edition 15.03.2003, Vol. 28, pp. 552-553 and in the Plastics Handbook, Technical Thermoplastics: Polyamides, Carl Hanser Verlag, 1998, Munich, pp. 42-47 and 65- 70 described.

In the first process step of the hydrolytic polymerization, part of the lactam used reacts by the action of water with ring opening to give the corresponding ω-aminocarboxylic acid. This then reacts with further lactam in polycondensation reactions to the corresponding polyamide. In a preferred variant ε-caprolactam reacts by the action of water with ring opening to aminocaproic acid and then on to polyamide 6. The hydrolytic polymerization can be carried out in one or more stages. As a rule, the polycondensation takes place in a vertical tube reactor (VK tube). The abbreviation "VK" stands for "simplified continuously". Optionally, a plant can be used with prepolymerization at elevated pressure, also referred to as Druckvorreaktor. The use of such a prereactor reduces the residence time required for the ring-opening reaction of the ε-caprolactam. At the end of the vertical tube reactor (VK tube), a polyamide melt having a composition close to the chemical equilibrium of polyamide, lactam monomer, oligomers and water is obtained. The content of oligomers and monomers can, for. B. 8 to 15 wt .-% and the viscosity number of the crude polyamide, which is directly related to the molar mass and thus the processing properties, is usually between 1 10 to 160 ml / g. For many uses, eg. For example, for the production of films for packaging materials, a low residual monomer content in the polyamide is required, so that the crude polyamide is generally subjected before its further processing at least a partial removal of monomers and oligomers.

To reduce the content of low molecular weight components of the product of the hydrolytic polymerization is usually first granules of crude polyamide particles produced and then extracted with an extractant to remove residual monomers and oligomers. This is often done by continuous or discontinuous extraction with hot water, as z. B. in DE 25 01 348 A and DE 27 32 328 A is described. For the purification of crude polyamide 6, the extraction with caprolactam-containing water (WO 99/26996 A2) or the treatment in the superheated steam stream (EP 0 284 986 A1) is also known. For reasons of environmental protection and economy, the extracted components, in particular in the case of polyamide 6, the caprolactam and the cyclic oligomers are recycled to the process. The extraction is usually followed by drying of the extracted polyamide. For many applications, higher molecular weight polyamides are needed which are not achieved by hydrolytic polymerization alone. To increase the molecular weight or the viscosity of the polyamide, subsequent condensation is then carried out after the extraction and drying, the polyamide preferably being present in the solid phase. For this purpose, the granules can be tempered at temperatures below the melting point of the polyamide, the polycondensation in particular progressing. This leads to the structure of the molecular weight and thus to increase the viscosity number of the polyamide. As a rule, the viscosity number of the polyamide 6 after extraction and post-polymerization is 180 to 260 ml / g.

Postcondensation and drying are often carried out in one process step (WO 2009/153340 A1, DE 199 57 664 A1).

DD 2090899 describes processes for vacuum melt monomerization in which a polyamide extraction is preceded, in which the polyamide melt is brought into contact with liquid caprolactam.

DD 227140 describes a process for the production of polyamide having a degree of polymerization DP> 200. The process is characterized by at least 5 post-polymerization nander following stages. At the beginning of each drying step, the surface of the molten polyamide is adjusted to> 4 cm ² / g of polyamide and the maximum diffusion path of the water in the melt to <3 mm. WO 03/040212 discloses a method for producing polyamide 6 by hydrolytic polymerization of ε-caprolactam under the action of water. Dewatering is achieved by increasing the surface area of the melt.

An alternative, industrially not yet heavily used route for the production of polyamides is the polycondensation of aminonitriles, eg. For example, the preparation of polyamide 6 from 6-aminocapronitrile (ACN). In a conventional procedure, this process involves nitrile hydrolysis followed by aminamidation, which are usually carried out in separate reaction steps in the presence of a heterogeneous catalyst, such as ΤΊΟ2. A multi-step procedure has proven to be practicable since both reaction steps have different requirements with respect to water content and completeness of the reaction. In this synthesis route too, it is often advantageous to subject the resulting polymer to a purification to remove monomers / oligomers. WO 00/47651 A1 describes a continuous process for the preparation of polyamides by reacting at least one aminocarbonitrile with water.

The known processes for the preparation of polyamides by hydrolytic polymerization are still in need of improvement. Thus, the content of residual monomer, especially ε-caprolactam, at the beginning of the postpolymerization below the melting point of the polyamide is far below the equilibrium value. Thus, a back reaction of the polycondensation (remonomerization) can take place during the concluding polymerization, so that the residual monomer content of the polyamide increases again in the last process step of the production process.

The present invention is therefore based on the object to provide an improved hydrolytic process for the preparation of polyamides, in which the aforementioned disadvantages are avoided. In particular, it should be possible according to this method to provide a product with sufficiently high molecular weight and at the same time very low residual monomer content available. Specifically, it should be possible to dispense with a post-condensation after extraction and drying at least partially or completely. Thus, a further increase in the residual monomer content after extraction can be reduced or avoided. It has been found that this object is achieved by subjecting the reaction mixture obtained in the hydrolytic polymerization, the polyamide, water, unreacted monomers and oligomers, a post-polymerization in a kneader having a free space zone subject. The product obtained therefrom may optionally be subjected to at least one extraction, whereby unreacted monomers and oligomers are at least partially removed. Subsequently, further workup steps, for. As a drying done. According to the inventive method, the discharge from the at least one kneader advantageously already substantially the target molecular weight, ie the desired viscosity number. Thus, even after the at least one kneader polyamides can be achieved with lower residual monomer content. During extraction, only small amounts of low molecular weight components have to be removed. The subsequent drying can be carried out at lower temperatures and / or a lower consumption of inert gas. The process step of a post-condensation following the extraction and drying is generally no longer necessary. Optionally, the process steps of extraction and drying are no longer necessary. The process according to the invention has shorter retention and throughput times than conventional processes. In particular, it is possible to provide polyamides which at the same time have a low residual content of monomeric lactam and of cyclic dimer.

SUMMARY OF THE INVENTION

The invention therefore provides a process for the preparation of polyamides, comprising the following process steps a) providing a monomer composition containing at least one lactam or at least one aminocarbonitrile and / or oligomers thereof, b) reacting the monomer composition provided in process step a) in a hydrolytic Polymerization at elevated temperature in the presence of water to obtain a reaction product containing polyamide, water, unreacted monomers and oligomers, and c) post-polymerization of the reaction product obtained in process step b) in at least one kneader (3), the process step c) used kneader (3) has at least two screws, wherein in the direction of the longitudinal axes of the screw at least one reaction zone and a discharge zone is arranged and in the at least one reaction zone on each of the screws Kneading elements are arranged one behind the other, characterized in that in the kneader (3) above the at least two screws, a free space zone is provided which has a free space volume in the range of 10 to 70%, based on the total volume of the at least one reaction zone.

Another object of the invention are polyamides, which are obtainable by the method described above. These polyamides are characterized by a very low residual monomer content, as can not be achieved by methods known from the prior art.

Another object of the invention is the use of the above polyamides for the production of granules, films, fibers or moldings.

DETAILED DESCRIPTION OF THE INVENTION

In the present case, monomer is understood to mean a low molecular weight compound as used in the preparation of the polyamide by hydrolytic polymerization to introduce a single repeating unit. These include the lactams and aminocarboxylic acid nitriles used. These include, for the preparation of the polyamides optionally used comonomers, such as ω-aminocarboxylic acids, co-amino carboxylic acid amides, co-Aminocarbonsäuresalze, ω-aminocarboxylic acid esters, diamines and dicarboxylic acids, dicarboxylic acid / diamine salts, dinitriles and mixtures thereof.

In the present context, an oligomer is understood as meaning a compound as formed in the preparation of the polyamides by reaction of at least two of the compounds forming the individual repeat units. The oligomers have a lower molecular weight than the polyamides prepared according to the invention. The oligomers include cyclic and linear oligomers, especially cyclic dimer, linear dimer, trimer, tetramer, pentamer, hexamer and heptamer. Common methods for the determination of the oligomeric components of polyamides generally detect the components up to the heptamer.

The viscosity number (Staudinger function, denoted by VZ, VN or J) is defined as VZ = 1 / cx (η - n _s ) / r \ s. The viscosity number is directly related to the average molar mass of the polyamide and provides information about the processability of a plastic. The determination of the viscosity number can be carried out according to EN ISO 307 with an Ubbelohde viscometer.

The process according to the invention has the following advantages: By using a kneader with free space zone for postpolymerization, the molecular weight build-up to the final molecular weight compared to conventional methods succeeds much earlier in the manufacturing process. As a result, the dwelling and throughput times of the production process can be reduced. After the kneader, no further step joins, which is associated with a stronger thermal load of the polymer, as z. B. occurs in the postpolymerization. Thus, the re-formation of monomers and / or oligomers, as occurs at higher temperatures than equilibrium reaction, avoided. Thus, very low residual monomer contents are made possible.

Through the open space zone, d. H. a space that is free of internals, an improved degassing over a kneader without the same can be achieved.

Postcondensation, for example in a dryer, as in conventional processes is not required. As a result, a process stage can be saved and the dwell and throughput times in the production process are shorter.

By the extraction upstream kneader extraction of less monomer and / or oligomer must be removed, for. B. carpolactam, d. H. it must be removed less low molecular weight components in the extraction stage.

Due to the kneader upstream of the extraction, less drying capacity is required during drying, ie. H. The dry performance of the process can be made smaller or the production can be increased while maintaining the drying performance.

Process step a)

In process step a) of the process according to the invention, a monomer mixture which comprises at least one lactam or at least one aminocarbonitrile and / or oligomers of these monomers and optionally further components is reacted under polyamide-forming reaction conditions, a polyamide being formed.

Polyamides according to the invention are understood as meaning homopolyamides, copolyamides and polymers which contain at least one lactam or nitrile and at least one other Contain monomer incorporated and having a content of at least 60 wt .-% polyamide basic units, based on the total weight of Monomergrundbausteine of the polyamide having. Homopolyamides are derived from an aminocarboxylic acid or a lactam and can be described by a single repeating unit. Polyamide 6 basic building blocks may be composed, for example, of caprolactam, aminocapronitrile, aminocapronic acid or mixtures thereof. Examples of homopolyamides are nylon 6 (PA 6, polycaprolactam), nylon 7 (PA 7, polyenantholactam or polyheptanamide), nylon 10 (PA 10, polydecanamide), nylon 1 1 (PA 11, polyundecanlactam) and nylon 12 (PA 12, polydodecanlactam).

Copolyamides are derived from several different monomers, wherein the monomers are interconnected by an amide bond.

Possible Copolyamidbausteine can be derived, for example, from lactams, aminocarboxylic acids, dicarboxylic acids and diamines. Preferred copolyamides are polyamides of caprolactam, hexamethylenediamine and adipic acid (PA 6/66). Copolyamides may contain the polyamide units in various ratios.

Polyamide copolymers contain, in addition to the polyamide basic building blocks, further basic building blocks which are not connected to one another by amide bonds. The proportion of comonomers in polyamide copolymers is preferably at most 40% by weight, particularly preferably at most 20% by weight, in particular at most 10% by weight, based on the total weight of the basic building blocks of the polyamide copolymer.

The polyamides prepared by the process according to the invention are preferably selected from polyamide-6, polyamide-11, polyamide-12, and their copolyamides and polymer blends thereof. Particularly preferred are polyamide-6 and polyamide-12, more preferably polyamide-6.

The monomer mixture provided in process step a) preferably comprises at least one C5 to C12 lactam and / or an oligomer thereof. The lactams are in particular selected from ε-caprolactam, 2-piperidone (δ-valerolatam), 2-pyrrolidone (γ-butyrolactam), capryllactam, enanthlactam, laurolactam, their mixtures and oligomers thereof.

Particularly preferred in process step a) is provided a monomer mixture which contains ε-caprolactam, 6-aminocapronitrile and / or an oligomer thereof. In particular, In particular, in method step a) a monomer mixture is provided which contains exclusively ε-caprolactam or exclusively 6-aminocapronitrile as monomer component. Furthermore, it is also possible that in process step a) a monomer mixture is provided which, in addition to at least one lactam or aminocarbonitrile nitrile and / or oligomer thereof contains at least one monomer (M) copolymerizable therewith. Suitable monomers (M) are dicarboxylic acids, for example aliphatic C ₄ -α-alpha, omega-dicarboxylic acids, such as succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid and dodecanedioic acid. Aromatic C8-2o-dicarboxylic acids, such as terephthalic acid and isophthalic acid, can also be used.

Diamines suitable as monomers (M) are α, ω-diamines of four to ten carbon atoms, such as tetramethylenediamine, pentamethylenediamine, hexamethylenediamine, heptamethylenediamine, octamethylenediamine, nonamethylenediamine and decamethylenediamine. Particularly preferred is hexamethylenediamine.

Among the salts of said dicarboxylic acids and diamines which are suitable as monomers (M), preference is given in particular to the salt of adipic acid and hexamethylenediamine, so-called AH salt. Suitable monomers (M) are also lactones. Preferred lactones are, for example, ε-caprolactone and / or γ-butyrolactone.

One or more chain regulators can be used in the preparation of the polyamides, for example aliphatic amines or diamines such as triacetonediamine or mono- or dicarboxylic acid such as propionic acid and acetic acid or aromatic carboxylic acids such as benzoic acid or terephthalic acid.

Process Step b) The reaction of the monomer mixture provided in process step a) in a hydrolytic polymerization in process step b) can be carried out by customary methods known to the person skilled in the art. Such a method is z. B. in the plastic manual,% engineering thermoplastics: polyamides, Carl Hanser Verlag, 1998, Mün- Chen, pp. 42-47 and 65-70. This disclosure is hereby incorporated by reference.

Preferably, in step b) for hydrolytic polymerization, a lactam is subjected to ring opening under the action of water. This z. B. the lactam cleaved at least partially to the corresponding aminocarboxylic acid, which then further polymerized in the subsequent step by polycondensation. If, in a preferred embodiment, a monomer mixture containing caprolactam is provided in process step a), it is at least partially opened to the corresponding aminocaproic acid under the action of water and then reacts under polycondensation to form polyamide 6. In an alternative embodiment, in process step b) Aminocarbonsäurenitril, especially 6-aminocapronitrile, a polymerization under the action of water and optionally in the presence of a catalyst subjected.

The reaction in process step b) is preferably carried out continuously.

Preferably, the hydrolytic polymerization in step b) in the presence of 0.1 to 25 wt .-% of added water, more preferably from 0.5 to 20 wt .-% of added water, based on the total amount of the monomers and oligomers used. Additional water formed in the condensation reaction is not included in this quantity.

The hydrolytic polymerization in process step b) can be carried out in one or more stages (for example in two stages). If the hydrolytic polymerization in step b) is carried out in one stage, then the initial concentration of water is preferably 0.1 to 4 wt .-% based on the total amount of the monomers and oligomers used. If the hydrolytic polymerization in step b) is carried out in two stages, then the VK tube is preferably a prepress, z. B. a Druckvorreaktor, upstream. In the prepress stage, the initial concentration of water is preferably from 2 to 25% by weight, more preferably from 3 to 20% by weight, based on the total amount of monomers and oligomers used.

In a specific embodiment, the monomer mixture prepared in process step a) consists of at least one lactam and the hydrolytic polymerization in process step b) takes place in the presence of 0.1 to 4% by weight of water, based on the total amount of lactam used. Specifically, the lactam is ε-caprolactam. The hydrolytic polymerization in process step b) can be carried out in the presence of at least one regulator, such as propionic acid. If a regulator is used in process step b) and the hydrolytic polymerization is carried out in two stages using a pressure pre-stage, then the regulator can be used in the pre-press and / or in the second polymerization stage. In a specific embodiment, the hydrolytic polymerization in process step b) does not take place in the presence of a regulator.

The polyamides prepared in the process according to the invention may additionally contain conventional additives such as matting agents, for. As titanium dioxide, nucleating agents, z. As magnesium silicate, stabilizers, z. As copper (L) halides and alkali halides, antioxidants, reinforcing agents, etc., contained in conventional amounts. The additives are usually added before, during or after the hydrolytic polymerization (process step b). The additives are preferably added before the hydrolytic polymerization in process step b).

The reaction in process step b) can take place in one or more stages (for example in two stages). In a first embodiment, the reaction in step b) is carried out in one stage. In this case, preferably the lactam or aminocarbonitrile and optionally oligomers thereof are reacted with water and optionally additives in a reactor.

Suitable reactors are the customary for the preparation of polyamides, known in the art reactors. The hydrolytic polymerization preferably takes place in process step b) in a polymerization tube or a bundle of polymerization tubes. Specifically, at least one so-called VK tube is used for the hydrolytic polymerization in process step b). The abbreviation "VK" stands for "simplified continuously". In a multistage embodiment of the reaction in process step b), at least one of the stages preferably takes place in a VK pipe. In a two-stage embodiment of the reaction in method step b), the second stage preferably takes place in a VK tube. In a two-stage embodiment of the reaction in process step b), the first stage can take place in a pressure pre-reactor. When using an aminocarbonitrile, the reaction in process step b) is generally carried out in several stages, the first step preferably taking place in a pressure pre-reactor.

In a suitable embodiment, polyamide 6 is prepared in a multi-stage process, especially a two-stage process. Caprolactam, water and optionally at least one additive, such as a chain regulator, are fed to the first stage and converted to a polymer composition. This posi- The lyric composition may be transferred to the second stage under pressure or through a melt discharge pump. This is preferably done via a melt distributor. The hydrolytic polymerization in process step b) is preferably carried out at a temperature in the range from 240 to 280 ° C. In a multi-stage embodiment of the hydrolytic polymerization in process step b), the individual stages can be carried out at the same or at different temperatures and pressures. When carrying out a polymerization stage in a tubular reactor, especially a VK tube, the reactor can have substantially the same temperature over the entire length. Also possible is a temperature gradient in the region of at least part of the tubular reactor. It is also possible to carry out the hydrolytic polymerization in a tubular reactor which has two or more reaction zones which are operated at different temperatures and / or under different pressure. The skilled person can the optimal conditions as needed, for. B. taking into account the equilibrium conditions.

If the hydrolytic polymerization in step b) takes place in one stage, the absolute pressure in the polymerization reactor is preferably in a range of about 1 to 10 bar, more preferably from 1, 01 bar to 2 bar. Particularly preferably, the single-stage polymerization is carried out at ambient pressure.

In a preferred embodiment, the hydrolytic polymerization in process step b) is carried out in two stages. By pre-switching a so-called pressure stage, process acceleration can be achieved by carrying out the rate-limiting cleavage of the lactam, especially of caprolactam, under elevated pressure under otherwise similar conditions as in the second reaction stage. The second stage is then preferably in a VK tube, as previously described. Preferably, the absolute pressure in the first stage is in a range of about 1.5 to 70 bar, more preferably in a range of 2 to 30 bar. Preferably, the absolute pressure in the second stage is in a range of about 0.1 to 10 bar, more preferably from 0.2 bar to 5 bar. In particular, the pressure in the second stage is at ambient pressure. Process step c)

In process step c) of the process according to the invention, the reaction product obtained in process step b) is fed to at least one kneader with free space zone and subjected to postpolymerization. The kneader used in process step c) heats up at least two screws, wherein in the direction of the longitudinal axes of the screws at least one reaction zone and a discharge zone is arranged and in the at least one reaction zone on each of the screws kneading elements are arranged one behind the other, characterized in that in the kneader ( 3) above the at least two screws, a free space zone is provided, which has a free space volume in the range of 10 to 70%, based on the total volume of the at least one reaction zone. It is essential to the invention that the post-polymerization is carried out in a kneader which is modified compared to conventional kneaders:

It is based on a conventional kneader, which has at least two substantially horizontal screws, wherein on the screws kneading elements are arranged one behind the other, which strip on the inner wall of an elongated horizontal housing.

The invention is not limited to the specific embodiments of the screws and kneading elements. Kneaders with eccentric kneading elements are preferred. Particularly preferred are self-cleaning kneaders, as described, for example, in EP 2732870.

Starting from the above conventional kneaders, it is essential to the invention that the kneader used for the postpolymerization is modified in such a way that it has a free space zone above the kneading structures (screws and kneading elements), ie. h., That the housing in an upper area of the same is free of Kneteinbauten.

According to the invention, this region should have a free space volume in the range from 10 to 70%, based on the total volume of the at least one reaction zone, which forms the interior of the housing or a subspace thereof.

Further preferred is a free space volume of 15 to 60%, in particular from 20 to 40%, in each case based on the total volume of the at least one reaction zone. In the postpolymerization, the temperature in the reaction zone is preferably in a range from 200 to 350 ° C, more preferably from 220 to 300 ° C. In the post-polymerization, the absolute pressure in the reaction zone is usually in a range from 1 mbar to 1, 5 bar, more preferably from 500 mbar to 1, 3 bar. The adjustment of the temperature in the reaction zone by indirect heat transfer using the usual heat exchangers. This can be flowed through by a conventional heat transfer medium. Examples of common heat transfer media are oils, water and superheated steam. The adjustment of the temperature in the reaction zone can also take place by means of electric heating or other suitable devices.

In a preferred embodiment, the postpolymerization takes place in process step c) in the presence of at least one inert gas, preferably nitrogen. In this case, the inert gas is fed directly to the polyamide in the kneader.

The inert gas may be preheated on entering the kneader, preferably at 200 to 350 ° C, more preferably at 220 to 300 ° C.

The preferred volume ratio of inert gas to polymer melt is in the range of 10: 1 to 100: 1, wherein the volume of the inert gas is given as a standard cubic meter.

In the context of the present invention, a standard cubic meter is understood to mean that amount of gas which is at a pressure of 1, 013 bar, an atmospheric humidity of 0% (dry gas) and a temperature of 288.15 K = 15 ° C. (standard atmosphere according to ISO 2533). would have a volume of one cubic meter.

The residence time of the reaction mixture in the kneader (s) used in process step c) is preferably from 5 to 300 minutes, particularly preferably from 10 to 240 minutes, very particularly preferably from 20 to 170 minutes.

According to a preferred embodiment of the kneader, the distance between the longitudinal axes of the at least two screws in the range of 10 to 3000 mm, preferably in the range of 50 to 2000 mm, more preferably in the range of 100 to 1000 mm, most preferably in the range of 200 to 800 mm.

According to a preferred embodiment, the at least two screws rotate in the same direction or in opposite directions. With more than two screws, all screws can rotate in the same direction or a desired number of screws in opposite directions. In the context of the present invention, it is understood in the same sense that the at least two screws rotate in the same direction. In the context of the present invention, opposing is understood to mean that the at least two screws rotate in the opposite direction. In contrary operation of the screw is in contrast to a same driving style, the shear and elongation of the reaction product more intense and the mixing homogeneous.

Preferably, the at least two screws rotate in the opposite direction.

According to a preferred embodiment of the kneader, the kneading elements successively arranged on the screws in the direction of the longitudinal axes have radial offset angles between the kneading elements in the range of 0 ° to 360 °, preferably in a range of 20 ° to 300 °, particularly preferably in a range of 40 ° to 240 °, more preferably in a range of 60 ° to 120 °.

According to a preferred embodiment of the kneader, the kneading elements arranged one behind the other on each of the screws in the direction of their longitudinal axis are arranged eccentrically.

According to a preferred embodiment, the kneader used in process step c) has kneading elements in the region of the reaction zone with a length to diameter ratio in the range from 1 to 20, preferably in the range from 3 to 10, particularly preferably in the range from 4 to 6.

In the present case, the term "length of a kneading element" is understood to mean the length of a kneading element in the axial direction and the term "diameter" to be understood to mean the maximum external diameter of a circular surface which is exceeded during a rotary movement of a kneading element.

The kneading elements are preferably selected from kneading disks, kneading blocks, kneading screws and combinations thereof.

Preferably, the kneading elements in an inner region are solid, hollow, with recesses, with webs, designed as combinations thereof.

Preferred are kneading elements with solid and fully filled interior and / or hollow and partially filled interior. Advantageously, the kneader has at least one degassing device.

In the present case, a degassing device is a device for removing gases and other volatile substances, such as solvents, moisture, water vapor, caprolactam monomer from liquids, solids and / or other media, in particular from the media conveyed in the kneader. The degassing can be done for example by mechanical surface enlargement and / or mixing of the conveyed in the kneader medium. In addition, the degassing of the medium conveyed in the kneader can also be subjected to a negative pressure. Examples of degassing devices are continuous degasser, driven shaft with combs and / or spatulas, vacuum pump (s), vent valve (s), combinations thereof.

Preferably, the gaseous effluent from the kneader is subjected to removal of volatile components contained therein, preferably selected from water, monomer, oligomers and mixtures thereof.

The discharge from the kneader in process step c) preferably has a viscosity number in the range from 120 to 300 ml / g, preferably in the range from 130 to 280 ml / g, particularly preferably in the range from 150 to 250 ml / g.

The viscosity number of the reaction product discharged in process step c) preferably has an increase in the range from 0 to 200%, preferably in the range from 10 to 150%, particularly preferably in the range from 30 to 120%, compared to the reaction product introduced in process step c).

The discharge from the kneader in process step c) preferably has a residual monomer content in the range from 0 to 5%, preferably in the range from 0.1 to 3%, very particularly preferably in the range from 0.2 to 1.5%.

The discharge from the kneader in process step c) preferably has a content of cyclic dimer in the range from 0 to 5%, preferably in the range from 0.1 to 3%, very particularly preferably in the range from 0.2 to 0.5% , Process step d)

In process step d) of the process according to the invention, the reaction product obtained in process step c) is optionally subjected to shaping to obtain polyamide particles. Preferably, the reaction product obtained in process step c) is first formed into one or more strands. For this purpose, devices known to the person skilled in the art can be used. Suitable devices are for. As perforated plates, nozzles or nozzle plates. Preferably, the reaction product obtained in process step c) is formed into strands in a flowable state and subjected to comminution as a flowable strand-like reaction product to form polyamide particles. The hole diameter is preferably in a range of 0.5 mm to 20 mm, more preferably 0.75 mm to 5 mm, most preferably 1 to 3 mm.

The shaping in method step d) preferably comprises granulation. For granulation, the reaction product obtained in process step c), formed into one or more strands, can be solidified and subsequently granulated. In z. B. Kunststoffhandbuch, Technical thermoplastics: polyamides, Carl Hanser Verlag, 1998, Munich, pp 68-69 suitable measures are described. A special method of shaping is underwater granulation, which is also known in principle to a person skilled in the art.

Process step e)

In process step e), the polyamide particles obtained in process step d) are subjected to a first extraction.

Suitable methods and devices for extracting polyamide particles are known in principle to the person skilled in the art.

Extraction means that the content of monomers and optionally dimers and other oligomers in the polyamide is reduced by treatment with an extractant. Technically, this can be done, for example, by continuous or discontinuous extraction with hot water (DE 25 01 348 A, DE 27 32 328 A) or in the superheated steam stream (EP 0 284 968 W1).

Preferably, for extraction in process step e) a first extractant is used which contains water or consists of water. In a preferred embodiment, the first extractant consists only of water. In a further preferred embodiment, the first extraction agent contains water and a lactam used for the preparation of the polyamide and / or its oligomers. In the case of polyamide 6, it is thus also possible to use caprolactam-containing water for the extraction, as described in WO 99/26996 A2. The temperature of the extractant is preferably in a range of 75 to 130 ° C, more preferably 85 to 120 ° C. The extraction can be continuous or discontinuous. Preferred is a continuous extraction.

During extraction, the polyamide particles and the first extractant may be passed in cocurrent or countercurrent. Preferably, the extraction is in countercurrent.

In a first preferred embodiment, the polyamide particles are continuously extracted in countercurrent with water at a temperature <100 ° C and ambient pressure. Preferably, the temperature is then in a range of 85 to 99.9 ° C.

In a further preferred embodiment, the polyamide particles are extracted continuously in countercurrent with water at a temperature> 100 ° C and a pressure in the range of 1 to 2 bar absolute. Preferably, the temperature is then in a range of 101 to 120 ° C.

For extraction, it is possible to use customary devices known to the person skilled in the art. In a specific embodiment, at least one pulsed extraction column is used for the extraction. The components contained in the loaded first extraction agent obtained in process step e), selected from monomers and optionally dimers and / or oligomers, can be isolated and recycled to process step a) or b). It is possible to subject the extracted polyamide obtained in process step e) to drying (process step f)). The drying of polyamides is known in principle to the person skilled in the art. For example, the extracted granules may be dried by contact with dry air or a dry inert gas or a mixture thereof. Preference is given to an inert gas, for. As nitrogen, used for drying. The extracted granules may also be dried by contacting with superheated steam or a mixture thereof with a gas other than that, preferably an inert gas. For drying conventional dryer, z. B. countercurrent, crossflow, plate, tumble, paddle, trickle, cone, shaft dryer, fluidized beds, etc. are used. A suitable embodiment is the discontinuous drying in the tumble dryer or conical dryer under vacuum. Another suitable embodiment is the continuous drying in so-called drying tubes, which are traversed with an inert gas under the drying conditions. In a special version, at least one shaft dryer is used for drying. Preferably, the shaft dryer is flowed through by a hot inert gas under the post-polymerization conditions. A preferred inert gas is nitrogen.

Preferably, the process is a continuous or batch process.

The drying in process step f) is preferably carried out at a temperature in the range from 70 to 220 ° C., preferably in the range from 100 to 200 ° C., very particularly preferably in the range from 140 to 180 ° C. The polyamide obtained by the process according to the invention preferably has a number-average molecular weight M _n in the range from 10,000 to 40,000 g / mol, preferably in the range from 12,000 to 30,000 g / mol, very particularly preferably in the range from 13,000 to 25,000 g / mol. The inventive method leads to polyamides having particularly advantageous properties, in particular to high viscosity at the same time very low residual monomer content.

The process is explained in more detail below by FIG. 1 and the examples.

Figure 1 shows schematically an embodiment for carrying out the method according to the invention.

In Figure 1, the following reference numerals are used:

1 pre-press reactor

2 VK pipe

3 kneader

4 granulator

5 extraction

6 drying

A monomer composition provided in process step a) is optionally fed via a pressure reactor 1 to a VK pipe 2. In optionally the pressure reactor 1 and / or the VK tube 2, the process step b). The reaction product obtained in process step b) is fed to process step c) at least one kneader 3 and subjected to postpolymerization. Optionally, the reaction product obtained in process step c) is subjected to shaping in a granulator 4 to obtain polyamide particles. Optionally, the polyamide particles obtained in process step c) or d) are treated with at least one extractant in an extraction 5. Optionally, the extracted polyamide obtained in process step e) is additionally subjected to drying 6.

EMBODIMENTS Embodiment 1

An industrial-scale polyamide 6 granulate with a viscosity number of 143 ml / g and a caprolactam content of 9.96% was melted and dried under nitrogen (80 Nl / h) in a KRC kneader from Kurimoto with a free space volume of 21% of a postpolymerization. The residence time in the kneader was 16 minutes at a temperature of 290 ° C. The discharge from the kneader was granulated by underwater granulation and then dried. The final product had a viscosity number of 156 ml / g and a caprolactam content of 3.986%.

Embodiment 2:

Embodiment 2 was carried out analogously to Example 1, but the residence time in the kneader was different from Embodiment 1 60 min.

The final product had a viscosity number of 225 ml / g and a caprolactam content of 0.39%. Embodiment 3

Embodiment 3 was carried out analogously to Example 1, but the residence time in the kneader was different from Embodiment 1 100 min.

The final product had a viscosity number of 254 ml / g and a caprolactam content of 0.264%.

Claims

claims

1 . Process for the preparation of polyamides with the following process steps

Providing a monomer composition containing at least one lactam or at least one aminocarbonitrile and / or oligomers thereof,

Reacting the monomer composition provided in step a) in a hydrolytic polymerization at elevated temperature in the presence of water to obtain a reaction product containing polyamide, water, unreacted monomers and oligomers, and

Postpolymerization of the reaction product obtained in process step b) in at least one kneader (3), the kneader (3) used in process step c) having at least two screws, wherein in the direction of the longitudinal axes of the screws at least one reaction zone and a discharge zone is arranged and in the at least one reaction zone on each of the screw kneading elements are arranged one behind the other, characterized in that in the kneader (3) above the at least two screws a free space zone is provided which has a free space volume in the range of 10 to 70%, based on the total volume of at least one reaction zone , having.

Process according to Claim 1, characterized in that the process comprises the further process step d) shaping of the reaction product obtained in process step c) to obtain polyamide particles.

3. Process according to claim 1 or 2, characterized in that the process comprises the further process step e) treatment of the reaction product obtained in process step c) or of the polyamide particles obtained in process step d) with an extractant having.

A method according to claim 3, characterized in that the method comprises the further process step f) drying (6) of the extracted polyamide f) obtained in process step e).

Method according to one of claims 1 to 4, characterized in that the provided in step a) monomer composition ε-caprolactam or 6-aminocapronitrile and / or oligomers of these monomers.

Method according to one of claims 1 to 5, characterized in that the free space zone has a free space volume in the range of 15 to 60%, preferably in the range of 20 to 40%, based on the total volume of the at least one reaction zone.

Method according to one of claims 1 to 6, characterized in that the kneader (3) in process step c) with an inert gas, in particular with nitrogen, flows through.

Method according to one of claims 1 to 7, characterized in that the drying in process step f) at a temperature in the range of 70 to 220 ° C, preferably in the range of 100 to 200 ° C, more preferably in the range of 140 to 180 ° C, is performed.

Polyamides obtainable by a process as defined in any one of claims 1 to 8.

10. Use of a polyamide according to claim 9 for the production of granules, films, fibers or moldings.