Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – From carboxylic acid or derivative thereof
Reexamination Certificate
2001-12-03
2003-04-15
Hampton-Hightower, P. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
From carboxylic acid or derivative thereof
C528S312000, C528S315000, C528S323000, C528S324000, C528S480000, C528S486000, C528S492000, C528S499000, C526S067000, C526S068000, C526S070000
Reexamination Certificate
active
06548626
ABSTRACT:
The present invention relates to a process for producing polyamides by reacting mixtures of at least one lactam and the extract obtained in the extraction of polyamides, under polyamide-forming conditions.
Polyamide which is mostly produced by polymerization of caprolactam has a caprolactam monomer and oligomer equilibrium content of from 8 to 15%, depending on the temperature. Caprolactam monomer and oligomer interfere with the further processing and are therefore usually removed by extraction with water, caprolactam-containing water or alcohol or by treatment with inert gases or by vacuum treatment after pelletization.
The extraction with water usually produces an extract water containing from about 2 to 15% by weight of caprolactam monomer and oligomer, and this extract water is recovered for economic and environmental protection reasons, the caprolactam monomer and oligomer being recycled back into the polymerization.
DE-A-43 21 683 and U.S. Pat. No. 4,049,638 describe processes for polycaprolactam production permitting the use of caprolactam having a water content of up to 15%.
EP-A-0 745 631 discloses the reuse of aqueous extract liquors through addition of small amounts of a di- or polycarboxylic acid, since the extract is otherwise slower to polymerize than caprolactam.
Since the extract also contains appreciable fractions of cyclic oligomers which remain unchanged during the polymerization, various processes have been proposed to crack these oligomers or convert them into linear oligomers. The oligomers are customarily cracked using phosphoric acid or high temperatures. For instance,: U.S. Pat. No. 5,077,381 describes a process for cracking the oligomers at from 220° C. to 290° C., preferably under elevated pressure.
Before recycling into the polymerization, the usually about 10% by weight extract liquor has to be first worked up, ie generally concentrated. The workup normally takes the form of the water being distilled off. DE-A-25 01 348 describes concentrating in the absence of atmospheric oxygen, the concentrating to more than 70% by weight being preceded by the addition of virgin caprolactam to the extract water. EP-A-0 123 881 discloses adding caprolactam to the extract liquor before the start of the concentrating to reduce oligomer separating out.
However, the use of the abovementioned processes for extract water recycling has revealed a serious disadvantage. The continuous recycling of the extract water is responsible for a substantial increase in the concentration of the oligomer and of the thermodynamically stable cyclic dimer not only in the reaction mixture but also in the polymer if, in the course of the continuous hydrolytic lactam polymerization, the cracking of the oligomers is not successful or the attainment of the chemical equilibria is too slow. In addition, the increase in the oligomer concentration is particularly high when the reaction mixture—to produce high molecular weight polyamides, for example—has a low water content.
It is an object of the present invention to provide a process for producing polyamide whereby the extract water from the extraction of polyamides may be further processed or recycled, the cracking of lactam dimers and higher oligomers and the attainment of the chemical equilibria during the melt polymerization is speeded up and polyamides having reduced total extractables and especially reduced dimer and oligomer content may be produced.
We have found that this object is achieved according to the invention by a process for producing polyamides by reacting at least one lactam and optionally further polyamide-forming monomers with aqueous monomer and oligomer extracts obtained during the extraction with water of the polymer product obtained in the production of polyamides, the water content of the reaction mixture being in the range from 0.5 to 13% by weight, which comprises conducting the reaction in the presence of metal oxides, beta-zeolites, sheet-silicates or silica gels, which may be doped, as heterogeneous catalysts, the heterogeneous catalysts being used in the form which permits mechanical removal from the reaction mixture and are removed from the reaction mixture in the course of the polymerization or after it has ended.
The polymerization is preferably carried out in at least 2 stages, the first stage being carried out under an elevated pressure at which the reaction mixture, with the exception of the heterogeneous catalyst, is present in the form of a single liquid phase and the second stage being carried out under a pressure in the range from 0.1 mbar to 1.5 bar for postcondensation, the heterogeneous catalyst being present in the first stage or both stages.
Preferably the reaction in the first stage is carried out at from 170° C. to 310° C. and from 5 to 40 bar. Preferably an adiabatic decompression is carried out in the second stage to discharge water and any lactam monomer or oligomer by flash evaporation.
The present invention accordingly provides a process for producing polyamides from at least one lactam, especially caprolactam, and optionally further monomers and optionally customary additive and filler substances which comprises reacting a reaction mixture comprising extract water from the extraction of polyamide and virgin lactam and having a water content of from 0.5 to 13% by weight in the presence of a heterogeneous metal oxide catalyst under polyamide-forming conditions. The process of the invention makes it possible to produce polyamides having a (compared with the prior art) distinctly reduced extractable content and especially reduced level of dimers and higher oligomers. The invention is advantageous in particular when the reaction mixture has a low water content, so that the (catalyst-free) oligomer cracking takes place particularly slowly.
The extract waters obtained in the polyamide extraction generally contain from 4 to 15% by weight of organics with or without inorganics. To be able to be recycled into the polymerization, these extract waters first have to be evaporated. This is effected in a conventional manner in a single- or multi-stage evaporation unit having a short residence time, for example in a Robert evaporator, a falling-film evaporator, a thin-film evaporator or a rotary evaporator. The evaporating is carried on to an extract content of not more than 85% by weight, since this concentration is still below that at which the dissolved constituents are observed to separate out. Preferably the evaporating is carried on to an extract content of 60 to 85% by weight, especially from 70 to 85% by weight. The evaporating temperatures are generally in the range from 103° C. to 115° C., preferably from 107° C. to 112° C. (at standard pressure). In general, the evaporating is carried out continuously.
Particularly preferably the extract water is admixed with virgin lactam even before the concentrating, especially when the extraction of the polyamide is not carried out with, caprolactam-containing water. This has the advantage that the extract concentrate is stabilized against oligomer separations even during the concentrating. The weight ratio of added caprolactam to extract content is set in the range from 0.1 to 1.5, preferably from 0.5 to 1.
The extract liquor concentrate from the concentrating generally has a temperature from 107° C. to 112° C. and is subsequently admixed with the lactam to be polymerized. Sufficient lactam is used for the resulting mixture to have a water content of from 0.5 to 13% by weight, preferably from 0.5 to 10% by weight, particularly preferably from 0.6 to 7% by weight, especially from 1 to 4% by weight, particularly preferably from 1.9 to 3.5% by weight. To set this water content, the concentrate is mixed with the lactam in a weight ratio which is generally in the range from 1:1 to 1:12, preferably from 1:1 to 1:10, especially from 1:1 to 1:8. The high lactam content (generally in the range from 79 to 95% by weight) improves the solubility of the oligomers present in the mixture, so that no separations are to be observed. The mixture is therefore stable and can
Hildebrandt Volker
Ludwig Alfons
Mohrschladt Ralf
BASF - Aktiengesellschaft
Hampton-Hightower P.
Keil & Weinkauf
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