EP2046863A2

EP2046863A2 - Method for the continuous production of ployamide granules

Info

Publication number: EP2046863A2
Application number: EP07724885A
Authority: EP
Inventors: Gerhard Schmidt
Original assignee: Uhde Inventa Fischer AG
Current assignee: Uhde Inventa Fischer AG
Priority date: 2006-05-04
Filing date: 2007-05-04
Publication date: 2009-04-15
Also published as: JP2009535468A; KR20090009231A; WO2007128521A3; CN101437870A; WO2007128521A2

Abstract

The invention relates to a continuous method for production of polyamide granules, comprising a hydrolytic polymerisation of a mixture of at least one lactam and water to give a low-viscosity polyamide melt at a pressure of at least 5 bar and granulation of the low-viscosity polyamide melt by direct droplet formation with maintenance of a pressure which corresponds to at least the vapour pressure of the water retained in the polyamide melt. The low-viscosity granules thus obtained can be condensed in a single-stage subsequent step by treatment with super-heated steam to give a higher viscosity and unreacted monomers and cyclic oligomers extracted and then dried. The invention is characterised by gentle conditions with reduced residence time and temperatures.

Description

Process for the continuous production of polyamide granules

TECHNICAL AREA

The present invention relates to a process for the continuous production of polyamide granules comprising the steps:

Hydrolytic polymerization of a mixture of at least one lactam and water to a polyamide melt, and

- Granulation of the polyamide melt. STATE OF THE ART

The preparation of polyamides is well known, see e.g. L. Bottenbruch and R. Binsack, "Polyamide", Carl Hanser Verlag, Munich, 1998. The standard process comprises several process stages in order to obtain a sufficient degree of polymerization and a virtually monomer-free polymer.

1 shows the process steps of the standard process for polyamide 6 using caprolactam as monomer with the following typical process parameters:

Hydrolytic polymerization of caprolactam and

Water in 1 or 2 stages at a temperature of 240 - 270 ⁰ C in vertical reaction tubes (so-called VK tubes), which are open at the top. Excess water is distilled off here.

- In single-stage polymerization, the pressure is 1.0 - 1.2 bar absolute and the residence time is 18 - 22 h.

- In the two-stage polymerization is the

Pressure in the first stage 2 - 4 bar and 0.5 - 1.2 bar in the second stage. The residence time is in the first stage 3 - 5 h and in the second stage 6 - 10 h. In the first stage, only a low-viscosity, low molecular weight prepolymer is formed.

The desired degree of polymerization with a correspondingly high molecular weight is achieved in the second stage.

Granulation of the resulting polyamide melt with a relative viscosity of 2.3-3.0 (measured in 1% strength by weight solution in sulfuric acid), for example by underwater granulation at a pressure of 1 bar.

- Extraction of the unreacted monomer and the resulting cyclic oligomers from the granules with water at a temperature of 80 - 110 ⁰ C, a pressure of 1 bar and a residence time of 16 - 24 h.

Concentration of the extract water, e.g. by evaporation, and recycling of the extract in the polymerization. Particularly economical is the direct recycling of the extract into the polymerization, i. the extract is not subjected to further purification steps such as distillation. On the other hand, additional purification steps may be required if e.g. In film or spinning applications special demands are placed on the quality of the recycled extract.

The reuse of the extract with or without additional purification steps in other production lines is known and practiced.

- Drying of the granules with nitrogen at 100 - 140 ⁰ C and a pressure of 1 bar for a residence time of 10 - 25 h. At higher temperatures, solid phase postcondensation (SSPC) takes place with increasing molecular weight, but this is not always necessary because the polymer has a sufficient molecular weight already after granulation for most applications. The solid-state postcondensation, which is typically carried out at a temperature of 140-180 ° and a residence time of 12-36 hours, the relative viscosity is exceeded. sometimes increased to 4.7 (see eg DE 195 106 98).

- Cooling of the granules with nitrogen to 40 ⁰ C at a pressure of 1 bar.

Disadvantages of the established process are, in general, the long residence times, inter alia, during the hydrolytic polymerization and the high temperatures which are involved, which can lead to thermal damage to the polymer.

Furthermore disadvantageous are the granulation methods used according to the state of the art, which are based on mechanical division of the polymer melt by means of rotating knives, because they cause high maintenance and wear costs. Examples of this are the strand granulation and the underwater hot break.

A process with a polymerization of caprolactam in two stages is described in WO 95/01389 A1, wherein in the first stage under a pressure of 5 to 30 bar, a caprolactam conversion of 85% is achieved over a residence time of 2 to 4 hours becomes. In the second stage, adiabatic expansion and further polymerisation takes place. During this phase, foaming of the polymer may occur. The further processing takes place either by hot water extraction and subsequent solid phase postcondensation or by

Extraction with simultaneous solid phase postcondensation by means of superheated steam, as is also known from EP 0 284 968 B1.

In EP 1 007 582 B1, a prepolymerization under pressure takes place in a closed tubular reactor in times less than 60 minutes in the presence of a vapor phase. After expansion to atmospheric pressure, the vapor phase, in which volatile constituents of the reaction product are contained, is deposited.

In DE 100 37 030 A1 a polyamide melt is discharged at a pressure of at least 3 bar from a pressure reactor into a cooling liquid and granulated therein by the mentioned underwater granulation. This form of granulation requires a not too low relative viscosity of the polymer material of at least 1.6.

On the other hand, lower viscosity polyamides can be granulated by dripping under vibration. Direct dropletization is understood to mean a process in which droplets directly from the molten liquid product, i. without the use of cutting or striking tools. For polyester, such dropping methods are described in WO 01/81450 A1 and DE 100 19 508 A1, which were carried out with drip devices from Rieter Automatik GmbH, D-63762 Grossostheim. These dripping devices are known under the type designation "DROPPO". A similar process for polyamide resins which are liquid at room temperature is described in DE 100 50 463 A1.

PRESENTATION OF THE INVENTION

The object of the invention is to specify a gentle process for the continuous production of polyamide granules, which manages with comparatively low residence times and which are run at comparatively low temperatures can.

This object is achieved by a method as characterized in claim 1 and in which, accordingly:

The hydrolytic polymerization of a mixture of at least one lactam and water to a low-viscosity polyamide melt is carried out under a pressure of at least 5 bar,

The granulation of the low-viscosity polyamide melt is carried out by direct dripping while maintaining a pressure which at least corresponds to the vapor pressure of the water contained in the polyamide melt.

In the process according to the invention, a substantial reduction in the residence time during the hydrolytic process results over the standard process

Polymerization by the applied pressure of at least 5 bar. Due to the increased pressure, the ring opening of the lactam is accelerated as a start reaction of the polymerization. On the other hand, the increased pressure causes the further reaction to the polymer

(Polycondensation reaction) impeded. Preferably, in the phase of the hydrolytic polymerization, only a low-viscosity, low molecular weight polymer which could also be referred to as a prepolymer is formed. However, it is possible to use the molecular weight after

Increase granulation by solid phase condensation and adjust to the desired value.

The applied during the hydrolytic polymerization pressure is limited upwards by the strength of the pressure vessels used, but could be up to amount to 20 bar. As a pressure vessel is suitable as in the prior art, a so-called. VK tube.

The procedure according to the invention presupposes that it is possible to granulate the preferably low-viscosity polyamide melt prepared by the hydrolytic polymerization under pressure. As mentioned, granulation methods, such as Underwater granulation, a certain minimum viscosity of the melt ahead, which is not achieved in the inventive method. Conventional granulation methods are therefore not usable in the context of the invention. In contrast, the above-mentioned dropletizing methods are fundamentally more suitable at low melt viscosities. However, these methods and the apparatus developed for them have hitherto been used or operated only under ambient pressure. In the present case, however, the melt is under a pressure of at least 5 bar. As a low-viscosity, low molecular weight melt, it therefore contains much more water than at lower pressures. Relieving the pressure to ambient pressure results in the problem of foaming of the polymer, which in turn would make it impossible to drip. According to another feature of

Although the granulation of the preferably low-viscosity polyamide melt is carried out by dripping, but while maintaining a pressure which at least equal to the vapor pressure of the water contained in the polyamide melt. As a result, the foaming is effectively prevented.

Preferred embodiments and further developments of the invention are characterized in the dependent claims. The minimum pressure required during the dripping is, depending on the steam pressure of the water contained in the polyamide melt, depending on the temperature traveled. In practice, it should not be below 3 bar. It should also be no more than 2 bar lower than the pressure under which the hydrolytic polymerization is carried out. However, in order to be on the safe side, one will choose a higher than the minimum required pressure and, if necessary, set it even higher than the pressure under which the hydrolytic polymerization is carried out.

As the lactone, e.g. ε-caprolactam and / or lauryl lactam can be used.

The mixture of at least one lactam and water may advantageously contain further polyamide-forming components, in particular a dicarboxylic acid, such as adipic acid, azelaic acid, sebacic acid, dodecanedioic acid,

Terephthalic acid or isophthalic acid and a diamine such as hexamethylenediamine, decanediamine, dodecanediamine or m-xylylenediamine.

Preferably, the mixture additionally contains chain regulators, such as mono- or dicarboxylic acids.

The water content of the polymerization supplied lactam / water mixture should be at least the saturation pressure at polymerization conditions correspond, for example in caprolactam> 10% at a temperature of 240 ⁰ C and a pressure of 15 bar.

In the case of caprolactam, a residence time of 0.5-5 h, in particular 1-3 h, is sufficient for hydrolytic polymerization under pressures> 5 bar. The hydrolytic polymerization of caprolactam can be carried out at a temperature of 200-250 ⁰ C. Compared to the hydrolytic polymerization of the above-mentioned standard method in which the

In any case, the temperature is greater than 240 ⁰ C, thus lower temperatures are possible in the inventive method in this phase.

It is sufficient if the polyamide melt obtained by the hydrolytic polymerization, a melt viscosity of 0.2 - 2.0 Pa's, in particular from 0.5 to 1.5 Pas. This corresponds to a relative viscosity of 1.3 - 1.5 for PA6.

The hydrolytic polymerization can be carried out efficiently in a single stage in a pipe reactor suitable for overpressure, e.g. a so-called VK pipe.

As is known per se from the dripping method mentioned above, the dripping can take place in a dripping apparatus under the influence of vibration in a gas atmosphere. Suitable as a gas is eg nitrogen. In the known dropping method, the melt is forced through a nozzle or a casting head and excited to vibrate. The granules obtained by the dropping in the form of substantially spherical melt droplets can then, as also already provided in the known Vertropfungsverfahren, after falling through a drop distance in the gas atmosphere in a cooling liquid are collected. The drop distance should be sufficiently long so that the melt drops have sufficient time for at least partial solidification. In particular, suitable as cooling liquid are ser, caprolactam or a mixture of both. In contrast to the known drip method, a pressure of at least 3 bar must be maintained, which also requires the use of a special pressure vessel for dripping.

Preferably, the granules are discharged together with the cooling liquid, with at least partial pressure relief from the pressure region of the dropletization and then separated from the cooling liquid. The separation of the cooling liquid can be carried out by means of a centrifuge, through a sieve or in a hydrocyclone.

For all known applications, it is necessary to increase the viscosity or the molecular weight of the granules yet. This is possible by aftertreatment with solid phase condensation, as known per se as a solid phase postcondensation for higher viscosity ranges.

By this post-treatment, the relative viscosity of a PA6 is preferably increased to a value of 2.4 - 4.2.

The post-treatment of a low-viscosity PA6 is more preferably carried out with superheated steam at a temperature of 140-180 ⁰ C, as is already known from WO 95/01389 Al or EP 0 284 968 Bl for other starting viscosities. The water vapor itself should have a temperature of 170-210 ⁰ C for this purpose.

In comparison, for example, to the second stage of the hydrolytic polymerization of said standard method, wherein the temperature is greater than 240 ⁰ C. is, the temperature of the granules during this post-treatment phase below the melting point of the granules and thus at PA6 in any case substantially lower than 220 ⁰ C.

The aftertreatment of a PA6 with the superheated steam results with particular advantage, as is also known from WO 95/01389 A1 or EP 0 284 968 B1, in addition to the increase in molecular weight simultaneously an extraction of unreacted monomer and of cyclic dimer the granules. The content of these components can be reduced to values of 0.1 to 1.0 wt .-%. In addition, the water content in the granules is reduced to values which require a considerably lower expenditure for the subsequent drying than is the case with the standard method described at the outset (example: 10 - 12% H ₂ O in the standard method, 0.4 - 1%). in the method according to the invention). The aftertreatment of the granules can be carried out in the context of the process according to the invention in an extractor in countercurrent to the superheated steam.

For the post-treatment of PA6 according to the invention with simultaneous solid-phase condensation, extraction and drying, a residence time of 5 to 50 hours is expediently selected. Together with the residence time during the hydrolytic polymerization, this results in a total residence time of only 25-54 h for the process according to the invention compared to a total residence time of at least 60 h in the standard process mentioned in the introduction.

From the monomer-laden water vapor released during the aftertreatment, the monomer is preferably recovered and introduced into the polymerization stage, dhzB in the aforementioned tubular reactor, returned. A particular advantage results from the way in which the unreacted caprolactone is separated off: While in conventional hot-water extraction the lactam is separated off in the liquid state, this is carried out in gaseous form during the extraction with steam. This includes a cleaning step for the reusable extract, without the need for additional equipment. Thus, it is known that water-soluble fractions of inorganic additives such as matting agents are co-extracted during hot water extraction and accumulate over time in polymerization and extraction. This accumulation of by-products leads to problems, which are, inter alia, in deficient

Report dispersing properties of the actual additives and in the form of undesirable deposits on product leading parts. Extraction into the vapor phase prevents such unwanted effects.

With the addition of caprolactam to the above-mentioned cooling liquid, a certain pre-extraction of cyclic dimer can already be achieved in the dripping apparatus. This has advantages in applications in which too high a content of cyclic dimer interferes with further processing.

The granules coming from the post-treatment is more preferably with nitrogen, for example, in a silo, even to a temperature of 40 - 50 ⁰ C cooled. A residence time of 4 - 6 h is sufficient for this purpose. Through the nitrogen, the granules are dried and the remaining water vapor is still displaced from the cavities of granules. A residual water content of 0.04 to 0.06 wt .-% is achievable here. BRIEF EXPLANATION OF THE FIGURES

In the drawing show:

Fig. 1, the process steps of the already explained

Standard procedure for polyamide 6 using caprolactam as monomer; and

Fig. 2 shows the process steps of a process according to the invention for polyamide 6 using

Caprolactam as monomer;

Fig. 3, the process steps of an inventive

Process for polyamide 6 with additional recovery and recycling of the monomer to the tubular reactor.

EXPLANATION OF FIG. 2 AND FIG. 3

According to FIG. 2, the process stages of a process according to the invention for polyamide 6 using caprolactam as monomer, including aftertreatment and cooling with typical process parameters, are as follows:

Hydrolytic polymerization of caprolactam and water in one stage at temperatures of 200 - 260 ⁰ C under pressures of 5 - 25 bar for 1 - 3 h residence time. The result is a low molecular weight, low viscosity prepolymer.

Granulation of the resulting polyamide melt by direct dripping at a pressure of 3 - 25 bar. Combined solid phase condensation, extraction of unreacted monomer and resulting cyclic dimer from the granules and drying of the granules with superheated steam of 160-190 ⁰ C at a pressure of 1 bar during a

Residence time of 20 - 45 h.

Cooling of the granules with nitrogen to 40 ⁰ C at a pressure of 1 bar and a residence time of 4-6 h.

In Fig. 3, the same procedure is shown, in addition, there is still the recovery and recycling of the monomers and optionally the other polyamide-forming substances in the tubular reactor.

example 1

By means of a tubular heat exchanger were 30 kg / h of a mixture of 85% caprolactam and 15% water to 150 ⁰ C and placed in a heated tubular reactor (VK tube). In the tubular reactor, a temperature of 242 ⁰ C and a pressure of 6.8 bar was set. This resulted in a polyamide melt with a relative viscosity of 1.3, measured in sulfuric acid, and a melt viscosity of 0.5 Pas during a residence time of about 2.5 h.

The low-viscosity polyamide melt was treated with a

Gear pump in a container in which a pressure of 10 bar prevailed, transported and there dripped under vibration in a nitrogen atmosphere and solidified into granules. The granules were collected in the lower part of the container in water and cooled to 65 ⁰ C. The granulate-water mixture was removed from the withdrawn and dosed with a volumetric pump in a centrifuge, in which the granules were separated from the liquid. By hydraulic promotion, the pressure was reduced to 3 bar to the centrifuge. After the centrifuge, a pressure of 1 bar prevailed.

From the centrifuge, the granules were fed into an extractor. There it was conducted in countercurrent to 88 kg / h of superheated steam, with a temperature of 190 ⁰ C, and brought to a temperature of 170 ⁰ C. The residence time of the granules in the extractor was about 32 hours. By solid phase condensation, the relative viscosity of the granules increased to 2.8. At the same time, the content of caprolactam in the granules was reduced from 7.9 to 0.2% by weight and the content of cyclic dimer from 0.5 to 0.05% by weight. The water content dropped to 0.3 wt .-%.

From the extractor the granules came over one

Rotary valve in a silo and was cooled in countercurrent to nitrogen to 50 ⁰ C. This resulted in a reduction of the water content to 0.05 wt .-%.

Example 2 (negative example)

Example 1 was repeated, but a pressure of 3 bar was set in the dropletizer. The polymer formed a foam mass that could not be granulated.

Example 3 (negative example)

Example 1 was repeated, but with a different setting in the extractor. The supplied steam had a temperature of 160 ⁰ C and the granules were heated to 140 ⁰ C. After extraction, the residual content of caprolactam in the granules was 0.6% by weight and the content of cyclic dimer 0.1% by weight. The water content reached 1.2 wt.%. With these values, the polyamide granules could not meet the usual requirements in terms of further processing (production of moldings, films, fibers or filaments).

Example 4

Example 1 was repeated, but instead of water, a solution of 30% by weight of caprolactam in water was used as a dropping agent. As a result, the content of cyclic dimer in the granules after extraction to 0.02 wt .-% could be reduced. The caprolactam content of the granules after extraction was 0.3% by weight and the water content was 0.3% by weight. At the outlet of the cooling silo, the water content was even only 0.04 wt .-%.

Claims

claims

A process for the continuous production of polyamide granules comprising the steps of:

Granulation of the polyamide melt,

characterized,

- That the hydrolytic polymerization of the mixture is carried out to a low-viscosity polyamide melt under a pressure of at least 5 bar, and

- That the granulation of low-viscosity polyamide melt is carried out by direct dripping while maintaining a pressure which at least corresponds to the vapor pressure of the water contained in the polyamide melt.

2. The method according to claim 1, characterized in that the dripping is carried out under a pressure of at least 3 bar.

3. The method according to any one of claims 1 or 2, characterized in that the dripping is carried out under a pressure which is not more than 2 bar lower than the pressure at which the hydrolytic polymerization ausge- leads.

4. The method according to any one of claims 1-3, characterized in that as lactam caprolactone and / or laurolactam is used.

5. The method according to any one of claims 1 - 4, characterized in that the mixture contains further polyamide-forming components, in particular a dicarboxylic acid, such as adipic acid,

Azelaic acid, sebacic acid, dodecanedioic acid, terephthalic acid or isophthalic acid and a diamine such as hexamethylenediamine, decanediamine, dodecanediamine or m-xylylenediamine.

6. The method according to any one of claims 1-5, characterized in that the mixture additionally contains chain regulators, in particular mono- or dicarboxylic acids.

7. The method according to any one of claims 1-6, characterized in that the water content of the supplied mixture at least equal to the saturation pressure at the polymerization temperature.

8. The method according to any one of claims 1-7, characterized in that the hydrolytic polymerization in caprolactam is carried out as a monomer during a residence time of 0.5 to 5 hours, in particular 1 to 3 hours.

9. The method according to any one of claims 1-8, characterized in that the hydrolytic polymerization in caprolactam as a monomer at a temperature of 200 - 250 ⁰ C. becomes .

10. The method according to any one of claims 1-9, characterized in that the low-viscosity polyamide melt has a melt viscosity of

0.2 - 2.0 Pa 's, in particular from 0.5 to 1.5 Pas.

11. The method according to any one of claims 1-10, characterized in that the hydrolytic

Polymerization is carried out in only one stage in a tubular reactor.

12. The method according to any one of claims 1-11, character- ized in that the dripping takes place under the action of vibration in a gas atmosphere.

13. The method according to claim 12, characterized in that by the dripping in

Form of melt drops obtained granules after falling through a drop distance in the gas atmosphere in a cooling liquid, in particular in water, caprolactam or a mixture of water and caprolactam, is collected.

14. The method according to claim 13, characterized in that the granules are discharged together with the cooling liquid from the pressure range of the dripping and then separated from the cooling liquid.

15. The method according to any one of claims 1-14, characterized in that the granules of a post-treatment with solid phase condensation for

Increase of its relative viscosity or Nes molecular weight and / or with extraction of unreacted monomer and cyclic oligomers and / or with drying is supplied.

16. The method according to claim 15, characterized in that the relative viscosity of the granules is brought by the solid phase condensation in PA6 granules at least to a value of 2.4.

17. The method according to any one of claims 15 or 16, characterized in that the granules with a superheated steam, in PA6 granules at a steam temperature of 170 - 210 ⁰ C, aftertreatment.

18. The method according to any one of claims 15-17, characterized in that the post-treatment of PA6 granules during a residence time of

5 to 50 h is carried out.

19. The method according to any one of claims 17 or 18, characterized in that the aftertreatment of the granules is carried out in an extractor in countercurrent to the superheated steam.

20. The method according to any one of claims 15-19, characterized in that the post-treated granules with nitrogen to a temperature of

40 - 50 ⁰ C is cooled.

21. The method according to claim 20, characterized in that the cooling of the aftertreated Granu- lats is carried out in a silo in countercurrent to the nitrogen.

22. The method according to any one of claims 17 to 19, characterized in that the extract or the water vapor laden with extract in the polymerization reactor of the same or another production line is performed.