EP1446439A1

EP1446439A1 - Production of polyamide

Info

Publication number: EP1446439A1
Application number: EP02792732A
Authority: EP
Inventors: Sven Gestermann; Ralph Ulrich
Original assignee: Bayer AG
Current assignee: Lanxess Deutschland GmbH
Priority date: 2001-11-09
Filing date: 2002-11-05
Publication date: 2004-08-18
Also published as: CN1585795A; KR20050043773A; US6916901B2; JP2005533871A; US20030125506A1; WO2003040212A1; DE10155242A1

Abstract

The invention relates to a method for producing polyamide-6 by hydrolytic polymerization of epsilon -caprolactam. According to the inventive method, a ring fission of caprolactam takes place in a first step, under the influence of water, and in subsequent steps a polycondensation at low temperatures and water-free conditions is carried out.

Description

Manufacture of polyamide

The invention relates to a method for the production of polyamide-6 by hydrolytic polymerization of ε-caprolactam in which the ring opening of caprolactam takes place under the action of water in the first step and in the following steps the polycondensation is carried out at low temperatures under anhydrous conditions.

Methods of making polyamide are well known, as in Kohan, Nylon

Plastics Handbook, Carl Hanser Nerlag, Munich, 1995 or in the plastics manual, 3. Technical thermoplastics, 4. Polyamides, Carl Hanser Nerlag, Munich, 1998 (pages 42-47 and 65-71). Accordingly, in a first step, caprolactam is at least partially cleaved under the action of water to give the corresponding aminocaproic acid, which is then removed in the subsequent step

Removal of water further polymerized by polyaddition and polycondensation.

On an industrial scale, PA is produced in an NK tube (NK = simplified continuous loan) in which liquid caprolactam with approx. 1-4% water is fed from above to one or a series of vertical tube reactors. Excess water is distilled off. The polymerization is carried out at temperatures between 240 and 270 ° C in 15 to 30 h. A significant acceleration of the process by a few hours can be achieved by switching a pressure stage in which the rate-determining cleavage of caprolactam is increased

Printing is carried out under otherwise similar conditions.

With this process, the achievable nicosity is determined by the water content of the melt. As a rule, relative viscosities around 2.6 - 3.0 (measured as a 1% solution in m-cresol at 25 ° C) are achieved. For thermodynamic reasons, this process limits sales. In equilibrium at 270 ° C there is about 10% residual content of low molecular weight species in addition to polyamide, essentially caprolactam and cyclic oligomers (dimer - tetramer). This residual content drops significantly with decreasing temperature. Since the residual content interferes with other applications, it is necessary to minimize the residual content. This can be done by aqueous extraction or by vacuum lactamization.

Viscosities higher than those mentioned above, which are necessary for certain applications (e.g. extrusion), are usually achieved in a subsequent solid phase post-condensation at temperatures 30-80 ° C below the polymer melting point in a vacuum or inert gas countercurrent. For example, starting from polyamide 6 with a relative viscosity of 2.8 in 24 h at 185 ° C a relative viscosity of 3.8.

More recently, alternatives to this tried-and-tested process concept have been described which enable significantly faster polymerization, in particular quicker melt post-condensation of (pressure stage) prepolymers and also make higher viscosities directly accessible in the melt.

WO-A 00/23501 and WO-A 00/23502 describe the melt post-condensation of prepolymers in horizontal tubular reactors while spreading out the surface, as a result of which large, self-renewing melt surfaces are produced. An inert gas stream is passed through the reactor in order to achieve a more effective dewatering of the melt by reducing the partial pressure of water in the gas phase. In this way, relative viscosities of up to 4.0 have been achieved in residence times of 4.5 hours. The procedure described in examples was carried out at 267 ° C. Demonomerization is also achieved with this process. In a preferred embodiment, the ring opening of caprolactam is likewise carried out in a reactor with large, self-renewing surfaces under the action of water in the gas phase (high H ₂ O partial pressures). A related process for integrated demonomerization and post-condensation is described in EP-A 137 884 and US-A 450 774, a significantly more effective demonomerization being achieved here by the vacuum mode (p <5 Torr) of the horizontal tubular reactor. Here the process in the examples is operated at 284 ° C.

DD-A 227 140 describes a multistage process for melt post-condensation of pressure stage prepolymer using a sequence of one melt-drying stage and the subsequent polycondensation stage.

The use of falling polymer threads with a small diameter and countercurrent nitrogen enables the polymer to dry quickly, so that the water content in the melt is far from equilibrium. This causes a high polycondensation rate in the subsequent condensation stage. The condensation releases water again, which is removed in a further melt-drying stage, so that high viscosity build-up speeds can be achieved again in the next polycondensation reactor. In addition to the use of falling polymer threads to achieve large surfaces, a film evaporator is also described in which the melt flows as a thin film over a vertical metal gauze, here too under an inert gas countercurrent.

The process described is carried out at 275 ° C. in the example mentioned.

An analogous process scheme is described in DE-A 19 506 407. Here, large self-renewing melt surfaces are generated and thus dewatered in counter-current flow on expanded metals, followed by a melt sump that takes over the function of the polycondensation reactors described above. These reactors are also cascaded in series. In this process, bottom residence times of <0.5 h are aimed for, the melt temperature in the example mentioned is 280 ° C. When using 3 such reactors, a relative viscosity of 3.8 is achieved in a total melt post-condensation time of 2 h. A similar process using various degassing reactors, the thin melt films with the help of vertical expanded metal perforated sheets is described in DD-A 234 430 without further details on exact process parameters.

DE-A 69 512 437 describes a process in which a rapid viscosity build-up of PA is achieved by mixing a stripping agent (N) into the polymer melt under pressure and then relaxing to outgas H ₂ O in vacuo. The foam formation caused by the relaxation also leads to large surfaces and thus to effective drainage of the melt.

The melt is then used to set the equilibrium at the same temperature, not mentioned. The preferred embodiment of this method is the use of an extruder.

All of the processes described are based on the following basic principles:

• Effective dewatering of polyamide melts results in very high levels

Polycondensation rates due to long distance from equilibrium.

This effective dewatering is principally achieved by increasing the surface of the melt and thus short diffusion paths for water and lowering the partial pressure of water in the gas phase to further increase the efficiency of the melt drying.

The temperatures described in the respective examples are above 265 ° C.

It is now known that side reactions which lead to decarboxylation and branching take place precisely under the conditions described. So describes

DE-A 22 55 674 that especially in polyamide melts, the water content of which is far below is half the equilibrium value, a clear decarboxylation and branching take place.

As an attempt to circumvent the disadvantages of the side reaction occurring, DE-A 22 55 674 proposes the use of inert gas, which has an increased water vapor partial pressure. However, this means that the viscosities that can be achieved are significantly lower than when driving without water vapor.

The object of the present invention was to develop a process concept for effective melt post-condensation using the above-mentioned principles - large melt surface, short diffusion paths and lowering of the water vapor partial pressure in the gas phase, which on the one hand supplies polyamide with a sufficiently high viscosity and on the other hand due to by-products largely avoided by decarboxylation and formation of chain branches.

Surprisingly, it was found that lowering the post-condensation temperature to T <260 ° C. resulted in melt post-condensation without a side reaction, in particular without end group degradation. The viscosity build-up that can be achieved in this way is still much faster than in the conventional VK pipe process.

This new process concept enables the production of medium to high viscosity polyamide from polyamide, preferably prepolymers, by melt post-condensation at mild temperatures, namely T <260 ° C, preferably T <255 ° C in very short times (e.g. η _re ι = 2.9 ( Injection molding viscosity) in <2.5 h; η _rel = 3.8 (film viscosity) in ~ 5.5 h starting from a prepolymer with η _re ι = 1.9). In addition to the reduction in the residence time, it is advantageous above all to avoid side reactions, in particular decarboxylation, and to reduce the caprolactam and cyclic dinning content significantly compared to the conventional process, which may simplify subsequent extraction steps. The invention relates to a process for the production of polyamide 6 or copolyamides of polyamide 6 in at least 2 stages, in which a polymer, preferably polyamide, very preferably prepolyamide from the first stage (s) is post-condensed in the melt, that the post-condensation in the

melt

• in the case of an enlarged melt surface with surface / volume ratios> 20 m ^"1, preferably> 100 m-1; particularly preferably> 200 m ^{" 1} , preferably also constant renewal of the melt surface and thus short ones

Diffusion paths (<5 cm, preferably <1 cm, particularly preferably <0.5 cm) for water from the melt into the gas phase,

• and with a significant reduction in the partial pressure of water in the gas phase by using an inert gas stream passed through the melt, by incorporating a stripping agent (inert gas) into the melt with subsequent foaming of the melt and / or by lowering the total pressure, preferably vacuum

• and in a temperature range melting point <temperature <melting point + 40 ° C, preferably in a temperature range melting point <temperature <melting point + 35 ° C

is carried out.

For example, for polyamide 6 the values are 220 ° C. <temperature <260 ° C. and preferably in the range between 220 ° C. and 225 ° C.

In this way, starting from polyamide, preferably prepolyamide, preferably with η _re ι <2.2, very fast medium viscosities, as are necessary for injection molding applications (η _rei = 2.6-3.0 in <2-3 h), but also high viscosities for for example extrusion applications (η _re ι = 3.5 - 6.0) accessible. The last-mentioned, highly viscous polyamides can of course also be obtained with this process starting from polyamide of a higher viscosity (for example injection molding viscosity ((η _re ι = 2.6 - 3.0)). The polyamide thus obtained is characterized in particular by a balanced end group balance, since the polymer does not experience any thermal damage due to the low temperature. An additional advantage of this procedure is the lower lactam and oligomer content compared to the conventional process, since for thermodynamic reasons the equilibrium contents of lactam and cyclic dimer are lower at a lower temperature.

In this process, one or more reaction stages can be used for the melt post-condensation of polyamides, at least one stage being operated according to the principles mentioned.

In a preferred embodiment, customary pressure stage prepolymers (η _re ι <2.2) are used. However, it is also possible to use prepolymers which are produced in another way by hydrolytic ring opening. Even higher-viscosity polyamides (e.g. injection molding viscosities η _re ι ~ 2.6 - 3.0) can be melted to higher viscosities (e.g. viscosities for film extrusion, η _rel ~ 3.6 - 4.2), be condensed.

The melt post-condensation itself can be carried out in one or more stages. The same but also different stages can be connected in series.

In a preferred embodiment, a sequence of alternating reaction zones with effective degassing of the melt and bottom zones is used to re-equilibrate the melt. However, a process with permanent drainage of the melt under the conditions mentioned is also possible. All types of reactors that enable effective melt dewatering according to the above-mentioned principles can be used for melt post-condensation; the following degassing apparatuses are preferred: flash evaporators, thin-film evaporators, degassing extruders, degassing centrifuges, falling-film evaporators and other degassing reactors which have the possibility of large melt film surfaces (melt) To generate foam) and for the reduction of the water vapor partial pressure in the gas phase with an inert gas stream (preferably N ₂ ) or vacuum can be applied.

In a further preferred embodiment, in particular for injection molding viscosities (η _re ι = 2.6 - 3.0), one or more of the melt post-condensation stages, expediently the last / last also for demonomerization / demimerization, is operated in this embodiment preferably under vacuum, optionally with the introduction of a stripping agent (inert gas).

A static or dynamic mixer or a screw connected downstream of the last stage can be used to directly process the polyamide still in the melt by adding additives / additives to form special compounds (eg fiber-reinforced polyamide) or to color the polyamide. Also a chemical modification of the polyamide is possible by adding reactive components.

In a preferred embodiment of the process, in particular when using the last condensation stage (s) for simultaneous demonomerization, the melt emerging from the last post-condensation stage is therefore directly in the

Connection compounded or provided with additives or additives.

In an alternative embodiment, an aqueous extraction of monomer and oligomers can follow. The subject matter is also a method in which the melt coming from the post-condensation is directly provided with additives and / or additives or chemically modified by means of suitable mixing elements, preferably twin-screw extrusion, static or dynamic mixers.

All basically known types of polyamide 6 and copolyamides of polyamide 6 with a relative solution viscosity of η _re ι = 2.4 to 4.5, preferably η _re ι = 2.5 to 3.5, particularly preferably η can be produced with the process _re ι = 2.6 to 3.2.

The process can also be used to produce polyamides from the monomer classes:

Lactams or aminocarboxylic acids or diamines and dicarboxylic acids or mixtures thereof.

Polyamides based on: ε-caprolactam and mixtures of ε-caprolactam and aminoundecanoic acid and the

Diamines 1,6-hexamethylene diamine, isophorone diamine and the dicarboxylic acids adipic acid, isophthalic acid and mixtures thereof with a proportion of ε-caprolactam of> 75% by weight in the finished polymer. Such polyamides based on: ε-caprolactam or. 1,6-aminocaproic acid.

Optional copolyamides are those based on: mixtures of ε-caprolactam and other lactams with 7 to 15 carbon atoms and α, ω-diamines with 4 to 20 carbon atoms, which are derived from alkylene or arylene, and the α, ω-dicarboxylic acids with 4 up to 20 carbon atoms derived from alkylene or arylene and mixtures thereof with a proportion of ε-caprolactam of> 50% by weight in the finished polymer.

The relative solution viscosity in the sense of the invention is measured as the relative viscosity (ratio of the throughput times in an Ubbelohde viscometer) of a cresol solution from 1 g PA sample, which is made up to 100 ml solution. Preferred areas of application for the polyamides obtainable from the process are:

1. Direct use as a molding compound for the production of moldings

2. Molding composition for the production of compounds in a separate process step by adding additives and additives in a melt compounding by means of twin-screw extrusion, which only then serves as a molding composition for the production of moldings.

The invention furthermore relates to molding compositions for the production of moldings, hollow bodies, (un) reinforced semi-finished products, films or fibers and monofilaments containing a polyamide composition according to the invention. The invention also relates to moldings, fibers or monofilaments which can be produced from the molding compositions according to the invention.

Examples

In a 250 ml round-bottom flask, 25 g of a prepolymer (pressure stage sample from a polyamide test facility / autoclave) are heated with rotation at a standstill (<30 rev / min) with periodic standstills 2 / min, 10 s each, while measuring the internal temperature for defined periods. This forms a thin melt film (0.1 cm - 1 cm), the surface of which is constantly renewed. A nitrogen stream heated to oil bath temperature (50 1 h; τ point: -60 ° C.) is passed onto the melt surface and discharged via a riser pipe. After the reaction is complete, the melt is quenched (dry ice), ground and analyzed. The measured

The melt temperature fluctuates by ~ ± 5 ° C, which is essentially due to the measuring arrangement.

The following series of experiments were carried out:

Results:

The experiments clearly show that very fast increases in viscosity and high final viscosities can be achieved if prepolymers are post-condensed in the melt under greatly enlarged, self-renewing surfaces and lower H O partial pressure by means of an inert gas stream. It is clearly clear that at temperatures> 265 ° C (Examples 1 and 3) there is significant damage to the polymer, as the increasing disturbance of the end group balance shows. The end group difference reaches a value of ΔEu _dg m _ppe n ^> 1 meq / lOOg after ~ 4 h. After prolonged periods of inactivity, the extent of the thermal damage to the polymer is so drastic that the viscosity is partially reduced (example 1).

At 245-255 ° C (Examples 2 and 4), compared to the standard method (Example 5), a significantly faster post-condensation is still achieved (at 245-255 ° C, for example, an injection molding viscosity of η _re ι ~ 3.0 2-3 times faster than achieved in the conventional method). Very high viscosities are also available here. Here, however, there is an almost balanced end group ratio even after 8 hours of post-condensation; the polymer is not thermally damaged. The End group difference Δ is significantly lower for Examples 2 and 4 than in the tests at high temperature, the maximum measured values also being below the end group differences of the comparative tests which correspond to the conventional method (Example 5; Δ = 0.42-0.43 meq /100 g).

The occurrence of negative end group differences (excess of carboxyl end groups) can be attributed to oxidative damage to the polyamide, which is caused by unavoidable inertization against atmospheric oxygen which is unavoidable on a laboratory scale.

Claims

claims

1. Process for the production of polyamide 6 or copolyamide based on polyamide 6 in at least 2 stages, in which a polyamide, preferably prepolyamide, is post-condensed from the first stage (s) in such a way that the post-condensation in the melt

• with an enlarged melt surface with a surface / volume ratio of> 20 nr ¹

Using an inert gas flow and / or lowering the total pressure,

is carried out and the post-condensation takes place in a temperature range between melting point and melting point + 40 ° C.

2. The method according to claim 1, wherein the post-condensation takes place at a temperature which is between the melting point and melting point + 35 ° C.

3. The method according to one or more of the preceding claims, wherein the post-condensation takes place at the same time a lactation.

4. The method according to one or more of the preceding claims, wherein the melt coming from the post-condensation is directly provided with additives and / or additives or chemically modified by means of suitable mixing elements, preferably twin-screw extrusion, static or dynamic mixers.

5. The method according to one or more of the preceding claims, wherein the polyamide was produced from the / the first stages at temperatures <240 ° C.

6. polyamide, producible according to one or more of the preceding claims.

7. Molding compositions that can be produced according to one or more of the preceding

Expectations.

8. Use of molding compositions according to claim 7 for the production of moldings.

9. Shaped body, producible according to one or more of the preceding claims.